The Night Sky This Week

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The Planets Mercury   Mercury is immersed deep in the glow of the Sun and cannot be observed until mid-January 2008, when it makes a brief appearance in the evening sky.   Venus   Early-morning skygazers will enjoy finding Venus. Our "sister planet" comes up in total darkness, about three hours before the first glimmer of dawn, and blazes in the southeast an hour before sunup.   Venus shines at magnitude -4.1, about ten times brighter than Sirius, the brightest of all stars. In fact, the planet is so bright, that you might try sighting it on very clear days with the naked eye after sunrise. If you can keep track of where it is through sunup, you should still be able to see it as a tiny white "speck" against the blue daytime sky.   Aim your telescope at Venus and you will see a 69-percent lit disk that spans 17" - the planet's phase waxes imperceptibly, to 71-percent lit, during the week. Simultaneously, the apparent size of Venus' disk shrinks from 17" to 16", as the planet's distance from Earth increases from 94 million miles to 97 million miles. Such changes are explained by the fact that Venus is on a smaller, faster orbit than Earth, and the morning star is now rapidly receding from us.   Finder map - one hour before sunrise, looking southeast.   Mars   The Red Planet reaches opposition this month, and appears bigger through a telescope than any time until 2016. NASA [larger image] The most spectacular planet this month has to be Mars. December brings the Red Planet closer to Earth than at any time since the winter of 2005. If you have a telescope, even a small one, now is certainly the time to get it out.   Mars pokes above the horizon during evening twilight, among the background stars of Gemini the Twins, and is high up in fine view in the east by 9 P.M. local time. It gleams so brightly and with such a distinct ruddy hue, you will not confuse it with any other object. Through a telescope, the planet measures 16" across - nowhere near the record- breaking apparent diameter of 25" in August 2003, but bigger than Mars will appear again until 2016.   Mars is never easy to study visually, but this will be your best chance! The best telescope for planetary observing is a large, high- quality refractor or a large Newtonian reflector with first-rate mirrors in perfect collimation (optical alignment). Fine planetary views can also be obtained with Schmidt-Cassegrains; there are no hard and fast rules in this respect.   In the end, the limiting factor is atmospheric seeing. Studying the planets means spending a lot of time at the eyepiece, waiting for brief glimpses through steady air. Just as important, the longer you watch, the better trained your eye becomes.   Finder map - 8 P.M. local time, looking east.   Jupiter   Jupiter is lost in the evening twilight; it sets just minutes after the Sun. Nevertheless, it is out there, on the other side of the Sun from Earth and 580 million miles away from our planet. It will return to view in mid-January 2008, low in the morning sky.   Saturn   This illustration shows a close-up of Saturn's ring system. The rings are believed to have formed after an icy moon was broken up by an impact with a comet or asteroid. NASA/CXC/M. Weiss [larger image] Saturn shines at magnitude +0.7 and remains in Leo throughout December, nearly due east of 1st-magnitude Regulus (Alpha Leonis). The planet rises an hour before midnight and climbs high enough by 5 A.M. local time to offer excellent views through a telescope. Saturn's disk measures 19" across this week, while the rings span 43" and appear closer to edge-on now than they have since 1997.   Saturn is the most striking member of our planetary family. With an equatorial diameter of about 75,000 miles, it is the second largest planet circling the Sun. Appearing as a bright yellowish object in our night sky, Saturn presents a maximum apparent diameter of 20" and reaches magnitude -0.2 at opposition, when it lies closest to the Earth. This is about ten times fainter than Jupiter.   The planet's most attractive feature is, of course, the magnificent ring system. The rings consist of countless particles of water ice, ranging in size from small grains to irregularly shaped pieces generally a few feet across. It is interesting to note that if all of the material of the rings of Saturn were formed into a single moon, the moon would be about the mass of Janus (one of the smallest of Saturn's moons) and only 1/20,000 the mass of the Earth's Moon.   Finder map - one hour before sunrise, looking southwest.   Uranus   A portrait of William Herschel, discoverer of Uranus. Library of Congress [larger image] Glowing at magnitude +5.9, Uranus pops into view in binoculars, even from moderately light-polluted sites. Look for it in Aquarius the Water-bearer, 2° southwest of 4th-magnitude Phi Aquarii. Through a telescope, the planet displays a disk 3.4" in diameter sporting a distinct blue-green hue.   The famous German-English amateur astronomer William Herschel discovered Uranus in 1781, and once astronomers realized it was an unknown world, the size of the solar system doubled. Uranus was the first new planet to be found since antiquity, but at the outset it was not at all clear that a new world had been spotted.   Herschel's telescope provided the world's best view of the new object, but even Herschel thought he had discovered a comet. His journal record for his first sighting on Tuesday, March 13, 1781, documents his suspicion: "In the quartile near Zeta Tauri the lowest of the two is a curious either Nebulous Star or perhaps a Comet". Four nights later, Herschel observed it again. Despite its lack of a tail, he concluded - because it had moved - that he had a comet on his hands.   Independent observatories soon verified the existence and motion of Herschel's new "comet", but attempts to determine its orbit and predict its position failed. It was obvious that this object was much more distant than anyone had thought, and calculations soon showed it to be a planet, moving far beyond the orbit of Saturn. After some discussion it was named Uranus, after the Greek god of the sky.   Finder map - field width 15°, stars to magnitude +8.   Neptune   Neptune is about six times dimmer than Uranus, but if you have access to a dark, clear sky and carefully examine the finder map below, you should have no trouble in finding it with a small telescope. The planet lies among the stars of Capricornus, the Sea Goat, a mere 2° northeast of 4th-magnitude Iota Capricorni.   Telescopes show little of Neptune - even though it is four times Earth's diameter, Neptune lies 2.85 billion miles away and so appears only 2.4" across. You are going to need at least a 4-inch telescope with a magnification of no less than 150-power, just to turn Neptune into a tiny blue dot of light. The best views are in the early evening, when Neptune lies in the southwest.   Neptune takes almost 165 years to complete one journey around the Sun, so that it was discovered less than one "Neptunian year" ago. Like Uranus, Neptune shows a light bluish color (from methane in the atmosphere). The main constituents are molecular hydrogen and helium; methane makes up a minor amount.   Neptune's atmosphere is marked by cirrus clouds and large storms, most notably the Great Dark Spot. This darker bluish-gray region circling the planet's Southern Hemisphere rotates counterclockwise in a few days. A region of high pressure, the Dark Spot is surrounded by white clouds of ammonia and is similar to storms on Jupiter.   Finder map - field width 15°, stars to magnitude +8.5.   Pluto   The dwarf planet is too deep in the solar glare and cannot be observed until early March 2008, when it will reappear in the morning sky.   The Deep Sky Taurus, the Bull, is an ancient constellation that dates back to when bulls were worshipped in the Middle East. His face is marked by the V-shaped cluster of stars called the Hyades, his glinting red eye is the bright star Aldebaran and his long horns are tipped by the stars Zeta and Beta Tauri.   The Hyades (also catalogued as Melotte 25) are one of the major tourist attractions in the sky. In Greek mythology they are sisters, the daughters of Aethra and Atlas (the giant who carried the heavens on his shoulders) and half-sisters to the Pleiades. The Greeks told that the Hyades nursed the infant Dionysus, a son of Zeus, and were rewarded with a Within the borders of the constellation Taurus are two celebrated star groups, the Hyades (lower left in the image) and the Pleiades (upper right). Lynn Laux [larger image] place in the sky. The name Hyades means "the rainy ones", because they were associated with wet and stormy weather.   Legend says that there were five Hyades, but fifteen shine brighter than 5th magnitude and are visible to the naked eye. Binoculars quickly raise that number into the dozens - in fact, hundreds of stars lie inside the cluster's 5.5° span. More than 130 of them are brighter than 9th magnitude and, therefore, visible in binoculars under reasonably dark skies.   The brightest star in the Hyades is Theta-2 Tauri, at magnitude +3.4. It teams with magnitude +3.8 Theta-1 Tauri to form a wide naked eye double star, just southwest of brilliant Aldebaran (Alpha Tauri). Aldebaran, an orange giant star about 150 times as luminous as our Sun, looks as though it is a member of the star cluster, but it is not. It is actually a foreground object that merely happens to lie about halfway between the Hyades and ourselves.   Because the Hyades are so scattered, telescopes cannot take the cluster all in at once; even some binoculars cannot squeeze its full extent into their fields of view. The best view of the Hyades will be through binoculars that have fields of view between 8° and 10° in diameter. Anything less and the clustering effect will be lost.   To astronomers, the Hyades are one of the most important star groups in the sky, because all of the cluster's stars share a common motion. They drift slowly through space toward a point a few degrees east of Betelgeuse in Orion, a fact first demonstrated by the American astronomer Lewis Boss in 1908.   From the movements of its stars, the distance to the Hyades Cluster has been accurately determined - it lies 151 light-years from us. This distance can be used to derive absolute magnitudes for cluster members of different kinds, and these values are in turn used as calibrators for determinations of distances to other stellar aggregates. In this way, the distance to the Hyades is the first stepping-stone in our scale of the Universe.