This summer belongs to Mars, Earth's neighbor in orbit, appearing at its biggest and brightest in a dozen years. The red planet, named for the oxidized iron-bearing minerals (i.e., rust) that cover its surface, rises in the east after sunset and adorns the zodiac all night. It's a small planet, about half the diameter of Earth, with a day about 24.5 hours long and a year of 780 days. It has two very small moons, Phobos and Deimos (Fear and Panic), named for the dogs of Mars, the god of war.
The desertlike surface of Mars is marked with craters from meteorite impact, huge valleys, and volcanic mountains that dwarf similar features on Earth. The largest valley, called Valles Marinaris, is longer than the United States is wide. Mars's Olympus Mons is the largest known volcano in the solar system. It's wider than Arizona and almost three times higher than the volcanoes of the Hawaiian Islands (which it structurally resembles).
The atmosphere of Mars is very thin, composed mainly of carbon dioxide, with some nitrogen and argon. Though the planet is too small and its gravitational force too weak to hold a substantial atmosphere, there's enough of an atmosphere to support huge dust storms. The temperatures on the surface of Mars vary with latitude and season, but even at the equators temperatures are below freezing most of the time. Like Earth, Mars has polar ice caps, but on Mars these are a combination of water ice and "dry" ice (frozen carbon dioxide). Mars has an axial tilt almost the same as that of Earth and so experiences seasons. The changing seasons produce variations in the prevalence of dust storms and the size and brightness of the polar caps.
Planets of the Week
Venus, putting in an appearance as the Morning Star, rises in the east before the sun. Mars rises in the southeast not long after the sun sets in the west and is visible the rest of the night. It sits in the constellation Ophiuchus, between Scorpius and Sagittarius. Mars will brighten and grow larger over the next week, reaching its peak in the middle of the month.
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. After adorning the sky so beautifully all winter, Saturn and Jupiter have disappeared behind the sun for a few weeks. Early risers can start looking for them again at the end of June.
At 9:00 p.m. on June evenings (10:00 p.m. daylight savings time), face northwest to see Leo the Lion plunging downward toward the horizon. Leo's bright alpha star, Regulus, is due west. The Great Bear, Ursa Major, most recognizable by the star group within its boundaries called the Big Dipper, is about halfway up the northwestern sky.
The northeastern part of the sky is dominated by the three bright stars of the Summer Triangle: Vega, the alpha star in Lyra (the Lyre), is highest; Deneb, the alpha star in Cygnus (the Swan), is toward the northeast; and Altair, the alpha star in Aquila (the Eagle), is somewhat lower and toward the east. Below and to the left of Altair is the distinctive, albeit tiny and dim, constellation Delphinus, which really does look like a tiny dolphin leaping the waves.
Low toward the north is the W of Cassiopeia, the Queen. Higher up is the Little Dipper (the constellation Ursa Minor, the Little Bear), standing vertically on its handle. The tip of the handle is Polaris, the North Star.
Face southeast to see Antares, the bright red star marking the heart of Scorpius, low in the sky. High in the southeast, near Arcturus (the orange alpha star in Bootes, the Herdsman), is the semicircle of stars called Corona Borealis, the Northern Crown. Below the crown to the east is the H-shape of Hercules.
Although Arcturus has just crossed the meridian into the southwestern quadrant of the sky, the rest of its constellation, Bootes, stands vertically overhead. Spica, the bright alpha star in the constellation Virgo, dominates the lower southwestern sky.