Mars shines down with a placid red glow on these July nights, visible even in light-polluted city skies. Its two moons, however, can be seen only through very large telescopes. Their names, Phobos (Fear) and Deimos (Panic), are said to be those of the dogs of Mars, the god of war, or of the horses that pulled his chariot.
Mars resides at the inner edge of the asteroid belt, and it seems likely that Phobos and Deimos are asteroids that were gravitationally captured by Mars. These puny, irregularly shaped moons resemble nothing so much as a pair of lumpy potatoes. They evidently lacked the gravity to pull themselves into the spherical shapes of most planets and moons. Both satellites are small and pockmarked, although Deimos has a much smoother surface than Phobos. They are very dark, composed mainly of carbon-rich rock, and not very reflective. Phobos reflects only 4 percent of the sunlight that falls on it.
The larger of the two, Phobos is about 17 miles long and just under 6,000 miles from Mars. It orbits the planet in about eight hours. Deimos measures only 9 miles across at its longest dimension and lies farther out, about 14,600 miles from its parent planet. With its larger orbit, it circles Mars once every 1.3 Earth days. Because they're small and nonreflective, these moons would appear from Mars as points of light among the stars, not as disks, as our moon appears to us.
Planets of the Week
Mars rises in the southeast a couple of hours before sunset and sets at about 3:00 a.m. It sits in the constellation Ophiuchus, between Scorpius and Sagittarius, and outshines every star in its part of the sky.
Early risers can look in the eastern sky for Venus, rising at approximately 3:00 a.m., Saturn at 3:30 a.m., and Mercury and Jupiter, sneaking in just before the sun at about 4:30 a.m.
The three most distant of the planets can also be seen this summer. The best way to find them is to plot the stars in the constellations they appear in over several nights' time. The planets are the "stars" that have moved out of position. Pluto appears in Ophiuchus, to the north of Mars. Uranus and Neptune are rising in the wee hours with the constellation Capricornus.
The jewels of summer, marking the three corners of the Summer Triangle, are the first stars visible as evening falls in July. But the skies hold plenty of other interesting sights this month as well. Ursa Major, the Great Bear, is the dominant figure in the northwestern quadrant of the sky when total darkness arrives. Most people don't recognize the full bear shape of this constellation, but nearly everyone can pick out a smaller shape, or asterism, within it: the Big Dipper. Trace a line northward from the two stars that make up the edge of the Dipper's bowl to reach Polaris, the North Star. This not-too-bright star marks the tip of the handle of the Little Dipper, which is another asterism, part of Ursa Minor, the Little Bear. Between the two bears twine the dim stars of Draco, the Dragon. This is a good month to look for Draco, as it's highest now, stretching from the northwest, above Polaris, into the northeast.
In the northeastern part of the sky is Deneb, one corner of the Summer Triangle. Deneb is the alpha (or brightest) star in the constellation Cygnus, the Swan, which is immersed in the hazy light of the Milky Way. To the left of Cygnus is Cepheus, the King, whose stars form a sort of crooked house shape. Below it is Cassiopeia, the Queen, an easily recognizable shallow W shape. The first stars of the Great Square of Pegasus are just rising above the northeastern horizon.
Sagittarius is low in the southeast. When you look toward Sagittarius you're looking in the direction of the center of our galaxy. Here the star-clouds of the Milky way are brightest, stretching up the southern sky across to the northeast, through Aquila, Cygnus, and Cassiopeia. Altair, the alpha star in Aquila, the Eagle, is in the east, and bright Vega, the alpha star in Lyra, the Lyre, is above it. They mark the other two corners of the Summer Triangle. Vega is a brilliant blue-white star, which indicates that it's young and very hot; it's the fifth-brightest star in the sky.
Arcturus, the bright orange alpha star in Bootes, the Herdsman, is in the west. Corona Borealis, the Northern Crown, forms a lovely half-circle of stars above the kite shape of Bootes. Antares, the red star marking the heart of Scorpius, the Scorpion, is due south. Like Sagittarius, it lies in front of the center of the Milky Way and has numerous star clusters and nebulas within its boundaries. It's a very interesting constellation to scan with binoculars or a telescope; pay close attention to the area around its tail. Spica, Virgo's bright alpha star, is low in the west. Between Scorpius and Virgo lie the stars of Libra, the Scales. Hercules, a rather nondescript constellation, is straight overhead. Look for the trapezoid shape called the Keystone formed by four of its stars. Along the western edge of the Keystone is M13, the brightest globular cluster (a roughly spherical group of very old stars) visible from the Northern Hemisphere. On clear nights it appears to the naked eye as a faint, slightly fuzzy star.