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Astronomy From Your Back Yard - 7/21 to 7/28 2010

By Dave Grosvold

This week, the planets Mercury, Venus, Mars, and Saturn continue to mark a nice straight line along the ecliptic, with Mercury in the lead as they sink into the western horizon. Mercury will set very soon after dusk, so you will have to watch carefully as the sun sets to catch it.

On Friday evening, Saturn's largest moon, Titan, is below and to the right of the Ringed Planet this evening. A small telescope will show it at magnitude 9. In the image at left, Titan is above and to the left of Saturn, which is the orientation that matches the mirror-reverse image found in a refracting telescope. Titan orbits Saturn every 16 days, so you can watch it swing from one side of Saturn to the other over a bit more than a two week period. Saturn's Rings, have widened a bit in the last several weeks to 3° from edge-on, even as the planet recedes from Earth.

Jupiter is at magnitude 2.6, in the constellation Pisces this week, and it rises around 12 AM CDT. Jupiter shines as the bright “Morning Star”, high in the southeast in the early morning hours before dawn. Jupiter's smaller, more distant cousin, Uranus is within 3° of the King of Planets at magnitude 5.8. In a telescope, Uranus is a tiny 3.6 arcseconds wide, compared to Jupiter's 44 arcminutes. Uranus will be hard to spot with the Moon reaching its Full phase by 8:37 PM CDT Sunday evening, July 25th.

The latter part of this week, Thursday and Friday evenings are a good time to look at some of the Moon's more prominent features. Most visible in small telescopes are the lunar maria (MAR-ee-ah,) or seas. These are darker areas, or plains on the surface of the Moon that stand out from the surrounding terrain because of their apparent smoothness and lack of albedo, or reflectivity. These maria are actually areas where basaltic lava has been laid on the lunar surface by volcanic activity.

Other prominent features include craters and mountain ranges. Copernicus and Tycho are very prominent craters since they both have relatively high albedos and are surrounded by ray systems, which are the remnants of ejected lunar material due to impacts. Other prominent craters include Ptolemaeus, Plato, Hercules, Atlas, and Endymion. Prominent mountain ranges include Montes Apenninus, Montes Caucasus, Montes Carpatus, and Montes Riphaeus. These mountain ranges are the rough edges that remain from huge, ancient impact craters.

The study of the surface and physical features of the Moon is known as Selenography, which is concerned not with geology so much as lunar mapping and the naming of lunar features. Although there had been earlier attempts to draw maps of lunar features. including a map by Michel Florent van Langren in 1645, the first real lunar atlas was produced by Johannes Hevelius in 1647 and titled Selenographia. Van Langren's map and the name of lunar features had distinctly Catholic origins, where Hevelius' work corresponded to features on Earth and on the Greek and Roman civilizations. If you're interested in Selenography, you should check out Virtual Moon Atlas, which is a great free software download you can use to learn about lunar features.
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