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Updated: 1 hour 42 min ago

S. N. Johnson-Roehr

3 hours 33 min ago

S. N. “JR” Johnson-Roehr is Sky & Telescope’s Observing Editor. She comes to S&T via University of Virginia, where she taught courses on the history of architecture and science.

JR at Kirkwood Observatory

Courtesy Kevin O. Mooney

JR began systematically sky watching the summer she turned twelve, when her family moved to a farm in north central Washington state. Her parents helped her buy her first telescope when she was fourteen (her mother worked in the apple orchard with her, her father matched her paychecks). Twin interests in astronomy and architecture eventually led her to a graduate program at University of Illinois, where she completed a doctoral dissertation on the astronomical observatories built by the Indian Maharaja Sawai Jai Singh II in the eighteenth century. After finishing her degree, JR held a Mellon Postdoctoral Fellowship at Rutgers University and an ACLS New Faculty Fellowship at University of Virginia, where she also served as Field Editor in South Asian Studies for Dissertation Reviews. She comments occasionally on the history of astronomy and architecture at her website, Observatories and Instruments. Look for her chapter on Sawai Jai Singh’s observatories in the upcoming edition of the Handbook of Archaeoastronomy and Ethnoastronomy (Springer, 2015).

When JR isn’t in the office, she can be found somewhere outdoors — on the trail, on the lake, or in a tent.

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Categories: Astronomy Headlines

Mutual Events of Jupiter’s Satellites in 2014–15

5 hours 56 min ago

During Jupiter's 2014–15 apparition, as it shines brightly down from the sky, its four big Galilean moons will occult and cast their shadows on each other quite often. A "mutual events season" like this happens about every 6 years, when Earth and Sun cross the plane of the satellites' orbits.

Some of these events will dim the shadow-eclipsed moon, or the combined light of two moons during a partial or total occultation, enough to detect the change visually in a small telescope for some minutes. Photometric recordings of the events, even shallow ones, provide a very accurate way to refine the gradually changing orbits of the satellites. Their orbits are morphing in interesting long-term ways due to tidal interactions among the satellites and between them and Jupiter, including tidal energy dissipation within the satellites themselves. For instance, this is why Io is so volcanic.

The international campaign to record light curves during this mutual-event season is being coordinated by the IMCCE (Institute of Celestial Mechanics and Calculation of Ephemerides) in Paris. Read all about it at

www.imcce.fr/phemu

In particular, to get the IMCCE's complete table of event predictions for this season, go to

www.imcce.fr/hosted_sites/saimirror/nsszph515he.htm

and click the "Show" button.

To condense the list to only your events — those visible when Jupiter is up during darkness at a location near you — use this list of observatories; do a Control-F search on the list for an observatory or city within a few hundred miles of you, and then back on the events table creation page, enter the 3-digit observatory code in place of the "500".

In the table you get, look for events with a large change in magnitude: the "Δm" column. A Delta-m of 0.7 magnitude or more (at least a 50% or so drop in light) ought to be fairly readily visible by eye.

The table also gives the altitude of Jupiter above the horizon and the depression of the Sun below the horizon at the time. See the explanation of the table's column headings.

There's lots more on the main site to browse, including more useful tools.

And here's a PowerPoint telling why and how to join the timing campaign, including the equipment you'll need, courtesy of the International Occultation Timing Association (IOTA).

 

 

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Categories: Astronomy Headlines

Fomalhaut – A Crazy-Wide Triple Star

6 hours 33 min ago

Lonely Fomalhaut turns out to have plenty of company. Learn how to find its two remarkably distant stellar companions.

Modest star, big reach

Fomalhaut is 16 times as bright as the Sun and lies 25 light-years from Earth. Its far-flung family covers a good chunk of the night sky!
NASA, ESA, DSS 2

Quick! Name the widest double star in the sky. If you chose Alpha Centauri and its faint, distant companion Proxima (separation 2.2°), you would have been correct ... prior to 2013. That year, Eric Mamajek (University of Rochester) and his colleagues announced the discovery of Fomalhaut C, a companion star located a whopping 5.7° northwest of Fomalhaut (Alpha Piscis Austrini) in a different constellation, Aquarius. 

Fomalhaut (FO-mal-ought) stands out as the only 1st-magnitude star among the fall constellations. For observers at mid-northern latitudes, it crouches low in the southern sky in the dim constellation Piscis Austrinus, the Southern Fish, and stands due south around 11:00 p.m. local time in early October (10:00 p.m. at mid-month).

Though it may appear isolated in the barren October sky, Fomalhaut has company. It feels the gravitational tug of the magnitude +6.5 star TW Piscis Austrini, 2° to the south. Both are 25 light-years distant and move in tandem across space, partaking of the same proper motion. They form a true double star with an actual separation of 5.5 trillion miles, or 0.91 light-years.

View of the Fomalhaut triple star system from Earth. The small inset shows a zoom of the newly discovered comet belt around Fomalhaut C as seen at infrared wavelengths by Herschel. The large inset shows a zoom of the much larger comet ring around Fomalhaut A as seen at optical wavelengths by Hubble. Grant Kennedy (Cambridge), Paul Kalas (UC Berkeley)

View of the Fomalhaut triple star system from Earth. The small inset shows a zoom of the newly discovered comet belt around Fomalhaut C as seen at infrared wavelengths by Herschel. The large inset shows the much larger comet ring around Fomalhaut A as seen at optical wavelengths by Hubble.
Grant Kennedy (Cambridge), Paul Kalas (UC Berkeley)

Better known as Fomalhaut B, TW PsA is three-quarters as massive as the Sun. The "TW" designation tells us that it's a variable star, and a fascinating one at that. As Fomalhaut B rotates, large starspots cause its light to vary in an irregular fashion. A pair of binoculars and a moonless sky will show it nestled next to 6th-magnitude SAO 214187. Look about 2° south of Fomalhaut to spot it.

Exploring the realm of Fomalhaut and its companions

Use this map to help you find Fomalhaut, the only bright star in the southern half of the autumn sky. Its bright companion, Fomalhaut B, is easily seen in binoculars, while Fomalhaut C will require at least a 6-inch telescope and dark sky. The detailed map below will guide you to this distant object. Click to enlarge.
Source: Stellarium

The 2° gap between Fomalhaut A (the main, bright star) and B is nothing to sniff at -- most double stars are much closer. But that's chump change compared to the 11 full moons (5.7°) of sky between Fomalhaut and its newly recognized second companion. When Mamajek and team carefully measured the proper motions and radial velocities (speed toward or away from Earth) of stars in the region, and compared them with precise distance measurements from the Hipparcos satellite, they uncovered yet another member of the Fomalhaut system, an obscure magnitude 12.6 red dwarf we now know as Fomalhaut C.

Hop, skip and a jump takes you to the fringe of Fomalhaut's domain

To ferret out Fomalhaut C, start with the naked-eye star Epsilon PsA and star hop about 2° northeast to a pair of 6th-magnitude stars (marked). Next, follow the "curl" of six stars (ranging in brightness from 8th- to 11th-magnitude) northwest to a compact triangle of 11th- and 12th-magnitude stars. Fomalhaut C lies a short distance northwest of the bottom right star of the triangle. If you prefer to print your own chart, Fomahaut C lies at R.A. 22h 48′, Dec. –24° 22′. Click to enlarge and print out for use at the telescope.
Source: Chris Marriott's SkyMap

A member of one of the widest, if not the widest (known) multiple star system in the sky, Fomalhaut C's true 3D separation is 2.5 light-years from Fomalhaut and 3.2 light-years from Fomalhaut B. While those distances seem impossibly vast, they're well within Fomalhaut's tidal radius of 6.2 light-years, where the star's gravity dominates that of the Milky Way.

 Amanda Smith

Artist’s impression of the Fomalhaut system. We've known for years that Fomalhaut A (right) is surrounded by a disk of cometary debris and at least one planet. A similar belt of comets and asteroidal debris was recently discovered around Fomalhaut C (at left). Notice that the belt around Fomalhaut A is offset slightly, possibly caused by past interactions with Fomalhaut C. Click for more information disk formation in the system.
Amanda Smith

When you next look at Fomalhaut twinkling above the fall leaves, put four fingers together and hold them up against the sky. They'll cover about 8°, or the amount of real estate spanned by the triple system. Incredible, isn't it? There are undoubtedly other stars with as wide or wider separations. But Fomalhaut's relative closeness to Earth gives those distances big play in the night sky, helping us appreciate just how prodigious a star's gravitational sphere of influence can truly be.

Need some help navigating the night sky? Check out the Pocket Sky Atlas!

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Categories: Astronomy Headlines

Citizen Scientists Probe Early Galaxies

Mon, 09/29/2014 - 06:00

New data collected by Galaxy Zoo show early galaxies with central bars, providing implications about how galaxies grow.

The NASA/ESA Hubble Space Telescope has taken a picture of the barred spiral galaxy NGC 1073, which is found in the constellation of Cetus (The Sea Monster). Our own galaxy, the Milky Way, is a similar barred spiral, and the study of galaxies such as NGC 1073 helps astronomers learn more about our celestial home. The Hubble Space Telescope is a project of international cooperation between ESA and NASA. NASA, ESA

The barred spiral galaxy NGC 1073, which is found in the constellation of Cetus (The Sea Monster).
NASA, ESA

The Hubble Space Telescope peers across the universe — imaging everything from the exquisite details of “nearby” galaxies millions of light-years away, to the blurry galaxies that formed only a few hundred million years after the Big Bang. Imaging the whole shebang allows astronomers to study the origin and evolution of galaxies, gaining insight into our own Milky Way Galaxy.

But many mysteries remain. Most spiral galaxies in the nearby universe, for example, have a bar in their center, with the spiral arms coming off it like streamers off the ends of a twirling baton. Although astronomers agree that these bars form when a galaxy passes from youth into adulthood, they disagree on the point in cosmic history at which this typically happens.

Now, astronomers using data collected by Galaxy Zoo — a crowd-sourced astronomy project that invites the public to analyze fuzzy images of distant galaxies — are peering deeper into the universe in search of these barred galaxies.

“Galaxy Zoo works because spotting features in galaxies is a task well suited to humans. We as a species are great at pattern recognition,” says project astronomer Brooke Simmons (Oxford, U.K.). “And you don't need to be an astrophysicist to recognize a boxy shape inside a rounded disk.”

The first disk galaxies were puffy and “hot,” with the gas and dust inside moving in highly turbulent ways. But gradually they “cooled” and the contents settled down, flattening into a thinner shape. At this point the thin disks grow magnificent spiral arms or barred features.

But astronomers remain unsure about when this settling occurs. Nearly 10 years ago astronomer Kartik Sheth (now at the National Radio Astronomy Observatory in Charlottesville, Virginia) found that the fraction of barred galaxies dropped from 50–70% in the nearby universe to 10% when the universe was only 6 billion years old. Further studies found roughly the same gradient.

So Simmons and her colleagues used Galaxy Zoo to probe even deeper. Extending the previous trend farther back in time, they expected the fraction of barred galaxies to drop to zero when the universe was 5 or 6 billion years old. Instead, they found that the number of barred galaxies when the universe was only 3 billion years old was still as high as 10%. The surprising results suggest that the young universe churned out bars relatively quickly.

Sheth worries about using volunteers’ eyes to determine the number of barred galaxies, instead of using statistical analyses. “Like Richard Dawkins says, we all see the patterns we want to,” he says. But he does agree that whatever the exact number, there are certainly some barred galaxies in the early universe, and this is unexpected.

These results are “new and fresh because no one really looks that hard,” says astronomer Bruce Elmegreen (IBM Corporation). These early barred galaxies might have formed either because they settled down earlier than expected, or because they were the result of two galaxies colliding.

Understanding the exact mechanisms that helped these galaxies grow their striking structures will shed light on how our own Milky Way became a starry behemoth with spiral arms and a central bar.

Reference:

B. D. Simmons et al. “Galaxy Zoo: CANDELS Barred Disks and Bar Fractions.” Monthly Notices of the Royal Astronomical Society, Accepted.

Find your own galaxies with our classic Pocket Sky Atlas.

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Categories: Astronomy Headlines

This Week’s Sky at a Glance, Sept. 26 – Oct. 4

Fri, 09/26/2014 - 10:34
Moon passing Saturn, Mars and Antares

The waxing crescent Moon works its way eastward above the star-and-planet display low in the southwestern twilight. (These scenes are plotted for the middle of North America.)

Friday, September 26

As early as 8 or 9 p.m. now look for Fomalhaut, the lonely 1st-magnitude Autumn Star, twinkling on its way up from the southeast horizon. It will be highest due south around 11 or midnight (depending on your location).

Saturday, September 27

Low in the southwest in twilight, Mars and Antares are passing 3° apart this evening and Sunday evening, as shown above. Meanwhile, off to their right, the waxing crescent Moon floats a couple degrees to the lower right of Saturn (for North America).

Sunday, September 28

Spot the Moon in the southwest as twilight fades. Use it as your guide to Saturn far to its lower right, and the Mars-Antares pair to its left (as shown above).

Monday, September 29

The thick waxing crescent Moon now stands above Mars and Antares at dusk (as shown above).

Tuesday, September 30

Arcturus is the bright star due west at nightfall. It's an orange giant 37 light-years away. Off to its right in the northwest is the Big Dipper, most of whose stars are about 80 light-years away. They're both sinking lower every week now.

Wednesday, October 1

First-quarter Moon (exact at 3:33 p.m. EDT). The Moon shines above the Sagittarius Teapot in the south at nightfall.

Thursday, October 2

Look high above the Moon at nightfall for Altair. Crossing the zenith (for the mid-northern latitudes) are the other two stars of the Summer Triangle: Vega and Deneb.

Friday, October 3

As early as 8 p.m. now look for Fomalhaut, the Autumn Star, on its way up from the southeast horizon. This evening it's very far lower left of the Moon.

Saturday, October 4

The W of Cassiopeia now stands vertically (on its dimmer end) high in the northeast around 10 or 11 p.m., depending on your location. By then the Big Dipper is lying level just above the north-northwest horizon — if you're in the mid-northern latitudes. As far south as San Diego and Jacksonville, the Dipper will lie partly below the horizon.

Make your plans for catching the total eclipse of the Moon next Wednesday, October 8th. It will happen before or during dawn for North America, and on the evening of the 8th local date for Australia and the Far East. Uranus will be near the eclipsed Moon. See the October Sky & Telescope page 50, or the version online: Wake Up to a Total Lunar Eclipse on October 8, 2014.

Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby; for an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

Pocket Sky Atlas

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means fairly heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet RoundupJupiter and Regulus before dawn, October 2014

It's only October, but already you can spot the Sickle of Leo before dawn. Jupiter marks the way.

Mercury (magnitude +0.2) has sunk deep into the glow of sunset.

Venus (magnitude –3.9) has sunk deep into the sunrise.

Mars (magnitude +0.8) is low in the southwest during dusk, passing above similarly colored but twinklier Antares (magnitude 1.0). They'll be 3° apart on September 27th and 28th, then will start to widen as Mars moves east.

Jupiter (magnitude –1.9, at the Cancer-Leo border) rises around 2 or 3 a.m. and shines brightly in the east before and during dawn. It forms a big triangle with Pollux above it (by about two fists at arm's length) and Procyon to their right. Look below Jupiter and a bit left for Regulus.

Saturn (magnitude +0.6, in Libra) is sinking away into the afterglow of sunset. Look for it well to the right of the Mars-Antares pair, and probably a little lower depending on your latitude.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are high in the southeast and south, respectively, by 10 or 11 p.m. See our Finder charts for Uranus and Neptune online or in the September Sky & Telescope, page 50.

All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 hours.

“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”

— Neil deGrasse Tyson, 2014.

 

The post This Week’s Sky at a Glance, Sept. 26 – Oct. 4 appeared first on Sky & Telescope.

Categories: Astronomy Headlines

John S. Gianforte

Fri, 09/26/2014 - 06:34

gianforte_bioBy night, John is an active astronomer and imager — by day, he is Sky & Telescope’s equipment editor. John has been an astronomer since he was seven years old and began his observing and astrophotography career in the early 1970s. His astronomical career has included observing, astro-imaging, science writing, astronomy outreach, and teaching astronomy for the University of New Hampshire System as well as for other colleges.

John’s main astronomical research interests include: observing and recording transits of extrasolar planets, cataclysmic variable stars, imaging comets, asteroids and supernovae. He also writes on astronomy on his website: www.theskyguy.org. John has appeared on public radio talk shows as well as on public television in his efforts to communicate the amazing discoveries in astronomy to anyone interested in and curious about our universe. He is the co-founder of the Astronomical Society of Northern New England (ASNNE), which formed in 1983. John has also assisted various colleges, universities, and planetariums with their astronomy programs and outreach efforts in astronomy education.

As S&T’s equipment editor, John is most excited about continuing S&T’s great tradition of bringing the modern tools of astronomy into the hands of today’s amateur astronomers and to help continue closing the gap between what professional astronomers can achieve and what amateurs can do for the science of astronomy and for their own enjoyment. He is also very excited to try out and test the latest innovations, both complex and simple, at his Blue Sky Observatory and bring the results of these investigations to S&T readers.

The post John S. Gianforte appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Wake Up to a Total Lunar Eclipse on October 8, 2014

Fri, 09/26/2014 - 05:45

Start your day with an eclipse of the full Moon! On the morning of October 8, 2014, a total lunar eclipse will be visible across most of North America.

Image of red Moon during lunar eclipse April 15, 2014

The total lunar eclipse of April 14-15, 2014, was visible all across North America.
Joel D. Tonyan

We’re approaching the second of four total lunar eclipses that come at half-year intervals in 2014 and 2015: a lunar-eclipse tetrad. All four can be seen from at least parts of North America.

The one before dawn on Wednesday, October 8th, will be visible from nearly all of the Americas. Moreover, the Moon, two days after perigee, will be 5% larger in diameter than it was during the first eclipse of the tetrad on April 14-15 earlier this year.

The map, diagram, and timetable below will tell what to expect at your location and when.If you’re in the central or western parts of the U.S. and Canada, you’ll see the total eclipse high in a dark sky well before sunrise. Easterners will find dawn brightening and the Moon sinking low in the west while the eclipse is in progress — offering particularly interesting photo opportunities. Viewers in Australia and eastern Asia get to view this event on the evening of October 14th,

For your location, see if the Moon will set or rise during any stage of the eclipse. Because an eclipsed Moon is always full, the Sun rises or sets at almost the same time on the opposite horizon. This means a lunar-eclipse moonset or moonrise always happens in a bright sky.

For your location, see if the Moon will set or rise during any stage of the eclipse. Because an eclipsed Moon is always full, the Sun rises or sets at almost the same time on the opposite horizon. This means a lunar-eclipse moonset or moonrise always happens in a bright sky.

How to Watch the Lunar EclipseStages of the October 14th lunar eclipse

Phases and timings for the total lunar eclipse, October 8, 2014.

A total lunar eclipse has five stages, with different things to watch during each interval over a roughly 3-hour period.

The first penumbral stage begins when the Moon’s leading edge enters the pale outer fringe of Earth’s shadow, the penumbra. But the shading is so weak that you won’t notice anything until the Moon has intruded about halfway into the penumbra. Watch for a slight darkening to become apparent on the Moon’s celestial eastern side. The penumbral shading becomes stronger as the minutes tick off and the Moon moves deeper in.

The second stage is partial eclipse. This begins much more dramatically when the Moon’s leading (eastern) edge enters the umbra, Earth’s central shadow, where no direct sunlight reaches. With a telescope, you can watch the edge of the umbra slowly engulfing one lunar feature after another, as the entire sky grows darker and darker.

An hour or so into partial eclipse, only a final bright sliver of Moon remains outside the umbra. And the rest is already showing an eerie reddish glow.

The third stage is the total eclipse itself, or totality, beginning when the last rim of Moon slips into the umbra. Although the Sun here is completely hidden, the Moon is sure to glow some shade of orange or red. This red light on the Moon is sunlight skimming and bending through Earth’s atmosphere: it’s the light of all the sunrises and sunsets that ring our world at any given moment. An astronaut standing on the Moon would see the Sun hidden and the dark Earth ringed with sunset- and sunrise-colored brilliance.

On rare occasions the eclipsed Moon does go almost black. Other times it appears as bright as a fresh penny. Sometimes it turns brown like milk chocolate.

Two factors affect an eclipse’s color and brightness. The first is simply how deeply the Moon goes into the umbra; the umbra’s center is much darker than its edges.

The other factor is the state of Earth’s atmosphere along the sunrise-sunset line. If the air is very clear, the eclipse is bright. But if a major volcanic eruption has recently polluted the stratosphere with thin global haze, the eclipse will be dark red, ashen gray, or almost black.

In addition, blue light refracted by Earth’s clear, ozone-rich upper atmosphere can also add to the scene, especially near the umbra’s edge, creating a subtle mix of changing colors. Time-lapse videos might show large “flying shadows” in the umbra, caused by changing cloud-shadowing effects around the sunrise-sunset line as Earth turns.

Totality on October 14th lasts 59 minutes. And then, as the Moon continues eastward along its orbit, events replay in reverse order. The Moon’s leading edge reemerges into sunlight, ending totality and beginning stage four: partial eclipse again.

When the entire Moon escapes the umbra, only the last penumbral shading remains for stage five. This final duskiness slowly fades away, leaving the full Moon as bright as ever.

Eclipse eventEDTCDTMDTPDTPenumbra first visible4:45 a.m.3:45 a.m.2:45 a.m.1:45 a.m.Partial eclipse begins5:15 a.m.4:15 a.m.3:15 a.m.2:15 a.m.Total eclipse begins6:25 a.m.5:25 a.m.4:25 a.m.3:35 a.m.Mid-eclipse6:55 a.m.5:55 a.m.4:55 a.m.3:55 a.m.Total eclipse ends7:24 a.m.6:24 a.m.5:24 a.m.4:24 a.m.Partial eclipse ends—7:34 a.m.6:34 a.m.5:34 a.m.Penumbra last visible?——7:05 a.m.6:05 a.m.How To Photograph the Lunar Eclipse

With the prospect of the Moon changing from bright to dull red-orange and back again, don't overlook the possibility of recording this dramatic celestial event with your still or video camera! Photographing a total lunar eclipse isn't difficult — but it does take a little preparation.

Total lunar eclipse of April 14-15, 2014

Alberto Levy captured last April's total lunar eclipse over a period of 3½ hours from his backyard in San Diego. He used a 4-inch f/8 refractor and exposures ranging from 1⁄1200 to 2 seconds at ISO 400.

Most importantly, you'll need a telescope or telephoto lens that enlarges the Moon to a good size. The minimum focal length for getting a good-looking Moon is about 300 mm. You'll also need a tripod to keep your camera rock-steady, or you can piggyback your camera on a tracking telescope mount.

Because the coloration and brightness of eclipsed Moon is different every time it plunges through the umbra — and even during totality itself — the best advice for photographing a lunar eclipse is to take lots of pictures at many different exposures. Count on using times of ½ second or longer during totality, so make sure your camera is able to make exposures that long, preferably in "manual" mode. Use a remote control (or the self-timer function) to minimize vibration during the exposures.

S&T's imaging experts offer more great tips on how to photograph a lunar eclipse elsewhere on this website.

Want to know more about the Moon? Look to the Sky & Telescope Moon Globe for all the details!

The post Wake Up to a Total Lunar Eclipse on October 8, 2014 appeared first on Sky & Telescope.

Categories: Astronomy Headlines

How to Never Miss an Aurora

Thu, 09/25/2014 - 07:19

Learn exactly how and when to expect the next display of the northern lights with a few easy-to-use online tools.

Burst of light in the night

A spectacular all-sky aurora. Using online resources, even beginners can know when to expect a show.
Bob King

I'm often asked when is the best time to see the northern lights. I usually point out that they're connected to solar flares and vast eruptions of solar plasma called coronal mass ejections or CMEs. While these violent events and the auroras they spawn are more common during the peak of the sunspot cycle, they're liable to happen anytime and can be as unpredictable as earthly weather.

Not a very satisfying answer, I'll admit. But before you throw up your arms and seek another seer, allow me to arm you with several essential tools. If consulted regularly, I guarantee your chances of seeing an aurora will be maximized. Don't flog me if clouds show up. They're not part of the deal.

Your first step is to drop by NOAA's Space Weather Prediction Center, register (it's free) and sign up to receive their alerts, forecasts and summaries of incoming blasts from the Sun otherwise known as space weather. Many e-mailed products are offered, but these are the ones I'd recommend to start:

* NOAA 3-day forecast: A plain-language look ahead at expected disturbances to Earth's geomagnetic field, the protective bubble of magnetism that sheds most the worst of what the Sun throws at us. Shocks to the field from arriving clouds of subatomic particles from the Sun can sometimes spark auroras. Issued twice a day. Access it directly here.

* Forecast discussion: A free-form summary covering recent and expected space weather. Included are brief explanations of what caused a particular disturbance in Earth's geomagnetic field. Solar flares, CMEs, a filament of hydrogen gas ejected from the Sun, and high speed streams of particles escaping through "holes" in the Sun's magnetic canopy (coronal holes) are the usual culprits. Occasionally, the forecasters won't know for sure — just like your local TV weathercaster.

* Geophysical alert message: Updated every 3 hours, or more often if all hell breaks loose. This message provides current and predicted space weather conditions.

All three of these sources will add some heft to your inbox, but I doubt you'll mind if it could mean standing stunned-face under one of nature's grandest sky spectacles.

Fever chart for magnetic moodiness

The Kp index measures the magnetic effects of incoming solar storms. When it's 5 or higher, auroras are possible depending on your latitude. This plot is from the recent storm on September 12th. The inset shows the Kp forecast every 3hours. The dotted line is 0h Universal Time or 7 p.m. CDT. The index is available anytime online.
Source: NOAA

Now that you're subscribed, you will need to familiarize yourself with two key concepts: the Kp  index and Bz. Every report includes a reference to them. The index, from the German planetarische Kennziffer or "planetary code number", indicates the severity of Sun-induced magnetic disturbances in Earth's vicinity. Rated on a scale from 0 to 9, it's compiled using magnetometers at 13 magnetic observatories around the globe and updated every 3 hours.

Like water off a duck's back

Gusty solar winds and blasts from CMEs usually pass Earth without rattling our magnetic defenses, but not always. If magnetic fields in the clouds point south (Bz is negative), get ready for potential auroras.
NASA

When the index is below 5 the chances of seeing aurora are low or nil. A Kp of 5 likely indicates a minor or G1 geomagnetic storm visible across southern Canada and the northern U.S. When the index hits 6 or 7, auroras intensify as the auroral oval spreads southward. Skywatchers across the middle U.S. then get a shot at seeing shimmering curtains of pink and green light dance across their normally aurora-free skies.

Every forecast that arrives in your e-mail includes the estimated Kp index. A quick glance is all it takes to see if any "fives" are on the way. "Sevens" indicate an intense storm.

Ready to move on to Bz? Embedded within the billowy clouds of solar plasma is the stamp of the Sun's magnetic field. Like a magnet, the swarm has regions of positive and negative polarity.

Even dawn couldn't put a stop to this auroral show.

An auroral curtain unfolds at dawn over Lake Superior in Duluth, Minnesota, on July 15, 2012. At right, Venus and Jupiter shine near the Hyades in Taurus.
Bob King

Most of the time, Earth's protective magnetic bubble deflects the disturbance and we're none the worse for wear. But if the south-pointing region of the cloud (said to have a southward or negative Bz) happens to brush against Earth's northward-pointing magnetic field, the two will link up like two magnets snapping together. A path now opens for electrons and protons to spiral down Earth's magnetic field lines and smack into atoms and molecules in the upper atmosphere above the polar regions. The atoms reach an excited state and then return to relaxing and smoking cigars by releasing photons of green and red light. This is what creates the aurora.

Wouldn't it be nice to know in advance if an incoming blast possessed a south-pointing (Bz)? Wish granted!

Since 1997, we've been getting help from NASA's Advanced Composition Explorer (ACE) which orbits at the L1 libration point, one of five places near Earth where the Sun’s and Earth’s gravity are in balance, allowing a satellite placed there to remain relatively stationary.

ACE acts as a distant early warning station 932,000 miles (1.5 million km) from Earth in the direction of the Sun.The probe detects the direction, strength and magnetic field particulars of incoming blasts of particles from the Sun and provides about an hour's advance warning of dangerous storms.

Profile of a solar storm

ACE plot of magnetic field direction or Bz from the September 12, 2014, aurora. You can see how the storm dissipated once the magnetic direction of the cloud changed from south (during the storm) to north (above the white horizontal line).
NASA

All this rich information, including the direction of the Bz, is plotted in near real time (updates every few minutes) at the ACE Dynamic Plots site. You can choose to see the Bz ups and downs at intervals from 2 hours to 7 days. Choose an interval and then click the "MAG SWEPAM" link.

The top plot, shown as a squiggly red line, represents the Bz. When it's positive or above the midline, the incoming solar wind points north and is unlikely to connect with Earth's field. But when it dips below the line, and in particular, when it sinks to –10 or lower, auroras are in the offing.

Like a living thing, the oval expands and contracts depending on how disturbed Earth's magnetic domain becomes.

In these Ovation plots, you can clearly see the difference in the extent of the northern auroral oval on February 18, 2014, when a storm was in progress and earlier this week during quiet conditions. The red line is the approximate viewing limit.
NOAA

One final tool I'm sure you'll find useful is a relatively new NOAA product called the Ovation Oval. Simply, it shows the extent of the auroral oval, the permanent caps of aurora centered on Earth's geomagnetic poles. Polar bears and other arctic residents get to experience auroras most dark nights of the year. The rest of us have to wait for more vigorous storms, when the oval expands southward to more populous latitudes.

I hope this little guide will help you in your planning. I've selected just a few of all the resources available on the Web — you will undoubtedly find more. However you plan, remember that space weather can often be as unpredictable as Earth weather. Happy forecasting!

Learn about the science of aurora in our recent cover story.

The post How to Never Miss an Aurora appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Dust Makes Cosmic Inflation Signal Iffy

Wed, 09/24/2014 - 06:48

A new analysis of Planck data provides the best measurements ever made of polarized dust emission across the sky — and bolsters the claim that the signal heralded as evidence for cosmic inflation is from dust instead.

Things are looking grim for the purported discovery of cosmic inflation’s fingerprints. Earlier this year, the BICEP2 team announced the detection of primordial B-modes, swirly polarization patterns imprinted on the cosmic microwave background (CMB) thanks to the hypothesized, split-second inflationary era. If inflation really did happen right after the universe’s birth, it should have spawned ripples in spacetime called gravitational waves, which in turn would have stretched and squeezed spacetime — and the plasma in it, ultimately fostering the creation of these polarization patterns in the CMB.

B-mode sky map

"Curly" B-modes of polarization in the cosmic microwave background. Each line is a measure of polarization at one point on the sky. This is an actual map of the sky near the south galactic pole, about 15° tall; the strongest curl patterns (emphasized with colors) are a couple of degrees wide, roughly the size of your thumb held at arm's length against the sky.
BICEP2

But other cosmologists quickly raised a red flag about BICEP2’s result, cautioning that the polarization detected might instead be from dust emission in the Milky Way itself. Irregular, charged dust grains interacting with our galaxy’s magnetic field can also produce the B-mode polarization patterns, and on the same angular scale as the theorized primordial ones.

Since the debate arose, the astronomy world has been waiting impatiently for the team of ESA’s Planck mission to cast the deciding vote. Planck’s all-sky CMB observations include polarization, and the mission’s researchers have been laboriously analyzing the data to separate the primordial signals from those originating closer to home. The team is being doubly careful because its preliminary data releases have inadvertently fueled the debate.

The galactic poles are the main regions of contention here. BICEP2 peered through the sparse galactic fog near the Milky Way’s south pole. More than a half dozen other B-mode experiments also focus on this Southern Hole. What everybody wants to know is, How much does dust emission contribute to the polarization signal in these galactic polar regions?

The Planck team has now released their preliminary analysis of the polarized dust emission near the galactic poles. The analysis doesn't include the detailed, multiple-frequency maps that will for certain settle the question, but it is still far and away the best measurement yet for this dust signal, says Planck scientist Charles Lawrence (JPL). What it shows is that dust is basically everywhere. "There is no escape from foregrounds, no part of the sky so clean that foregrounds can be ignored," Lawrence says.

Here's the punchline: the Planck team found that the strength of the dust signal is roughly of the same magnitude as the polarization signal reported by BICEP2.

No Final Answer Yet for B-modes

But we can’t write off the BICEP2 result just yet.

dust polarization emission at galactic poles

These kaleidoscope-esque maps show the estimated signal from polarized dust emission around the north (left) and south galactic poles, based on Planck data. Dark blue marks the “cleanest” areas. The maps are based on a fit to the angular power spectra, essentially a plot of how the strength of the dust signal changes with frequency across the sky. The black box in the south pole map roughly outlines the BICEP2 experiment’s field of view.
Planck Collaboration

First of all, the Planck analysis has sizable error bars, which the team is quick to point out. Second, there’s the ongoing issue of extrapolation. BICEP2 observed the CMB at a frequency of 150 GHz. The Planck analysis, on the other hand, uses 353-GHz data from the spacecraft’s High Frequency Instrument (HFI) and extrapolates down to what the emission should look like at 150 GHz. (See my detailed blog from June on why this is an important caveat.)

To do the extrapolation, the researchers used HFI data from 100, 143, and 217 GHz, as well as maps from various data subsets, to make sure they understood how the dust signal changes as they look at different frequencies. With this information in hand, they then took the 353-GHz data and created rough "150-GHz" maps based on how the dust signal's strength changes with frequency across the sky. The team stuck with 353 GHz instead of using the lower-frequency data to make the plots directly because at 353 GHz dust emission dominates over other polarization signals, meaning data at that frequency give the clearest picture of galactic dust.

The team stresses that the emission maps are estimates. But the analysis is exceptionally careful, and the results imply that polarized dust is the dominant signal in the BICEP2 field, says David Spergel (Princeton), who coauthored one of the cautionary analyses earlier this year. “There could be a weaker signal from gravitational waves, but I don’t think that the current data are good enough to separate out this weaker signal and make a statistically significant detection,” he sums up.

The Planck and BICEP2 teams are doing a joint analysis to get to the bottom of things. This analysis will hopefully be done in time for the big Planck conference in December, at which the Planck team will discuss the mission’s full temperature and polarization data (set to be released in November). These will include maps at all frequencies, Lawrence says, which will reveal features the averaging doesn't catch.

The new Planck analysis also picks out a few regions of sky that look the “cleanest” in terms of polarized dust emission. The largest region is, as expected, near the galactic south pole, as far from the dust-laden spiral disk as possible. Unfortunately, the BICEP2 field of view appears to have just missed the sweet spot. But other experiments — notably the Atacama B-mode Search (ABS) and the balloon-borne Suborbital Polarimeter for Inflation (SPIDER) — look right at it. (At least, if I’m doing my galactic-to-equatorial coordinate conversions correctly.)

Even if the BICEP2 result proves a gun-jumper, the experiment has helped to create the most sensitive polarization map for this sector of the sky, Spergel says. But it looks like it’s going to take even more sensitive measurements to discover primordial B-modes.

 

Reference: Planck Collaboration. “Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes.” Posted to arXiv.org September 19, 2014.

Read more about primordial B-modes and the race to detect them in our October 2013 issue.

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Categories: Astronomy Headlines

What’s Next for Inflation Cosmology – New Updates

Wed, 09/24/2014 - 06:34
The July 2014 Sky & Telescope

The July 2014 Sky & Telescope cover

Our July 2014 cover story was the apparent discovery of gravitational waves from the instant of inflation when the Big Bang took shape. Just as the article was printed, a serious challenge to the discovery appeared: the researchers had underestimated the amount of interstellar dust that could be contaminating their data.

Here are more links regarding what may yet be the biggest cosmology discovery of the 21st century, and its dust-contamination problems, and how soon the finding might be confirmed or disproved.

  • The original BICEP2 2014 Results Release page — with the preliminary discovery papers, a public FAQ, image gallery, videos of the March 17th announcement at Harvard Observatory, and websites at the institutions involved.
  • Cosmology: Polar Star. Backstory on team leader John Kovac and the BICEP project, by Ron Cowen in Nature.
  • Multiverse Controversy Heats Up over Gravitational Waves, in Scientific American. Among some cosmologists, the multiverse is disreputable and politically touchy. And further evidence for inflation pushes it more into the spotlight.
  • Tabitha Powledge’s review of the immediate media coverage and scientific reaction, at her PLoS “On Science” blogsite.
  • BICEP isn’t the only project in the race. Here are ten B-mode searches underway in Antarctica, the Andes, the upper atmosphere, and in space, with links to their websites. Which will be the first to confirm or refute BICEP? Some of the searches will be much wider, deeper, higher-resolution, and/or multi-wavelength.
  • Added May 13: As the excitement died down, on May 9th the Kavli Institute held a roundtable discussion of where things stand and what comes next. Among the cosmologists in the discussion was skeptic Paul Steinhardt as devil's advocate. Transcript.
  • Added May 14: A Nature review of the picture now, and especially the future microwave-background projects that are being discussed — and their iffy funding prospects: Cosmology: First Light, by Joanne Baker (May 14, 2014).
  • Added May 29: Just dust?? Nature reports on two new papers that challenge the BICEP team's claim to have ruled out dust in the Milky Way as the cause of their polarization signal: “No evidence for or against gravitational waves,” subtitled "Two analyses suggest signal of Big Bang ripples announced in March was too weak to be significant," by Ron Cowen.
    The two papers are:
    – 1. Mortonson, M. M. & Seljak, U.; preprint at http://arxiv.org/abs/1405.5857
    – 2. Flauger, R., Hill, J. C. & Spergel, D. N.; preprint at http://arxiv.org/abs/1405.7351
    Also in Nature, the story of the supposedly misapplied dust correction: Gravitational wave discovery faces scrutiny (May 16).
  • Added May 30: Tabitha Powledge rounds up the evolving controversy as of May 23rd at her PLoS "On Science" news blog: Inflationary Universe data in question.
  • Added June 1: The dust-contamination situation. Science magazine has published the clearest account, as far as I've seen, of the possibility that much or all of the polarization signal may come from magnetically-aligned dust in the Milky Way filtering the cosmic microwaves. Unfortunately it's behind a $20 paywall, but you may be able to get free access through a library that subscribes. It's also in the print magazine, May 23rd issue, page 790.
  • Added June 5: Our own super-clear explanation: Big Bang Inflation Evidence Inconclusive, by Camille Carlisle.
  • Added June 21: The BICEP team's formal publication, with a description of their mistaken application of the preliminary Planck dust map, and why they still think most of their signal is cosmological: Detection of B-Mode Polarization at Degree Angular Scales by BICEP2 (June 19). With a link to a good review article by Larry Krauss. Here's a New York Times story on the paper's publication: Astronomers Hedge on Big Bang Detection Claim, by Dennis Overbye (June 19).
  • Added June 24: Does the Higgs boson rule out the BICEP claim? Two theoretical physicists say that if the inflation-era fluctuations of spacetime were as strong as the BICEP team originally announced (that is, r = 1.6 or so), the Higgs boson might have ended up in a different state that would have caused the universe to immediately recollapse: Electroweak Vacuum Stability in Light of BICEP2 (June 24). The official article is behind a paywall, but the preprint is free. Here's a press release explaining things.
  • Added Sept. 11: BICEP and Planck teams are collaborating and sharing data, and in her Backreaction blog, physicist Sabine Hossenfelder gets wind that Planck is going to announce something by the end of September about dust contamination... perhaps something inconclusive.
  • Added Sept. 22–24: Planck's announcement: Yes, lots of dust. But not necessarily enough to rule out a cosmological component in BICEP's signal. Clearer results may come later this year from the ongoing Planck-BICEP collaborative analysis. See our article, Dust Makes Cosmic Inflation Signal Iffy. Also, an excellent New York Times article (by former Sky & Tel staffer Dennis Overbye): Criticism of Study Detecting Ripples From Big Bang Continues to Expand. Here is the Planck team's paper. When asked for comment, Princeton cosmologist David Spergel told Sky & Telescope, "The Planck data implies that polarized dust is the dominant signal in the BICEP2 field. There could be a weaker signal from gravitational waves, but I don't think that the current data is good enough to separate out this weaker signal and make a statistically significant detection. Discovery of gravitational waves will await more sensitive measurements — fortunately, there are many groups pushing towards higher sensitivities."

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Categories: Astronomy Headlines

India’s Mars Orbiter Mission Arrives Safely

Tue, 09/23/2014 - 20:48

On September 24th, after a convoluted, 10-month, 400-million-mile flight, India's first-ever interplanetary explorer fired braking rockets and slipped into orbit around the Red Planet.

MOM before launch

The Mars Orbiter Mission spacecraft, seen here prior to its launch in November 2013, employs a design used for other Indian spacecraft.
ISRO

It's a historic day for India's maturing space program, as its first-ever interplanetary explorer successfully slipped into orbit around the Red Planet. The Mars Orbiter Mission, informally called Mangalyaan (Hindi for "Mars-craft"), became the planet's newest robotic satellite at 7:30 a.m. India Standard Time on September 24th (10:00 p.m. EDT on the 23rd).

MOM operated autonomously throughout the orbit-insertion sequence. At the time Mars was 139 million miles (224 million km) away, so radio confirmation of the craft's arrival took 12½-minutes to reach Earth. Adding further to the drama, the spacecraft was both out of view from tracking stations on Earth and hidden in the planet's shadow during portions of the engine firing.

Mission engineers and scientists, who'd gathered at the the Indian Space Research Organization's control center in the southern city of Bangalore, applauded when the main engine and eight smaller thrusters flared to life.

India's space control center

Applause erupts from flight controllers in Bangalore celebrate the arrival of India's Mars Orbiter Mission at its destination on September 24, 2014.
ISRO

They cheered even louder when Doppler shifts in the arriving telemetry showed that the craft had slowed by nearly 2,500 miles per hour (1.1 km per second), exactly as planned. The initial orbit is a looping, polar-crossing 263-by-50,000-mile (423-by-80,000-km) ellipse that takes 3.2 days to complete.

MOM is the second craft to reach Mars this week (NASA's MAVEN arrived two days ago), and India now joins select company — along with the U.S., Russia, and the European Space Agency — to make the trek. Team members boasted that never before has a nation's spacecraft successfully reached Mars on the very first try. "History has been created today," prime minister Narendra Modi told the flight team.

Emboldened by the success of its 2008 lunar orbiter, Chandrayaan 1, ISRO's managers quickly set their sights on Mars. The MOM spacecraft follows that same basic design, utilizing a cube-shaped structure about 5 feet (1.5 m) on a side and a mass of about 1½ tons. Indian culture is imbued with the notion of jugaad — finding innovative but frugal solutions to life's challenges — and ISRO has taken that embraced that concept. Remarkably, MOM's entire mission cost is only about $75 million, a fraction of teh $670 million that NASA is spending on MAVEN.

MOM's path to Mars was not simple. After launch on November 13, 2013, the spacecraft made a half dozen trips around Earth in ever-larger loops, boosted higher and higher by repeated rocket firings. A final maneuver on December 1st pushed it free of Earth and sent it on a 10-month cruise toward Mars.

Success: Just Getting There

The mission's primary objective is not to make pioneering scientific discoveries about the Red Planet. Instead, it's simply to demonstrate the ability to design a craft capable of interplanetary travel. Science is secondary. But MOM's instrumental payload, while modest, can potentially deliver important discoveries.

A color camera will record the Martian surface. Also aboard is a photometer that will record Lyman-alpha emissions to deduce the relative abundance of deuterium in the planet's uppermost atmosphere. A spectrometer will determine the abundance of atmospheric methane — a gas at the center of a controversial debate — down to parts-per-billion levels. Rounding out the payload are a mass spectrometer (for atmospheric composition) and a thermal-emission spectrometer (for assessing surface composition and mechanical properties).

The mission is not without its detractors. Despite ISRO's jugaad-esque approach, India is a country trying to deal with endemic poverty. Still, science journalist Manoj Kumar Patairiya put the mission in perspective in a recent New York Times column. "We know how to embrace two ideas at once," he concludes, pairing "tradition and science, frugality and innovation — just as we can deal with issues like poverty at the same time as taking a giant leap into interplanetary space."

You can journey to Mars from the comfort of your favorite reading spot, thanks to our special issue, "Mars: Mysteries & Marvels of the Red Planet." It offers a timely, comprehensive look at this intriguing world next door.

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Categories: Astronomy Headlines

Moon and Jupiter conjunction.

Mon, 09/22/2014 - 06:50

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Categories: Astronomy Headlines

Comet Jacques

Mon, 09/22/2014 - 06:40

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Categories: Astronomy Headlines

The 2014 Autumnal Equinox Arrives

Mon, 09/22/2014 - 06:08

What is "the fall equinox" and how do we know when it happens?

To those who’ve unpacked their winter coats, closed their windows at night, and felt that telltale crispness in the air, it seems that autumn has already started. Astronomically speaking, however, the fall season only comes to the Northern Hemisphere on Tuesday, September 23, 2014 at 02:29 UTC (Monday, September 22 at 10:29 p.m. EDT). At that moment, the Sun passes over the Earth’s equator heading south; this event is called the autumnal equinox.

This seems awfully precise for seasons that gradually flow from one to the next, but the reason we say this event means the “End of Summer” and “Beginning of Fall” is because it is marked by a key moment in Earth’s annual orbit.

 the celestial-coordinate system is tilted with respect to the ecliptic (the path followed by the Sun through the stars over the course of a year). The equinoxes occur when the Sun crosses from one hemisphere to the other. S&T / Gregg Dinderman

The Earth's spin axis isn't at right angles to the plane of its orbit around the Sun. One consequence: the celestial-coordinate system is tilted with respect to the ecliptic (the path followed by the Sun through the stars over the course of a year). The equinoxes occur when the Sun crosses from one hemisphere to the other.
S&T / Gregg Dinderman

The apparent position of the Sun in our sky is further north or further south depending on the time of year due to the globe's axial tilt. Earth's rotational axis does not point straight up and down, like the handle of a perfectly spinning top, but is slanted about 23.5° with respect to our orbit around the Sun.

Another way to think of this is that the plane drawn by Earth's orbit around the Sun (called the ecliptic) is tilted with respect to the planet's equator. From the perspective of Earthlings like us, the Sun follows the ecliptic in its path through the sky throughout the year. Each day the apex of the Sun's arc moves depending on the time of the year. To observers at northern latitudes (e.g., the continental United States), the Sun appears to sneak higher in the sky between late December and late June, only to drop down again from late June through late the next December. The equinox occurs when the Sun is halfway through each journey.

Earth’s axial tilt also produces our seasons. When Earth is on one side of its orbit, the Northern Hemisphere is tipped towards the Sun, receiving more direct solar rays that produce the familiar climes of summer. When Earth is on the opposite side of its orbit half a year later, the Northern Hemisphere is tipped away from the Sun. The slanting solar rays heat the ground less, producing the colder winter season.

The Sun rises due east and sets due west on the equinoxes in March and September. At other times of year it comes up and goes down somewhat to the north or south. This illustration is drawn for mid-Northern latitudes.

The Sun rises due east and sets due west on the equinoxes in March and September. At other times of year it comes up and goes down somewhat to the north or south. This illustration is drawn for mid-Northern latitudes.

The same is true in reverse for the Southern Hemisphere, of course. Christmas is a warm holiday in Sydney, Australia. For those living in equatorial regions, however, there are usually only two recognizable seasons: wet and dry; and the days themselves vary less in length.

Aside from the aforementioned celestial arrangement, several other noteworthy things happen on the equinox date:

  • For the Southern Hemisphere, the seasons are reversed so September’s equinox marks the beginning of spring, while March’s equinox signals the start of fall.
  • Day and night are nearly the same length; the word “equinox” comes from the Latin aequinoctium meaning “equal night,”according to the Oxford English Dictionary. However a poke around your almanac will show that day and night are not precisely 12 hours each, for two reasons: First, sunrise and sunset are defined as when the Sun’s top edge—not its center—crosses the horizon. Second, Earth’s thick atmosphere distorts the Sun’s apparent position slightly when the Sun sits very low on the horizon.
  • The Sun rises due east and sets due west, as seen from every location on Earth—the equinoxes are the only times of the year when this occurs.
  • Should you be standing on the equator, the Sun would pass exactly overhead at midday. Should you be at the North Pole, the Sun would skim around the horizon as the months-long polar night begins.

Curious about what's happening in the sky? Check out the 2015 Sky & Telescope Observing Wall Calendar!

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Categories: Astronomy Headlines

MAVEN Makes It to Mars

Sun, 09/21/2014 - 21:29

NASA's latest interplanetary spacecraft has settled into orbit around the Red Planet. Its year-long atmospheric studies could reveal how and why Mars lost so much of its primordial atmosphere.

This image shows an artist concept of NASA's Mars Atmosphere and Volatile Evolution (MAVEN)  spacecraft, which reached the Red Planet on September 21, 2014.Lockheed Martin

This image shows an artist concept of NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which reached the Red Planet on September 21, 2014.
Lockheed Martin

When it comes to interplanetary exploration, you've got to trust your hardware. That was the case this evening, when the scientists and engineers for NASA's latest deep-space sortie could do little more than wait anxiously, fingers crossed, at a control center in Littleton, Colorado. Out there, 138 million miles (222 million km) and 12½ light-minutes from Earth — too far away to control directly — the Mars Atmosphere and Volatile Evolution spacecraft (MAVEN) successfully fired a cluster of six engines for 33 minutes and slipped into orbit around the Red Planet.

The spacecraft didn't exactly shout "I'm here" after the 10-month, 442 million-mile cruise that began last November 18th. But Doppler shifts in a weak radio beacon showed that the engines had reduced the approach velocity by about 4,000 feet (1.23 km) per second, slowing the craft enough for the planet's gravity to snare the spacecraft at 10:24 p.m. EDT (2:24 Universal Time on September 22nd). MAVEN had arrived at Mars.

For now, the spacecraft will follow a looping polar orbit that varies from 240 to 27,700 miles (380 to 44,600 km) in altitude. Over the next 6 weeks, the engines will fire again to shrink a 4½-hour-long orbit ranging from 95 to 3,850 miles, and then small thrusters will trim that further to a final, 3½-hour loop.

Unlike NASA's other Martian explorers, which have largely focused on the state of the planet's surface and its geologic evolution, MAVEN will study the Martian atmosphere exclusively. It carries eight instruments, six of which will measure charged particles, electromagnetic fields, and plasma waves in the solar wind as it sweeps past the planet. An imaging ultraviolet spectrograph and a mass spectrometer, both mounted on a steerable platform at the end of a short boom, will assess the upper atmosphere's chemical makeup.

What Happened to Mars?

Over the next year, flight controllers will command MAVEN to make five "deep dips", dropping it to altitudes as low as 77 miles (125 km) to sample directly the uppermost wisps of the planet's already tenuous air. These observations hope to answer a longstanding puzzle among planetary scientists. There's ample evidence that, early in its history, the Red Planet had a much denser atmosphere. Rain fell from its sky, and water coursed across its landscape.

But then something happened to the atmosphere: it basically vanished and, with it, the brief era when Mars might have been suitable as an abode for life. Mars quickly became the desolate, frigid world we see today. Researchers led by Bruce Jakosky (University of Colorado), MAVEN's principal investigator, want to know what happened to all that gas (most of it carbon dioxide) and, especially, to the ample water that once existed on the Martian surface.

One leading theory is that the gas escaped irrevocably to space, stripped away by the solar wind rushing past. Here on Earth, our planet's magnetosphere serves as an obstacle to the solar wind, keeping it from interacting directly with our atmosphere. But once Mars lost its global magnetic field, billions of years ago, the upper atmosphere became vulnerable.

MAVEN's spectrometers will attempt to determine if hydrogen atoms, torn from water molecules by ultraviolet sunlight, are escaping to space, and at what rate. "The stripping of gas from the atmosphere to space might have been the driving mechanism for climate change on Mars," Jakosky says.

For now, he and his team will ready the spacecraft to begin observations in early November. Results will not come quickly, he cautions, because it will take months to build up enough measurements to have a clear sense of what's going on — or going away.

However, one early, unexpected, and unprecedented opportunity will come relatively soon, when Comet Siding Spring (C/2013 A1) brushes within 82,000 miles of the Red Planet on October 19th. Because any cometary particles will strike at 35 miles (56 km) per second, there's some concern for the safety of MAVEN and other orbiters circling Mars. They'll be positioned on the back side of the planet during the time of greatest danger.

A few days before and after the comet's closest approach, MAVEN's ultraviolet spectrograph will measure both the abundance of gases within C/2013 A1's coma and also its effects on the Martian upper atmosphere (heating from cometary dust impacts or a temporary increase in water-vapor content). "We should have some pretty spectacular results," Jakosky promises.

Our special issue, "Mars: Mysteries & Marvels of the Red Planet," is loaded with spectacular photos and a must-read for anyone interested in this intriguing neighboring world.

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Categories: Astronomy Headlines

This Week’s Sky at a Glance, September 19–27

Fri, 09/19/2014 - 02:05
Moon and Jupiter at dawn

Look east at dawn for the waning crescent Moon passing Jupiter, then Regulus.

Friday, September 19

In early dawn Saturday morning, Jupiter shines upper left of the waning Moon in the east, as shown at right. How long has it been since you turned your scope on either Jupiter or the maria-covered waning crescent?

Saturday, September 20

In bright twilight, Mercury and fainter Spica are in conjunction 0.6° apart just above the west-southwest horizon. Use binoculars to scan for them about 20 minutes after sunset.

The eclipsing variable star Algol (Beta Persei) should be at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple of hours centered on 10:55 p.m. EDT.

In early dawn on Sunday the 21st, the waning crescent Moon shines far below Jupiter and closer to the right of Regulus, as shown above.

Sunday, September 21

Aquila's dark secret: If you're blessed with a really dark sky, try finding the big dark nebula known as "Barnard's E" near Altair in Aquila, using Gary Seronik's Binocular Highlight column and chart in the September Sky & Telescope, page 45.

And if you have a sky that dark, also use binoculars to investigate the big, dim North America Nebula and its surroundings near Deneb in Cygnus using the September issue's Deep-Sky Wonders article, page 56.

Monday, September 22

The September equinox comes at 10:29 p.m. on this date EDT (2:29 September 23rd UT). This is when the Sun crosses the equator heading south for the year. Fall begins in the Northern Hemisphere, spring in the Southern Hemisphere. Day and twilight-plus-night are nearly equal in length. The Sun rises and sets almost exactly east and west.

As summer ends, the Sagittarius Teapot is moves west of due south during evening and tips increasingly far over, as if pouring out the last of summer.

Tuesday, September 23

Arcturus is the bright star fairly high due west at nightfall. It's an orange giant 37 light-years away. Off to its right in the northwest is the Big Dipper, most of whose stars are about 80 light-years away.

Algol is at minimum light again for a couple hours centered on 7:44 p.m. EDT.

Wednesday, September 24

Mars is within 4° of Antares (passing north of it) from this evening through the 30th. Mars is just a little brighter and almost the same color as its namesake star; "Antares" is Greek for "anti-Mars."

Thursday, September 25

With the coming of fall, Deneb slowly replaces Vega as the bright star nearest to the zenith just after nightfall (for mid-northern latitudes).

Friday, September 26

As early as 8 or 9 p.m. now look for Fomalhaut, the lonely 1st-magnitude Autumn Star, twinkling on its way up from the southeast horizon. It will be highest due south around 11 or midnight (depending on your location).

Saturday, September 27

Low in the southwest in twilight, Mars and Antares are passing 3° apart this evening and Sunday evening, as shown below. Meanwhile, off to their right, the waxing crescent Moon floats a couple degrees to the lower right of Saturn (for North America).

Moon passing Saturn, Mars and Antares

The waxing crescent Moon works its way eastward above the star-and-planet display low in the southwestern twilight. (These scenes are plotted for the middle of North America.)

Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby; for an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).

Pocket Sky Atlas

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means fairly heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet RoundupSaturn, Mars and Antares at dusk

Follow the Antares-Mars-Saturn lineup as Mars moves leftward from its position here day by day.

Mercury (magnitude 0.0) remains very deep in the sunset. Scan for it with binoculars just above the west-southwest horizon about 20 minutes after sundown. Fainter, twinklier Spica is right nearby. Mercury and Spica appear closest together, 0.6° apart, on Saturday evening the 20th.

Venus (magnitude –3.9) is barely above the horizon due east shortly before sunrise. Bring binoculars.

Mars (magnitude +0.8, in Scorpius) glows low in the southwest at dusk near similarly colored Antares (magnitude 1.0). They'll pass 3° apart on September 27th and 28th.

Jupiter (magnitude –1.9, in Cancer) rises around 3 a.m. and shines brightly in the east before and during dawn. It forms a roughly equilateral triangle with Pollux above it (by about two fists at arm's length) and Procyon to their right. Farther to the right or lower right of Procyon sparkles brighter Sirius.

Saturn (magnitude +0.6, in Libra) is sinking low into the afterglow of sunset. Look for it well to the right of the Mars-Antares pair, and perhaps a little lower depending on your latitude.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are high in the southeast and south, respectively, by 11 p.m. See our Finder charts for Uranus and Neptune online or in the September Sky & Telescope, page 50.

All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 hours.

“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”

— Neil deGrasse Tyson, 2014.

 

The post This Week’s Sky at a Glance, September 19–27 appeared first on Sky & Telescope.

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