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The stars of northern winter linger in the west — as celestial bears, a lion, and a snake climb in the east. Meanwhile, Jupiter and Venus sparkle overhead.
With full Moon (and a total lunar eclipse) coming early in the month, it won’t be long before that big bright orb is gone from the evening sky, letting you enjoy the stars of spring in relative darkness.
Use brilliant Venus, in the west after sunset, as a benchmark to find the delicate star cluster called the Pleiades, the bright star Capella (nearly overhead), and the reddish star Aldebaran in Taurus. To the left of Venus, you’ll run into a distinctive three-star belt of Orion, the Hunter, with Betelgeuse above and Rigel below. To the belt’s lower left is Sirius, the brightest star in the nighttime sky.
Right now Sirius is outshone by both Venus and Jupiter, which is high up in the south as darkness falls. To Jupiter's left are the stars of Leo, the Lion.
Look high in the north, where the Big Dipper is positioned with its bowl at upper left and its handle curving toward lower right. The Big Dipper is what astronomers call an asterism, an obvious group of stars. (Orion's belt is an asterism too.)
But the Big Dipper is part of a constellation, Ursa Major, Latin for the Big Bear. And nearby is the Little Dipper in Ursa Minor, the Little Bear. Slithering up in the southeast is long, winding Hydra, the Sea Serpent. Hydra was quite carnivorous in Homer’s Odyssey. Spring is coming, and all these sky critters are on the march!
There's lots more to see by eye in the April evening sky. To get a personally guided your, download our 8½-minute-long stargazing podcast below.
There's no better guide to what's going on in nighttime sky than the April issue of Sky & Telescope magazine.
Alan MacRobert, Senior Editor
617-864-7360 x22151, macrobert@SkyandTelescope.com
(Nights, weekend: 781-275-9261)
J. Kelly Beatty, Senior Editor
617-864-7360 x22168, kbeatty@SkyandTelescope.com
An unusually brief total eclipse of the Moon will be visible before dawn this Saturday, April 4th, from western North America. The eclipse happens on Saturday evening for Australia and East Asia.
Whenever the Sun, Earth, and Moon form a near-perfect lineup in space, the Moon glides through Earth's deepest shadow — creating a total lunar eclipse. Earth's shadow gradually intrudes across the face of the full Moon until the entire lunar disk glows dim orange or red. Then events undo themselves in reverse order, until the Moon returns to full brilliance. This dramatic sequence can be seen on Saturday, April 4th — if you're looking from the right part of the world at the right time.
As was the case last October 8th, this weekend's lunar eclipse favors westerners in the U.S. and Canada. And once again most will need to look low in the west as dawn brightens — lower, in fact, than last time.
The timetable below tells what to expect at your location and when. (See also the diagram and map at the end.) Weather permitting, those near the West Coast will have the best view, with the total phase of the eclipse happening when the Moon is still fairly high in a dark sky before dawn even begins. Skywatchers in the Plains states will find dawn growing bright and the Moon sinking low in the west around totality. For the eastern half of North America, the Moon sets and the Sun rises before the total phase even begins. New England misses even the earliest partial phase.
Meanwhile, from Hawai'i or New Zealand, the eclipse happens deep in the night and high in the sky. For Australia, Japan, China, and Southeast Asia, the total eclipse occurs on the evening of April 4th.
Unlike a total solar eclipse, which can only be seen from a narrow path across Earth's surface, a lunar eclipse can be watched from the entire half of the world facing the Moon at the time.Total Eclipse of the Moon, April 4, 2015 Eclipse event UT (GMT) EDT CDT MDT PDT HAST Penumbra first visible? 9:35 5:35 a.m. 4:35 a.m 3:35 a.m. 2:35 a.m. 11:35 p.m. (4/3) Partial eclipse begins 10:15 6:15 a.m. 5:15 a.m. 4:15 a.m. 3:15 a.m. 12:15 a.m. Total eclipse begins 11:54 7:54 a.m. 6:54 a.m. 5:54 a.m. 4:54 a.m. 1:54 a.m. Mid-eclipse 12:00 — 7:00 a.m 6:00 a.m. 5:00 a.m. 2:00 a.m. Total eclipse ends 12:06 — 7:06 a.m. 6:06 a.m. 5:06 a.m. 2:06 a.m. Partial eclipse ends 13:45 — — 7:45 a.m. 6:45 a.m. 3:45 a.m. Penumbra last visible? 14:25 — — — 7:25 a.m. 4:25 a.m. What to Look For
You only need your eyes to see the drama unfold during a total lunar eclipse, though the view is enhanced when seen through binoculars or a small backyard telescope.
April 4th's total eclipse is unusual in that the Moon just barely skims through Earth's inner shadow, the umbra, and then only briefly. Because of this, the Moon's northeastern edge will remain much brighter than the deep red that is typically seen all across the eclipsed Moon's face.
The first sign that an eclipse is under way comes when the Moon is about halfway across Earth's pale outer fringe of shadow, the penumbra. A slight darkening on the Moon's left or lower-left side becomes stronger as the lunar disk moves deeper into the penumbra.
The partial eclipse that follows is more dramatic, because the Moon is partly within the umbra, where no sunlight directly reaches the lunar surface.
Totality comes about 1½ hours after the partial phases begin. However, the edge of the umbra is not sharp, due to Earth's semitransparent atmosphere, so there is some uncertainty on exactly how to define the umbra's edge. The U.S. Naval Observatory, the source used by Sky & Telescope, calculates that totality will last a bit more than 12 minutes, whereas some other sources say just 9 minutes.
In any case, during totality most of the Moon will likely glow some intense shade of orange or red — even though it is completely immersed in the umbra. This light comes from all the sunrises and sunsets around the Earth's rim at that moment: sunlight that has skimmed through Earth's atmosphere and been refracted (bent) by the atmosphere into the umbra.
After totality ends, the Moon's edge emerges into sunlight, and the sequence of partial and penumbral events plays out in reverse order. It will take 3½ hours from when the partial phase begins until it ends. About 45 minutes after that, the Moon appears fully bright white again and nothing unusual remains.
This eclipse comes just two weeks after the total solar eclipse on March 20th, which occurred at new Moon and was seen from parts of the North Atlantic and Arctic oceans. It is also the third of four total lunar eclipses during 2014–15 spaced about six months apart; the fourth will occur on the evening of September 27, 2015, for all of the Americas except northwestern Canada and Alaska. Such "eclipse tetrads" are not very common — the last one occurred a decade ago, but the next won't begin until 2032.
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The post (Brief) Total Lunar Eclipse Before Dawn on April 4th appeared first on Sky & Telescope.
Before sunrise on Saturday, April 4th, the Moon skims just inside Earth's deepest shadow during a total eclipse that favors western North America.
The next total eclipse of the Moon, on Saturday, April 4th, will be the third in the current "tetrad" of four in a row at half-year intervals.
In one sense it will be a lot like the total lunar eclipse last October 8th. Most North Americans will again need to get up early and look low in the west as dawn brightens. And, again, the farther west you are the better — more so this time, because the eastern half of the continent misses the total phase completely.
But in another way it will be different. This weekend's eclipse will be just barely total — in fact, you might even get the impression that it never becomes quite total at all. The Moon's north-northeastern limb squeaks so slightly inside the umbra of Earth's shadow that it will remain much brighter than the deep red we can expect across the rest of the Moon's face.
The map, diagram, and timetable here show what to expect at your location and when. Weather permitting, those near the West Coast will the total phase fairly high in a dark sky long before sunrise. Skywatchers farther east will find dawn brightening and the Moon sinking low in the west around totality. For easterners, the Moon sets (and the Sun rises) during the partial phase before totality begins. New England misses this one altogether.
Meanwhile, as seen from Hawai'i or New Zealand, the eclipse happens deep in the night and high in the sky. For Australia, Japan, China, and Southeast Asia, it comes on Saturday evening, local time.Total Eclipse of the Moon, April 4, 2015 Eclipse event UT
EDT CDT MDT PDT HAST Penumbra first visible? 9:35 5:35 a.m. 4:35 a.m 3:35 a.m. 2:35 a.m. 11:35 p.m. (4/3) Partial eclipse begins 10:15 6:15 a.m. 5:15 a.m. 4:15 a.m. 3:15 a.m. 12:15 a.m. Total eclipse begins 11:54 7:54 a.m. 6:54 a.m. 5:54 a.m. 4:54 a.m. 1:54 a.m. Mid-eclipse 12:00 — 7:00 a.m 6:00 a.m. 5:00 a.m. 2:00 a.m. Total eclipse ends 12:06 — 7:06 a.m. 6:06 a.m. 5:06 a.m. 2:06 a.m. Partial eclipse ends 13:45 — — 7:45 a.m. 6:45 a.m. 3:45 a.m. Penumbra last visible? 14:25 — — — 7:25 a.m. 4:25 a.m. What to Watch for During April 4th's Lunar Eclipse
A total lunar eclipse has five stages, with different things to watch for at each. You only need your eyes to see this celestial drama unfold, though the view is enhanced when seen through binoculars or a small backyard telescope.
The first 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 see anything of the penumbra until the Moon is about halfway across it. Watch for a slight darkening to become apparent on the Moon's celestial southeastern side (likely on the left or lower left as seen in the sky). The penumbral shading becomes stronger as the Moon moves deeper in.
The second stage is the partial eclipse. This begins much more dramatically when the Moon's leading edge enters the umbra, Earth's inner shadow, where no direct sunlight reaches. With a telescope, you can watch the edge of the umbra slowly engulf one lunar feature after another. Also note how the entire sky begins to grow darker as what had been a full Moon gradually disappears.
On April 4th, it'll take a good hour and a half before only a final bright sliver remains outside the umbra. By this time, the rest will already be showing a foreboding reddish glow.
The third stage is total eclipse, beginning when the last rim of Moon slips into the umbra — depending on how the edge of the umbra is defined! The edge is not sharp, since Earth has a semitransparent atmosphere. In a grazing instance like this, so critical is the adopted definition that the U.S. Naval Observatory's Astronomical Almanac (which S&T uses) lists totality as lasting 12.3 minutes, whereas Fred Espenak's Fifty-Year Canon of Lunar Eclipses says 8.6 minutes.
Most of the Moon is sure to glow some shade of intense orange or red. That red light shining onto the Moon is sunlight that has skimmed and bent through Earth's atmosphere: that is, from all the sunrises and sunsets that ring the world at any given moment.
Two factors affect a lunar eclipse's color and brightness. The first is simply how deeply the Moon goes into the umbra as it passes through; the center of the umbra is much darker than its edges. The second factor is the state of Earth's atmosphere along the sunrise-sunset line. If the air there is very clear, the eclipse is bright. But when a major volcanic eruption has recently polluted the stratosphere with thin global haze, a lunar eclipse will be dark red, ashen gray, or occasionally almost black.
In addition, blue light refracted through 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 moves and turns.
And then, as the Moon continues eastward along its orbit, events replay in reverse order. The Moon's edge re-emerges into sunlight, ending totality and beginning stage four: a partial eclipse again. When all of the Moon escapes the umbra, only the last, penumbral shading is left for stage five. By about 45 minutes later, nothing unusual remains.
The Moon's face is rich with geologic wonder. To track down all the details with your telescope, you'll want to have Sky & Telescope's laminated Moon map at your side.
Dozens of galaxy cluster collisions confirm that dark matter particles probably slip right past each other within messy cluster mergers.
The existence of dark matter would solve many problems in our universe: it enables spiral galaxies to rotate quickly at their outer edges, keeps galaxy clusters from flying apart, and forms the universe’s cosmic web structure.
But dark matter, or at least our understanding of it, creates its own set of problems. Simulations show that dark matter pools in gravitational wells, creating galaxies and clusters of galaxies with far denser cores than observed. That pooling happens when dark matter particles are slippery. Weakly interacting massive particles (WIMPs), physicists’ most popular dark particle, are downright antisocial — they glide right past one another rather than interacting.
One way to overcome the pooling problem is to make dark matter less slippery. In alternative models, so-called “hidden-sector” dark matter particles make contact, says Jonathan Feng (University of California, Irvine). Just like people at a party, he explains, particles that constantly bump into each other tend to spread out, rather than all congregating in the kitchen. Many of these models have the added benefit of explaining a mysterious X-ray spectral line seen in some galaxies and galaxy clusters.
Perfectly slippery dark matter particles should have a self-interaction cross section (a measure of how strongly they interact, measured in square centimeters per gram) of 0. Hidden-sector models propose cross-sections that range between 0.1 and 10.
But now these sticky dark matter models face a challenge. David Harvey (Observatoire de Sauverny, Switzerland, and University of Edingburgh, UK) and colleagues studied 30 galaxy clusters and found that dark matter particles appear to be quite slippery, with a cross section no higher than 0.47 cm2/g.When Clusters Collide
Astronomers can indirectly measure the stickiness of dark matter by looking at galaxy cluster collisions. When clusters collide, most galaxies sweep right past one another — space is after all very, very empty. But the tenuous gas halos surrounding the galaxies crash into each other and produce beautiful and complex X-ray emission.
If it’s slippery, dark matter will stay closely aligned with the galaxies. (The dark matter isn’t seen directly, but by its gravitational distortion of background light.) But if dark matter particles interact with one another, they’ll do one of two things: the dark mass will slow down, lagging behind the galaxies’ motion, or the dark particles will scatter, displacing the dark mass from the galaxies.
By far the best-studied cluster collision, the Bullet Cluster has already provided one estimate on dark matter slipperiness — observations limit the cross section to less than 1.25 cm2/g, which still leaves plenty of wiggle room for hidden-sector theories. Observations of other clusters haven’t narrowed the possibilities, mostly because the 3D geometry of the systems is difficult to understand.
So Harvey’s team set out to take a statistical approach instead, averaging out the measurements of lots of galaxy clusters to do away with any 3D uncertainty. Drawing from the archives of the Hubble Space Telescope and Chandra X-ray Observatory, Harvey and his colleagues looked at 30 nearby galaxy clusters. In each cluster, the team measured the offset between the hot gas (observed in X-rays) and the galaxies (seen in the visible-light images), which gives the direction of the collision.
Then the team measured the offset between the galaxies and the dark mass, which they pinpointed thanks to its gravitational lensing effect on background light. The astronomers found that across 30 clusters, the offset was essentially zero. Dark matter must be slippery indeed, with a cross section less than 0.47.The Fine Print
There are limitations to this approach, says Douglas Clowe (Ohio University). For one, it’s all archival data, collected for disparate science goals. So Hubble imaged some clusters through multiple filters and some through only a single filter. To level the playing field, the team opted to limit their analysis to a single filter for all the clusters. But that makes it difficult to separate cluster galaxies from foreground and background galaxies. While necessary, the decision added uncertainty to their measurements.
Nevertheless, Clowe adds, “Within the limitations of their data, they have produced an excellent analysis.” In addition to improving on the Bullet Cluster measurements, he says, these types of statistical studies, which pull in huge amounts of information on dozens of galaxy clusters, show what we can look forward to in five to ten years, when next-gen telescopes such as WFIRST, LSST, and Euclid come online.
But don’t count out hidden-sector models just yet, says Feng. Even with the tighter limit from Harvey’s study, these models have enough wiggle room to remain viable. “The really interesting message here,” Feng adds, “is that these observations are getting tighter and tighter in a very interesting region of parameter space.” In the near future, studies like this one might rule out self-interacting dark matter . . . or better yet, find evidence for its existence.
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South Shields Astronomical SocietyADDRESS
South Tyneside College
Saint George's Avenue,
Tyne and Wear
firstname.lastname@example.orgURL NUMBER OF MEMBERS
Society founded October 1957.
Meetings held every other Thursday from September to May in Block L Lecture Theatre 19:00 – 20:00