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The Sky This Week, 2013 January 15 - 22

Looking over the overlooked Moon.
Moon_091122_GalScope_01small.jpg

The Moon, 2009 NOV 22
Imaged with a 50mm (2-inch) "Galileoscope"
for the 2009 International Year of Astronomy 


The Moon waxes in the evening sky this week, passing through autumn’s dim constellations before ending the week among the bright stars of the Great Winter Circle. First Quarter occurs on the 18th at 6:45 pm Eastern Standard Time. Luna manages to avoid bright encounters until the 21st, when she drifts less than a degree south of Jupiter during the course of the evening. The pair will be closest at around 11:00 pm.

Moonlight will gradually start to flood the evening sky with light, extinguishing all but the brightest stars as the Moon climbs toward the winter constellations. Indeed, this will be one of the best times of the year to examine Luna’s rugged surface through the telescope. A popular phrase among amateur astronomers is that the Moon is "looked over, then overlooked" by new telescope owners and seasoned veteran observers alike. However, she is also our closest neighbor in space, and as such reveals lots of detail in even the smallest telescopes. Each night offers views of new lunar vistas as the "terminator" that divides the lunar sunrise line slowly creeps across the lunar landscape from night to night. The Moon’s features are dominated by impact craters ranging in size from the vast lunar "seas" caused by impacts from large asteroids in the early chaos of the solar system to small "pit" craters that challenge the resolution of your telescope. It is almost impossible to appreciate the scale of these formations, though. The smallest craters we can routinely see in our 12-inch refractor here at the Naval Observatory are comparable in size to the famous Meteor Crater in Arizona, which is a very impressive formation to see in situ. Most of the craters seen in amateur telescopes are tens of kilometers across, and there are literally thousands of them. The "southern highlands" of the Moon are a jumble of craters and other tectonic features which bear mute testament to the violence of the first billion years of solar system history. Careful examination will show many different kinds of landforms as well, such as great isolated mountain peaks jutting out of the smooth lava plains of Mare Imbrium or vast cracks and sinuous rilles caused by expansion of lunar crust over the eons. So give our only natural satellite a good look this week. She is a fascinating companion.

The bright stars of the Great Winter Circle compete with the Moon and Jupiter for your attention. Take the time to look at them, especially if you have a pair of binoculars. You’ll be surprised at how much a little optical aid will bring out the colors of these bright stars, from the gold glimmer of Capella to the ice-blue of Sirius and Rigel and the warm ruddy tint of Betelgeuse and Aldebaran. On chilly nights the view of Orion’s belt stars almost seems to shimmer in my view. The middle star, Alnilam, is one of the most intrinsically bright stars in the galaxy, shining with a luminosity of some 250,000 Suns across a gulf of over 2000 light years. If Alnilam were located at the distance of Sirius, just over 8 light years away, it would appear brighter than the Full Moon in our nighttime sky!

Jupiter outshines all the other stars in the winter sky and after the Moon is the most rewarding object for telescopic view. You’ll have a particularly good evening of exploration on the 21st, when the Moon glides just south of the giant planet. Once again the sense of scale is distorted if you view both objects in a low-power field. Jupiter appears tiny compared to the pockmarked landscape of Luna, yet its true size is miniscule compared to the distant planet. Jupiter’s four large moons, discovered 403 years ago this month by Galileo, are comparable in size to the Moon, but they appear as tiny dots in all but the largest telescopes. If you have a three-inch or larger aperture telescope, you can watch the shadow of the largest moon, Ganymede, drift across the southern edge of Jupiter’s disc between 10:00 pm and midnight EST on the night of the 17th.

Saturn now rises at around 1:30 am in the southeastern sky and nears the meridian in the southeast as morning twilight begins to gather. This planet is not as dynamic as Jupiter, but the dullness of his disc is more than made up for by the graceful arc of the planet’s famous rings. If you look carefully at the planet’s vicinity before the sky begins to brighten you should see several tiny pinpoints of light that seemingly swarm around the planet. A good four-inch telescope should reveal five of Saturn’s icy moons, and an 8-inch instrument should show enigmatic Enceladus when it is at elongation. This latter moon is remarkable for the water-ice geysers that the Cassini space probe discovered near its south pole.

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