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This Week’s Sky at a Glance, September 23 – October 1

Fri, 09/23/2016 - 02:12
Mars, Saturn, and Antares, Sept. 23, 2016

In the southwest at dusk, Saturn and Antares continue to pull farther away to the right of Mars. Summer ended yesterday, so this is a good time to declare that the triangle they make, the nightly celestial emblem of Summer 2016, is breaking up and losing its identity.

Friday, September 23

• The starry W of Cassiopeia stands high in the northeast after dark. The right-hand side of the W (the brightest side) is tilted up.

Look along the second segment of the W counting down from the top. Notice the dim naked-eye stars along that segment (not counting its two ends). The one on the right is Eta Cassiopeiae, magnitude 3.4, a Sun-like star just 19 light-years away with an orange-dwarf companion — a lovely binary in a telescope.

The "one" on the left, fainter, is a wide naked-eye pair: Upsilon1 and Upsilon2 Cassiopeiae, 0.3° apart. They're orange giants unrelated to each other, 200 and 400 light-years from us.

• Last-quarter Moon (exact at 5:56 a.m. Eastern Daylight Time on this date). The Moon rises around midnight or 1 a.m. on the morning of Saturday the 24th. Once it's fairly well up you'll see that it's in Gemini, with Castor and Pollux to its left. Orion is much farther to its right.

• Algol is at its minimum light, magnitude 3.4 instead of its usual 2.1, for about two hours centered on 11:19 p.m. EDT. Info and comparison-star chart.

Saturday, September 24

• This is the time of year when the rich Cygnus Milky Way crosses the zenith in the hour after nightfall is complete (for skywatchers at mid-northern latitudes). The Milky Way rises straight up from the southwest horizon, passed overhead, and runs straight down to the northeast.

Sunday, September 25

• About a half hour after sunset, you shouldn't have much trouble spotting Venus very low in the west-southwest through the twilight, if you have a clear view down that low.

But can you see twinkly little Spica 2½° beneath Venus, perhaps as twilight fades further? You'll likely need binoculars or a telescope, the more so the farther north you live. Spica is only magnitude +1.0 compared to Venus's –3.9. In other words, it's only 1% as bright. (And that's before the more severe atmospheric extinction for lower Spica.)

Monday, September 26

• Arcturus shines in the west these evenings as twilight fades out. Equally-bright Capella (they're both magnitude 0) is barely rising in the north-northeast, depending on your latitude; the farther north you are, the higher it will be. Late in the evening, Arcturus and Capella shine at the same height in their respective compass directions. When will this happen? It depends on both your latitude and longitude.

Moon, Regulus, and Mercury at dawn, Sept. 27-29, 2016

As dawn brightens in the east, the crescent Moon wanes and steps lower past Regulus and Mercury on successive mornings.

• Early Tuesday morning the 27th, the waning crescent Moon is about 6° upper right of Regulus (for North America), as shown at right. Look 17° below or lower right of Regulus for Mercury.

Tuesday, September 27

• This is the time of year when, during the evening, the dim Little Dipper "dumps water" into the bowl of the Big Dipper way down below. The Big Dipper will dump it back in the evenings of spring.

• As dawn brightens Wednesday morning the 28th, spot the thin crescent Moon between Regulus above it and Mercury below it, as shown at right.

Wednesday, September 28

• As dawn brightens Thursday morning the 29th, look for a super-thin crescent Moon near Mercury very low in the east. Start looking about 45 minutes before your local sunrise time. Binoculars will help as dawn grows bright.

Thursday, September 29

The Two Top Miras. Chi Cygni now overhead in the evening, and Mira (Omicron Ceti) visible late at night, are the two brightest Mira-type stars in the sky: long-period red variables. Chi Cyg should be at or just past its maximum brightness, 5th magnitude or so. Mira should be nearly at its minimum, 8th or 9th mag. Follow them through the coming months with the article and finder charts in the October Sky & Telescope, page 49. As one brightens and the other dims, when will they pass each other in brightness?

Friday, September 30

• This is the time of year when the Little Dipper extends left from Polaris after dark. The Little Dipper's only two bright stars are Polaris, the end of the Dipper's handle, and Kochab, the lip of its bowl. Both are 2nd magnitude. They're exactly level with each other about a half hour after dark now, depending on your latitude.

• New Moon (exact at 8:11 p.m. Eastern Daylight Time).

Saturday, October 1

• As Deneb takes over from Vega as the star at the zenith after dark (for mid-northern latitudes), dim Capricornus takes over from Sagittarius as the zodiacal constellation standing due south. It is ever thus.

_________________________

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.

Pocket Sky Atlas, jumbo edition

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown above is the new Jumbo Edition for easier reading in the night. Click image for larger view.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or new Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5, nearly three times as many. The next up, once you know your way around, is 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, or the bigger Night Sky Observer's Guide by Kepple and Sanner.

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 (meaning heavy and expensive). And 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 Roundup Mercury under Regulus at dawn, late September 2016

Mercury emerging at dawn. It's growing brighter daily.

Mercury has jumped up to shine low as dawn brightens. Look due east about 45 minutes before your local sunrise time. Mercury triples in brightness from magnitude +0.4 on the morning of September 24th to – 0.8 a week later (October 1st). Don't confuse it with Regulus 15° or 20° above it.

Venus (magnitude –3.9) is low in the west-southwest in twilight. Look for it there starting about 25 or 30 minutes after sunset.

Mars (magnitude 0.0) shines in the south-southwest at dusk. It's passing above the Sagittarius Teapot.

Jupiter is out of sight in conjunction behind the Sun.

Saturn (magnitude +0.5) shines far to the lower right of Mars at dusk, in the southwest. Look for Antares (twinkling at magnitude +1.0) 6° below and perhaps a bit left of Saturn, as shown at the top of this page.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are fairly well up after nightfall is complete, in the east and southeast, respectively. Info and finder charts.

__________________________

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

The post This Week’s Sky at a Glance, September 23 – October 1 appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Globular cluster Terzan 5

Thu, 09/22/2016 - 23:21

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Categories: Astronomy Headlines

Orange sea

Thu, 09/22/2016 - 22:52

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Categories: Astronomy Headlines

Blue Plasma

Thu, 09/22/2016 - 22:42

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Categories: Astronomy Headlines

RV Guide for the 2017 Total Solar Eclipse

Thu, 09/22/2016 - 10:56
Take an RV to see the total solar eclipse

Total solar eclipse.
RV Monte

Monday, August 21, 2017 is a day that’s probably already on the calendar of every amateur astronomer. It’s the day the Moon will completely eclipse the Sun, its shadow crossing the continental U.S. for the first time since 1979. The path of totality arcs across 14 U.S. states, and hotel spaces are already filling up. Taking an RV can provide a good alternative.

If you’re planning on RVing your way to totality, be aware — you’ll need to do some planning well in advance! Eclipse chasers from all over the world will be arriving on the eclipse path in the days and weeks before and after the main event. So book plane tickets and rent RVs and campsites well in advance. Follow this RV guide to the 2017 total solar eclipse to plan for the big event.

Best Campgrounds for Total Solar Eclipse Viewing

I’ve chosen the best campgrounds within the states in the path of the moon’s complete shadow based on either the number of cloud-free days annually or proximity to highways so that you can move easily if clouds threaten your view of the Sun.

For local eclipse times at any of these locations, see eclipse2017.org.

RV Guide for the 2017 eclipse path across the U.S.

This RV guide follows the path of the 2017 total solar eclipse (visible from within the dark gray line), which will sweep completely across the United States. Click image for bigger view.

Oregon

The Oregon coast tends to be cloudy, so if you’re hoping to be among the first to experience the eclipse, Central Oregon (just east of the Cascade Mountains) is your best bet for a cloud-free experience. The town of Madras will offer a unique experience for eclipse watchers: the Oregon SolarFest at Jefferson County Fairgrounds. They’re booking RV campsites both at the fairgrounds and at “Solar City,” a farm field exactly on the centerline of the path of totality near Madras.

Another public event planned for 2017 Eclipse Day that includes camping will be held at the Oregon State Fairgrounds in Salem: the Oregon Museum of Science and Industry will host the 2017 Eclipse Viewing Party. Tickets are on sale now and camping information will be available soon.

Idaho / Wyoming

Brigham Young University – Idaho in Rexburg, Idaho is welcoming eclipse watchers to reserve free viewing spots on campus for the event. If you arrive early enough to grab a parking spot, you can park your RV on campus as well.

Another good option for local RV camping during the eclipse is at Beaver Dick Park, run by the county. Those spots are first-come, first-served as well, and there’s a five-day limit on camping there.

While the path of totality barely brushes the southern border of Montana, you’re better off camping in Idaho or Wyoming. In Wyoming, the town of Casper lies in the optimal viewing path, with amateur astronomers from all around the globe flocking there next August for the yearly Astronomy League Conference. RV parks abound around Casper. A couple of good bets for superb viewing are Lodge Pole Campground on the city’s south side and Fort Casper Campground along the North Platte River.

You’ll also see the eclipse clearly from the southern end of Grand Teton National Park, especially in the Jackson, Wyoming area. There are too many campgrounds to name in the Jackson/Teton Village area, but here are a few to contact:

  • Hoback, Granite Creek or Kozy Campground – Bridger Teton National Forest (no reservations, so you’ll have to arrive early)
  • Jackson Hole/Snake River KOA
  • Gros Ventre Campground, Jackson
Nebraska / Kansas / Missouri / Illinois

The Sandhills region of west central Nebraska will receive some of the longest viewing times of the entire solar eclipse. Towns around North Platte, including Stapleton and Tryon, are gearing up to welcome thousands of eclipse viewers next August. You’ll even discover private landowners offering RV space for the event!

Easily accessible on I-80, this vast, isolated area of grass-covered sand dunes promises unfettered views. Start your planning by contacting these RV campgrounds for reservations: Lake Maloney State Recreation Area or Hidden Valley Campground in Arnold, Nebraska. You can find plenty more eclipse related info at these two websites: Eclipse on the Range and 2017 Nebraska Eclipse.

Like Montana, Iowa has just a whisper of the state in the path of totality. Travel to adjoining Missouri or Kansas for views of a lifetime. In Kansas, the towns of Atchison, Fairview and Sabetha near the Missouri border will enjoy more than two minutes of totality. Campgrounds in the area include Lewis & Clark Village State Park, just east across the Missouri state line from Atchison, or Ron’s RV Park on Bean Lake at Rushton, Missouri

Following the eclipse path east into Illinois, you’ll find yourself surrounded by throngs of fellow enthusiasts near Carbondale, near the point of the eclipse’s greatest duration. Set up camp at Lake Murphysboro State Park, a few minutes west of Carbondale, or Little Grassy Lake Campground, also within 15 miles of the city. By the way, Southern Illinois University-Carbondale will be hosting an eclipse viewing at their football stadium in addition to two days of related events.

Kentucky and Tennessee

According to KYSolarEclipse.com, Hopkinsville is the best place in the state to view the 2017 solar eclipse, but several other good viewing spots lie nearby. Paducah, Bowling Green, and Russellville can also deliver a memorable view, and Land Between the Lakes National Recreation Area offers RV campers the chance to experience the eclipse in an enormous wilderness area straight west of Hopkinsville. Four developed campgrounds there will take reservations up to six months in advance, so add an alert to your February calendar.

If you prefer a setting closer to civilization for your eclipse experience, take your RV toward Nashville, also in the path of totality. RV camping at J. Percy Priest Lake’s Seven Points Campground will situate you between Nashville and Lebanon, Tennessee, where the center point of the eclipse will cross. The town of Clarksville, Tennessee will also be under the moon’s shadow, with Clarksville RV Park in a natural setting outside the city limits.

Georgia, North and South Carolina

With less than 15 miles of northeast Georgia crossed by the path of totality, you’ll have to pick your campsite and viewing area carefully. Focus on the Blue Ridge Mountain towns of Clayton, Dillard, and Sky Valley. Here are some campgrounds to consider:

  • River Vista Mountain Village in Dillard, Georgia
  • Cross Creek Campground near Clayton, Georgia
  • Tate Branch Campground in the Chattahoochee National Forest

You should also know that North Carolina’s eclipse path is brief but spectacular, crossing the far west side of Great Smoky Mountains National Park. The town of Franklin, North Carolina is slated to have more than two minutes of total eclipse time, so Mi Mountain Campground north of town on US-23 is a good bet for August 21st.

If you choose to camp in South Carolina next August, you’ll be chasing the last views of the total solar eclipse as it leaves the U.S. From Abbeville to Woodford, there’s the chance to experience the full eclipse for two minutes or more, with the city of Columbia, South Carolina, right in the heart of the action. Camp at Lake Murray’s Siesta Cove near Columbia and enjoy the lake as you wait for the eclipse. If you’re coming from North Carolina, you’ll also find a pleasant Corps of Engineers campground at Lake Hartwell near Greenville and on the path of totality.

There you have it, a quick list of camping ideas to get you started as you plan your 2017 RV camping trip to view the American total solar eclipse. Remember, some of these areas will fill up quickly, so don’t delay in booking your RV rental, reserving your campsite(s), and planning for mobility in case you’ve got to chase good weather. It’s going to be an adventure you’ll never forget.

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Categories: Astronomy Headlines

Daily Alert for Asteroid Flybys

Thu, 09/22/2016 - 08:54

A new e-digest from the International Astronomical Union’s Minor Planet Center gives the public a head’s up on passing asteroids.

daily minor planet, classic view

The IAU's Minor Planet Center, in collaboration with volunteers from the Oracle Corporation, has put together a daily e-alert for passing near-Earth asteroids. This is the "classic" view of the website.
Daily Minor Planet

Asteroids buzz Earth all the time. Most you never hear about. But for those folks itching with curiosity — or who want a level-headed take on whatever object has blazed its way into the news — the Minor Planet Center’s new initiative is for you.

It’s called the Daily Minor Planet. (No, it’s not staffed by mini versions of Clark Kent.) The Daily Minor Planet is an alert service that sends an e-mail once a day to your inbox with information about any passing near-Earth objects. It includes the object’s name, time of closest approach (in Universal Time), speed, size, distance (compared with the Moon), and an orbit diagram. It also includes “asteroid fast facts” — stuff like how scientists calculate an asteroid’s orbit. A link takes you to the object’s entry in the Minor Planet Center’s database, where the data-enthused can find more details.

On days when there’s no interplanetary visitor whizzing past Earth, the report will feature a recently discovery asteroid and highlight an article in the popular press. (Pick mine, pick mine!)

Daily Minor Planet, modern view

"Modern" view of the Daily Minor Planet.

The web version lets you view the Daily Minor Planet in two modes: minimalist web style (“modern”) and old-timey newspaper style (“classic”).

The idea behind the project is to provide the public with no-nonsense info about passing asteroids, to counter hype in the media. However, currently there’s no public archive, so you’ll need to either check the website daily or save your e-mails to keep up. If you’re not looking too much in the past, you can use the table that’s tucked away on the right side of the Asteroid Hazards video page. It’s at the bottom of the column labeled Running Tallies. This table lists close approaches within the last week and those upcoming in the next couple of weeks, but it only includes the name, date/time, distance, and size.

The digest isn’t designed for observers; it doesn’t include things like apparent magnitude. For those looking to spot these objects, you’ll need an ephemeris generator. The Minor Planet Center has one, as does JPL and the European NEODys-2 site.

 

Read more about the new initiative in the press release from the Harvard-Smithsonian Center for Astrophysics.

Find oodles of fun in Sky & Telescope's online store.

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Categories: Astronomy Headlines

Rosetta’s Final Days of Comet Exploration

Wed, 09/21/2016 - 10:08

ESA's historic Rosetta mission to explore Comet 67P will end in dramatic fashion on September 30th.

Rosetta

An artist's concept of Rosetta approaching Comet 67P/Churyumov–Gerasimenko.
ESA/ATG-Medialab; comet image: ESA/Rosetta/NavCam

An amazing mission of cometary exploration is about to come to a climactic end next week, as the European Space Agency's Rosetta spacecraft comes to rest on Comet 67P/Churyumov-Gerasimenko on September 30.

Launched on March 2, 2004, from Kourou, French Guiana, atop an Ariane 5 rocket, it took Rosetta 10 years to arrive at Comet 67P. This epic journey included flybys of Mars, Earth and asteroids 2867 Steins and 21 Lutetia, after which Rosetta was placed in a risky hibernation mode for several years. Reawakened on January 20, 2014, Rosetta successfully phoned home and got to work.

Thanks to Rosetta, Comet 67P is now arguably the most studied comet in the history of planetary science.

Don't Call it an Impact

Up until now, Rosetta has kept a good distance from Comet 67P, but in its last days, the final orbits of Rosetta will bring it as close as a kilometer from the surface of the comet.

Rosetta will execute its low-speed collision maneuver 20 kilometers above the comet's surface late on the evening of Thursday, September 29th. Contact is set to occur later the next day during a 20-minute window centered on 10:40 Universal Time (UT) — Comet 67P is 40 light-minutes distant, so it will take that long for ESA to receive . Rosetta is expected to come to rest on the comet at a velocity of about 1 meter per second. That's about 3.6 kilometers per hour (2.2 mph, equivalent to a slow walking pace).

Rosetta's final week around Comet 67P

Rosetta's final week orbiting Comet 67P.
ESA

“We have observations/measurements of the comet at all scales, from kilometers down to 100s of nanometers and this is giving us a wonderful insight into how the comet was created from the interstellar dust,” says project scientist Matt Taylor (ESA) “We have shown that the comet was formed from two smaller similar cometesimals that collided at low velocity.”

The Ma'at region that Rosetta is targeting is dotted with several active sink hole-style pits, measuring about 100 meters wide by 50 meters deep (imagine a football field-sized hole as deep as a typical water tower is high). Researchers plan to bring Rosetta down on a smooth plain between the Ma'at 02 and Ma'at 03 depressions, in hopes of peering inside them.

Ma'at 02 region on Comet 67P

Rosetta's final home, imaged by the OSIRIS narrow angle camera from a distance of 28 kilometers.
ESA / Rosetta / MPS for OSIRIS Team / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Pits on Comet 67P show strange meter-sized nodules sometimes called "goosebumps." Scientists believe these lumps could be cometesimals which merged together to create the comet during the origin of the early solar system.

Instruments will continue to measure gas, dust, and plasma all the way down during Rosetta’s descent, including during the 2 kilometers closest to the comet, where some particles and ions in the coma begin accelerating toward the comet's tail. “This is something we have NEVER done with Rosetta,” Taylor says.

The mission team christened the Ma'at 02 pit as Deir el-Medina, after an ancient pit in Egypt that proved to be a modern archaeological treasure trove.

Science and the Final Days of Rosetta

The Rosetta mission provides proof that big projects spanning decades can pay off. We've learned about the dynamic processes on comets that turn them into beautiful celestial spectacles.

Rosetta has also shown that water ice on Comet 67P contains three times more deuterium than water on Earth, disputing primordial comets as the source of Earth's water. Rosetta did, however, discover glycine and other complex compounds on Comet 67P, offering a possible source for delivering organic compounds to early Earth.

And just within a month of the mission's end, Rosetta amazed us once again on September 5th, as the team released images of the Philae lander wedged in a dark crack on the comet's surface. This answered a lingering mystery as to just why Philae had a such rough time phoning home, as it received little sunlight to charge its batteries.

Will we hear from Rosetta again, if it survives surface contact? “No, the spacecraft will be commanded to not try to re-contact Earth,” Says Taylor. “As Jim Morrison once said: 'This is the end... beautiful friend.'”

Still it's fun to wonder just what the final fate of Rosetta and Philae might be over the coming millennia. Discovered in 1969, Comet 67P orbits the Sun once every 6.44 years, its distance ranging from 1.2 to 5.7 astronomical units (a.u., the distance between Earth the Sun). Most likely, the twin lobes of Comet 67P will one day break apart, and perhaps, Philae and Rosetta will once again drift free and derelict around the Sun.

Congrats to ESA and the Rosetta team on an amazing and inspiring mission, as Rosetta joins Philae on a final strange and exotic resting place on the surface of a comet.

Watch the ESA hangout discussing the final fate of Rosetta.

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Categories: Astronomy Headlines

We Cross the Equinox Tomorrow

Wed, 09/21/2016 - 09:58

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

If you've gotten your coat out of the closet, closed your windows at night, and felt that telltale crispness in the air, it seems like autumn has already begun. Astronomically speaking, however, fall comes to the Northern Hemisphere this year on Thursday, September 22nd, at 14:21 Universal Time (GMT). That's 10:21 a.m. Eastern Daylight Time.

Why? This is the moment when the Sun crosses precisely over Earth’s equator as it heads south for the season. Or to put it another way, it's when the Sun crosses the celestial equator as astronomers define coordinates on the sky: declination 0°. For us northerners, this is called the autumnal equinox.

Timing the start of fall to one-minute precision may seem pretty obsessive for seasons that, around us on Earth, gradually flow from one to the next. But from a celestial standpoint, such precision naturally falls out of the wheeling of our tilted planet around the Sun.

Explaining the autumnal equinox

From our point of view, Earth is at the center of the "celestial sphere": the astronomer's sky. Earth's equator is tilted with respect to our orbit around the Sun — or equivalently, as shown here, our equator projected onto the sky (the "celestial equator") is tilted with respect to the Sun's annual path around our sky (the "ecliptic"). Equinoxes occur when the Sun crosses the celestial equator.
Sky & Telescope illustration

If you were watching the scene from far away in space, you'd see Earth's Northern Hemisphere tipped sunward during our hot months, and tipped away from the Sun when Earth is on the opposite side of its orbit: winter. The spring and fall equinoxes are the instants halfway between, when the Sun shines equally on both hemispheres.

From our own point of view here on the ground, what we see is the Sun following an arc across the sky each day, from east to west, as Earth turns. During the months when the Sun is north of the equator, its daily route is longer and higher for those of us in the Northern Hemisphere. That's why summer is hot, and why the Sun of June and July passes so high at midday.

In December and January we see the Sun's daily arc at its shortest and lowest, which is why winter is cold.

The same thing happens in reverse for the Southern Hemisphere. There, the September equinox marks the beginning of spring. Christmas in Australia is a hot summer holiday for the beach, and July is when you shovel snow.

And if you live on Earth's equator? There the traditional seasons don't really happen; people think more in terms of local cycles like “wet” and “dry.”

Sunset at the solstices and equinoxes

Watch where the Sun sets every day, and you'll see that the sunset point on your horizon cycles far left and right during the course of the year.
Sky & Telescope illustration

Several other noteworthy situations happen for us on the equinoxes:

  • Day and night are nearly the same length; the word “equinox” comes from the Latin aequinoctium, meaning “equal night” (says the Oxford English Dictionary). That's if you define "night" as starting right at sunset and ending at sunrise, rather than as the period of complete darkness. However, your almanac will show that sunrise and sunset on Thursday are not precisely 12 hours apart. This is 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 at the horizon refracts the Sun’s apparent position upward by about ½° around sunrise and sunset. The “equal night” business would be truly correct only if the Sun were a point rather than a disk, and if Earth had no atmosphere.
  • On this day the Sun rises due east and sets due west (for anywhere on Earth), as shown above. The equinoxes are the only times of the year when this happens (again with a slight fudge factor for the two reasons above).
  • It you're standing on the equator, the Sun will pass exactly overhead at midday and you'll cast no shadow. Stand at the North or South Pole, and you'll see the Sun skim completely around the horizon over the course of 24 hours.

The post We Cross the Equinox Tomorrow appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Will the Real Albireo Please Stand Up?

Wed, 09/21/2016 - 07:48

A stunning double star, Albireo is also a bit of an enigma. Is it a true binary or the result of a chance alignment? We explore the possibilities.

High and Invitingly Beautiful

Albireo stands high in the southern sky at the foot of the Northern Cross on late September evenings.
Stellarium

Ask any amateur astronomer what his or her favorite double star is and chances are they'll sing the praises of Albireo. Also known as Beta (β) Cygni, this bright and colorful duo resides at the foot of the Northern Cross, an asterism within the traditional figure of Cygnus, the Swan. Medieval astronomers knew it as the "hen's beak," which well describes its position at the bill-end of the swan.

Whenever I show friends and family Albireo through the scope, they're struck by the vivid color contrast between the brighter, orange primary and sapphire secondary star located 34″ to its northeast. The pair is set in a rich field aglitter with faint stars, which only deepens its appeal.

The stars' hues fit their spectral classifications rather well: Albireo A is an class K3 orange giant 50 times more massive than the Sun, while Albireo B is a hotter, smaller B8 (blue-white) dwarf with a mass of 3.3 solar.

Either way, any telescope or binoculars will split them. For observers in mid-northern latitudes, vibrant Albireo remains a go-to star for anyone wanting a look at one of the sky's finest jewels. Its northern declination keeps it visible most months of the year, though spring through early fall is best.

Stellar Beauties

Even a small telescope at low magnification will split Albireo and show its distinctive, contrasting hues.
Both stars are located about 380 light-years from Earth.
Stefan Binnewies / Josef Pöpsel / Capella Observatory

Many of us consider Albireo the quintessential binary star, but do these suns actually orbit about each other, or are we seeing a chance alignment called an optical double? Looking into the literature, we learn that no change in the separation between the two stars has been observed since they were first measured 261 years ago. Nor has the position angle (PA) of the secondary star changed much:  58° to 54°. This slight difference might easily fall within the margin of uncertainty. Albireo's fixed appearance implies an orbital period of at least 75,000 years and probably upward of 100,000 years.

Assuming that's true, we'd have to twiddle our thumbs a long time to measure even a niggling fraction of the secondary's orbital arc. Perhaps there's another way to confirm or deny Albireo's gravitational bond. Both stars occupy the same region of space as they orbit around the galaxy, but their proper motions, that is, the direction each star travels across our line of sight, reveals something very interesting. Instead of coming together, these two appear to be drifting apart.

Stars in the Albireo Horde

The entry for Albireo in the Washington Double Star Catalog, maintained by the U.S. Naval Observatory, lists some 14 companions, many of which are field stars. Click to enlarge.

If you go to the double and multiple star bible, the Washington Double Star Catalog (WDS), and scroll down to 19307+2758 (the star's right ascension [R.A.] and declination [Dec.]), you'll see that Albireo appears to have not one but 14 companions in all! Most are line-of-sight pairings and not physically related. The stars labeled D through L appear to be faint, unrelated field stars. No change in position angle or separation has occurred between observations. Only 11th-magnitude Albireo C has shifted a bit in the past 68 years, but it appears the star is leaving Albireo A behind, so is likely unrelated.

Has Anyone Ever Seen Albireo Ac?

Use this diagram to determine the resolving power of your telescope and the chances of seeing Albireo A's close 5th-magnitude companion. A clean, dust-free 12- or 16-inch scope under ideal conditions might do it.
Andrew James

The others? Albireo Aa (the primary) has two additional physical companions named Ab and Ac. Both the Aa-Ab and Aa-Ac pairs were nabbed in the mid-1970s using speckle interferometry. At discovery, Ab glimmered just 0.1″ from the primary and Ac at 0.4″. Both are 5th magnitude. There's no way anyone's going to split Ab, since it's currently 0.0″ seconds from A, but there exists a small possibility that 12-inch and larger scopes under superb seeing conditions might wrest Ac from Albireo's dazzle. I've never known anyone who's successfully tackled the challenge. If you're game and succeed, we'd love to hear how you did it.

In 2002, Albireo B, the bright, blue companion of Albireo, was also found to be a physical binary. The Ba-Bb pair is composed of the bright secondary (magnitude 5) with a 9.2-magnitude companion snugged up 0.4″ to its southeast. Given the four magnitude difference between them, this is well nigh impossible to cleave in even large amateur telescopes.

Once Two, Now Five

In this greatly enlarged view, we see that the familiar bright Albireo A-B pair is joined by three additional close companions. Separations are 34″ for Aa-Ba, 0.0″ for Aa-Ab, 0.4″ for Aa-Ac, and 0.4″ for Ba-Bb.
Illustration: Bob King, Source: Stellarium

When you tally up its true companions, Albireo unfolds into a quintuple star! But hold on. There's still the niggling question of whether the familiar gold and blue suns that charm our eyes off are physically connected. To attempt to get an answer, let's take a closer look at the stars' individual proper motions.

According to the the WDS, every year, the right ascension of Albireo A decreases by 2.0 milliarcseconds (mas) and its declination by 1.0 mas, which means it travels to the west-southwest in PA 244°. Meanwhile, Albireo B, with a yearly proper motion of 1.0 mas to the west and 2.0 mas to the south is headed toward PA 206°, a significantly different direction from the primary. A check on their proper motions in the Simbad database gives an even more extreme value for Albireo A of 7.2 mas (decreasing R.A.) and 6.2 mas (south).

That's not all. The radial velocity — the direction toward or away a star is moving from us — is 24 km/sec for Albireo A and 18.8 km/sec for Albireo B. Both are heading in our direction, but at different speeds.

Stellar Couple Or ...

Amateur astronomer Jeremy Perez sketched this excellent likeness of Albireo as seen through the eyepiece. He used an 8-inch telescope.
Jeremy Perez

From all appearances then, the two stars, while relatively near one another and moving through space together, appear to be on separate paths and moving at different velocities, which implies they're unrelated. I don't offer this as absolute proof because measurements can vary, but to date, our favorite double appears to be more like two strangers eyeing one another across the room rather than lovers in binary embrace.

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Did Ancient Explosions Rock Eta Carinae?

Mon, 09/19/2016 - 18:40

New observations suggest this unstable star let off some steam before its famous 19th century “Great Eruption.”

eta-carinae-341px

The Hubble Space Telescope has been keeping an eye on Eta Carinae since 1993. The Homonculus Nebula seen here hides two stars on a tight, oval-shaped orbit within.
Nathan Smith / Jon Morse / NASA

In 1843 the giant, unstable star known as Eta Carinae began shedding its outer layers, ultimately expelling at least 10 Suns’ worth of gas. That is roughly 10% of the star’s total estimated mass. The star brightened from obscurity to become the second-brightest star in the sky before fading away again to the limit of the naked eye.

Needless to say astronomers took notice, and ever since then they’ve been trying to explain this star’s sudden and mysterious violence. Sky & Telescope recently provided an overview of recent research on Eta Carinae — check out Keith Cooper’s feature article in the October 2016 issue on newsstands now — but now there’s one more surprise to add: the possibility of explosions before the famous Great Eruption of the 19th century.

Ancient Eruptions? Eta Carinae simulation

In Eta Carinae's core two massive stars orbit each other, coming within about 1½ astronomical units of each other every 5½ years. This frame comes from a computer simulation of the stars' orbits and shows their interacting stellar winds.
NASA / Goddard Space Flight Center / T. Madura

Eta Carinae lies hidden behind the gigantic, still-expanding Homunculus Nebula, a double-lobed mass created during the Great Eruption. Yet astronomers are certain the star cloaked within isn’t alone. Its stellar companion weighs in at 30 solar masses and orbits the primary every 5.5 years in an oval-shaped orbit. Perhaps, some astronomers theorize, there may even have been a third star, which met an untimely death when it plunged into the primary star and brought about the 19th century explosion.

To peer deeper into this mysterious system, astronomers have been monitoring Eta Carinae for years. The Hubble Space Telescope, for one, has been keeping an eye on Eta Car since 1993, with the most recent observation taken in 2014. Megan Kiminki (University of Arizona) and colleagues are taking advantage of Hubble’s persistence to watch the star’s ejected gas fly outward in real time.

Kiminki’s team is studying the proper motions of gaseous clumps at the periphery of the Homunculus, measuring the clumps’ motion across the sky. Combining this information with spectra, which reveal motion toward or away from Earth, gives us the clumps’ three-dimensional movement in space.

Unsurprisingly, the team finds that the ejecta are flying away from Eta Carinae at hundreds, and in some cases more than 1,000, kilometers per second (2 million mph). What is surprising, though, is that if all the ejected clumps have always been traveling at the same speed, then they can’t have all come from the Homunculus-creating event: one, or perhaps two, eruptions went off before the 19th century.

 Eta Carinae Ejecta Speeds

Hubble imaged Eta Carinae in 2014 (shown in reverse color). To best see the faint clumps of gas at the outskirts of the nebula, the Hubble images have least 9 minutes of exposure time per image, which saturates the Homunculus Nebula at the center of the frame. The arrows show how fast each clump is moving through space, and the arrows are color-coded by the year of their ejection, calculated as if they have always traveled at the same speeds they're traveling now.
Kiminki & others, Monthly Notices of the Royal Astronomical Society

The more likely eruption may have occurred around AD 1250, just before Marco Polo's first visit to the Court of Kubla Khan. The other ejection would have happened in AD 1550, just after Copernicus published his De Revolutionibus.

Ifs, Ands, or Buts

But constant speeds for outward-flung ejecta are far from a sure thing. For example, it’s possible that the incredibly fast wind now coming from the secondary star also existed in ancient times, whipping the clumps to higher speeds early on. If the constant-velocity assumption is untrue, then the theory of multiple outbursts stands on shaky ground.

“I'm open to the possibility of earlier eruptions if that is what the observations really tell us, but I am not 100% convinced yet,” says Thomas Madura (San Jose State University).

Another oddity: the gas clumps supposedly ejected in the thirteenth century lie entirely on the northeast side of the system, traveling toward Earth at an angle of 20-40° out of the plane of the sky. There’s nothing similar on the star’s west side.

“It seems curious to me that if there were another major eruption in the 1200s, it would be so extremely asymmetric,” Madura adds. “This could perhaps be a sign that the mechanisms for each eruption were drastically different.”

Understanding those mechanisms is difficult in part because there’s no firm estimate of the mass ejected in ancient times. If Eta Carinae experienced earlier explosions before the Great Eruption, that leaves astronomers once again with the question: what caused Eta Carinae to blast away its outer layers without going whole hog supernova? And if the star is so unstable, why is it still here?

“I'm sure we'll still learn a lot about the late stages of massive star evolution by continuing to study [Eta Carinae],” Madura says. “It has a habit of surprising us.”.

Reference:

Megan Kiminki et al. "Ancient eruptions of Eta Carinae: A tale written in proper motions." Monthly Notices of the Royal Astronomical Society, 2016 November 21. Full text.

 

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Messier 27

Mon, 09/19/2016 - 16:22

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