As Earth makes its way around its orbit each year, the same constellations come into view at night at the same times of year. We can see Sagittarius and Scorpius in the evening sky every summer and Orion and Taurus every winter. The planets, however, do not appear with seasonal regularity. Their movements are complex. Not only are all moving around the sun in their own orbit and at their own pace, Earth is zipping along, too.
The planets orbit the sun in a counterclockwise direction (as seen from the "north side" of the solar system). Planets closer to the sun move faster in their orbits than do the outer planets. It's this continual ballet of planets, some faster than others, that gives rise to the changing positions of the planets in our skies. Since we're viewing the sky from a moving platform (i.e., Earth), we perceive both the movements of the other planets and our own movement.
The changing positions of the planets relative to the position of Earth affect what time of night (or day) we can see the other planets. When a planet is east of the sun as seen from Earth, it appears in the evening sky after sunset. Venus is currently east of the sun, making a brilliant appearance over the western horizon. Likewise, when a planet is west of the sun it appears in the morning sky before sunrise, as Mercury did last week.
The planets that are farther from the sun than Earth are best seen when they're in a position called opposition. That's when they're in the same relative position as the moon when it's full, with Earth aligned between them and the sun. They're closest to Earth, fully illuminated, and in the sky all night. Saturn reached opposition on November 19, and Jupiter reaches opposition on November 27.
The giant planets will steadily decrease in size and brightness over the coming months as they move farther from Earth and Earth moves farther from them. Eventually, they'll disappear into the glare of the sun and pass behind it from our point of view. But Earth will catch up with them once more, and in about a year's time both will return to opposition and again shine brightly all night long.
Planets of the Week
Saturn and Jupiter are found in the constellation Taurus, the Bull, and can be seen in the evening. (The September 18 and September 25 Sky Guides provide more information on viewing our solar system's largest planets and the array of moons that surround them. Click here to read the former; click here to read the latter.)
Venus is setting about three hours after sunset now. Look in the southwestern sky after the sun goes down to see if you can spot the Evening Star. It's the brightest object in the sky after the sun and the moon. Mars is dimly visible in the very early morning hours in the constellation Virgo.
Neptune, Uranus, and Pluto reached optimal viewing positions this fall. Pluto, difficult to see even with a mid-sized telescope, lies in the constellation Ophiuchus. Neptune is quite a bit brighter, and Uranus is brightest of the three. Neptune and Uranus can be found in the constellation Capricornus with a small telescope or binoculars and a pair of sharp eyes.
At 9 p.m. on November evenings, the Summer Triangle, now low in the northwestern quadrant of the sky, is the last remnant of the warmer months as the nights turn colder. Its three corners — Deneb, the alpha (or brightest) star in the constellation Cygnus, the Swan; Vega, the bright blue-white alpha star in Lyra, the Harp; and Altair, the alpha star in Aquila, the Eagle — are the most noticeable stars in the northwest. The Little Dipper, or Ursa Minor (the Little Bear), hangs down from its alpha star Polaris. Cepheus, the King, shaped like a peak-roofed house, is about halfway up the northern sky.
Orion, the Hunter, is rising above the eastern horizon as darkness falls. The three conspicuous stars of his belt point just north of the red star Aldebaran (the alpha star in Taurus, the Bull), about a third of the way up the eastern sky. The Pleiades, or Seven Sisters, star cluster lies just above Aldebaran. A third of the way up in the northeast is Capella, the alpha star in Auriga, the Charioteer. Near the horizon, the twin stars Castor and Pollux, in the constellation Gemini, are just rising. The Great Bear, Ursa Major, familiarly known as the Big Dipper, is very close to the horizon in the north.
There are no bright stars in the southeastern portion of the sky. Cetus, the Sea Monster, and parts of Pisces, the Fish, Aries, the Ram, and Eridanus, the River, all faint constellations, run through this part of the sky. Look low toward the southwest to see Fomalhaut, the bright alpha star in Piscis Austrinus, the Southern Fish. Between Pegasus and Fomalhaut are the stars of Aquarius and Pisces. Aquila and Capricornus are low in the west.
The Great Square of Pegasus, formed by three stars in the constellation Pegasus, the Winged Horse, and one star in Andromeda, the Princess, dominates the southwestern part of the sky. November is the best time of year to look for the Great Andromeda Galaxy. On a very dark, clear night (with no moon) look for a fuzzy patch of light above the middle of the two lines of stars that form Andromeda. This spiral galaxy is the most distant object the eye can see unaided by binoculars or a telescope.