The Night Sky     The Planets Mercury   Mercury is currently washed out in the solar glow, but makes its return to the dawn skies within a week.   Venus   For the past several months, dazzling Venus has been prominent in our morning sky. During this month, however, the planet will slowly fall back toward the Sun and will sink deeper into the twilight each consecutive morning.   If you watch the sky around 6:30 A.M. local time every morning, you can still notice Venus low above the southeast horizon. You might want to check your horizon in advance, to make sure that there are no tall trees or buildings that might obstruct your view of the planet. At magnitude -4, Venus will be the brightest star-like object in the sky, and will appear to be "gibbous phase" in even the smallest telescopes.       One good way to tell stars from planets is that looking at them with the unaided eye, stars twinkle and planets do not. The twinkling of stars, technically known as stellar scintillation, is caused by the Earth's atmosphere. Because stars are so incredibly distant from us, any disturbances in the atmosphere will bounce around the light from a star in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle".   On the other hand, planets are much nearer to the Earth and they look like small disks of light rather than point sources. The size of a planet on the sky in a sense "averages out" the turbulent effects of the atmosphere, and the total effect is one of steady light.   Finder map - 30 minutes before sunrise, looking southeast.   Mars   Phobos, the larger and innermost of Mars' two moons, is heavily cratered and resembles a potato in shape. G. Neukum/ DLR/ESA [larger image] Mars is moving eastward in direct motion (as it does most of the time) through the stars of Taurus and will cross over into Gemini next month, on March 5th. This brilliant yellow-orange world shines very high in the southeast sky after dusk, high above Orion and near the fairly bright star Beta Tauri, also known as Elnath.   Seen in a telescope, Mars is 11" wide this week. At moderately high magnification it appears as a small, distinct reddish ball displaying subtle dark markings and a bright white South Polar Cap. The red color comes from rust - iron oxide - that makes up about 10-percent of Martian soil. It is thought the oxide was created far back in the planet's history, when surface water was abundant and reacted with iron in the rocks on Mars' surface.   Mars is a small planet, closer in size to our Moon than to the Earth. It has two natural satellites, colorfully named Phobos (Fear) and Deimos (Panic), after the horses that drew the chariot of the Roman war god. They are rather unimpressive as moons go, resembling large asteroids. Phobos, for example, is 17 miles across its longest dimension, but it is 13 miles and 11 miles across its other dimensions. Thus, it is shaped like a potato. Deimos is even smaller and is similarly shaped.   Finder map - 7 P.M. local time, looking southeast.   Jupiter   Gas giant Jupiter, imaged by the Voyager spacecraft in 1979. NASA/JPL [larger image] Giant Jupiter returns to view in the morning sky low above the southeast horizon, and gets more conspicuous each morning. The planet spans 33" this week and grows to 47" by the time it reaches opposition in July. The telescopic view of Jupiter will improve markedly as it climbs higher in the coming months, but you can still practice observing this planet and training your eye to see detail.   Jupiter is the King of the Planets, and not just because of its enormous size - this monster planet is 88,700 miles in diameter. Jupiter also reigns over the other members of the solar system in the affections of amateur astronomers, because this great ball of gas is just so consistently interesting.   There is always a tremendous variety of interesting phenomena to see when Jupiter is in the sky. The planet shows detail in backyard telescopes and even the smallest optical aid will show its four bright moons. In small telescopes you can make out two or three of the darkest cloud bands, and as the scope gets bigger, the more you will see.   Four-inch telescopes can show multiple bands and the Great Red Spot. Larger scopes can see details in the bands such as texture, loops, and ovals, often in vivid color. Also visible in larger telescopes are transits of the moons across Jupiter's surface as well as the inky black dots of the moons' shadows as they transit the planet.   Finder map - 30 minutes before sunrise, looking southeast.   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] The ringed planet rises in the east around 7 P.M. local time and is highest in the south in the early-morning hours. It shines at magnitude +0.3 and remains in Leo throughout February, lower left of 1st- magnitude Regulus (Alpha Leonis) in the late evening. Saturn's disk measures 20" across this week, while the rings span 45" and tilt only 8° to our line of sight, allowing us to see more of the planet's globe.   Saturn is the most distant of the five planets known to ancient stargazers, and 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 -0.4 at favorable oppositions, 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 - 9 P.M. local time, looking east.   Uranus   Uranus is lost in evening twilight in the west-southwest sky. The planet will be visible again in early May, only now in the morning sky.   Neptune   Neptune is hidden in the glow of the Sun. It will return to view in late March, low in the morning sky.   Pluto   The dwarf planet is too deep in the solar glare and cannot be observed until early March, when it will reappear in the morning sky.

An artist's concept of Voyager 2, the only spacecraft to have visited far-off Neptune and its 13 moons. Voyager 2 is now 7.5 billion miles from Earth, more than twice the distance from the Sun as Pluto. NASA/JPL [larger image] Neptune Seek out Neptune in the constellation Capricornus the Sea Goat, 2° northwest of the 4th-magnitude star Gamma Capricorni. The distant world lies 2.8 billion miles from Earth and glows dimly at magnitude +7.9. Because Neptune is pretty low in the morning sky you will need at least a 4-inch telescope to see it and resolve its disk, only 2.4" in diameter. Neptune is a cold, dark world. With a mean solar distance of 2,798 million miles (30.11 astronomical units), light levels at Neptune are more than 800 times dimmer than they are on Earth. Noon on Neptune would appear no brighter than what a human would experience at dusk on Earth. Even the Sun would take on an appearance more like a star than the bright disk seen from our home planet. Because Neptune is so distant, so far, only one spacecraft, Voyager 2, has visited the planet - a flyby in 1989. The spacecraft observed Neptune almost continuously between June and October of that year. It measured the planet's radius and interior rotation rate, confirmed that Neptune has rings, and discovered six new moons. Voyager 2 also recorded pulses of radio emission, zonal cloud bands, and large scale storm systems with up to 1,500 miles per hour winds - the strongest measured on any planet. The strength of the radio signals received from Voyager 2 measured just one ten-quadrillionth of a watt (1/1016). It took 40 huge radio antennas on four continents to absorb enough energy to interpret the signals. Finder map - field width 15°, stars to magnitude +8.5. Pluto The dwarf planet Pluto lies in northern Sagittarius and is highest above the horizon in the predawn sky. Pluto glows at magnitude +13.9, making it similar to many stars in this Milky Way region. As a result, it is a challenge to spot. An 8-inch telescope on a perfect night brings Pluto to the edge of visibility. For a direct view, however, you will want to use at least a 10-inch scope. Pluto was discovered in 1930 as a result of an extensive search by American astronomer Clyde Tombaugh. Astronomers have argued since the late 1990s that Pluto's small size, less than one-fifth the diameter of Earth, and a weird tilted orbit that takes it inside Neptune's orbital path every couple hundred years make Pluto more like a Kuiper Belt body than a full-fledged planet. On August 24th 2006, the International Astronomical Union (the organization responsible for classifying planets) passed a new definition of planet that excludes Pluto and puts it in a new category of "dwarf planet". The two finder maps below will help you identify Pluto. First locate its general position on the coarse finder map, and after you have identified the 10th-magnitude guide star Tycho 6250-745-1 use the fine finder map which shows the position of the dwarf planet on each night of the week. Coarse finder map - field width 4°, stars to magnitude +12. Fine finder map - field width 1°, stars to magnitude +14.5.