All five naked eye planets gather in the western sky soon after sundown now. Is this a rare occurrence? Which of the planets are changing their position the most from one night to another, the ones higher in the sky or the ones nearest the horizon?
The evening before last, I took advantage of the relatively clear skies and warm temperatures and found a suitable place to look at the western quadrant of the sky. Even though the daytime temperatures were in the low 90s F (daily records were broken at a number of cities), fortunately, the humidity was sufficiently low so that haze didn't dramatically effect star gazing, as it often does in mid summer. With a chorus of spring peepers and other frogs in the background, and few mosquitoes to ruin the hour, I was able to sit back in a not so comfortable chair and gradually see the planets and stars emerge as the sky darkened.
After the Sun set, the crescent moon commanded the western sky, and a few minutes later, Venus, down low, and Jupiter, up high, appeared. Sirius came into view approximately 30 minutes after sunset, and shortly afterwards, Saturn could be seen trying to snuggle up against the moon. Saturn (Roman god of agriculture) now lies about half way between Venus (Roman god of love) and Jupiter (the head honcho of the Roman gods). At nearly the same time, earthshine could be discerned. This happens when the darkened portion of the moon is faintly illuminated by reflected sunlight off the daylight side of the Earth - sometimes referred to as "the old moon in the arms of the new." Tonight the Moon is just about at quarter phase and will be close to Jupiter.
Because it was murky near the horizon, I wasn't able to see Mercury (Roman god of commerce). It sets about an hour after the Sun now, and it's only about 10 degrees above the horizon (one fist width held at arm's length), so it's not an easy target. However, it'll be higher in the sky by month's end. Nearly an hour after the Sun set, ruddy Mars (Roman god of war) finally appeared. It was positioned about midway between Saturn and Venus. Currently, it's light is the most feeble of the 5 naked eye planets (magnitude of about 1.5), but it's easier to see than Mercury since it's higher in the sky. It's rather faint now, because it relatively a long way from us, about 209 million miles (334 million km) away.
Jupiter is in the constellation of Gemini now and shines with a brightness of -2.1. The principal stars of Gemini (the twins) are Castor and Pollux, which are nearly equally bright - they're twins after all. Pollux is just slightly brighter than Castor (about 1.1 magnitude for Pollux as opposed to 1.6 magnitude for Castor), but note how much brighter Jupiter is than Pollux, which is the 12th brightest star visible from the Northern Hemisphere. Jupiter appears to be about 16 times brighter than Pollux, although with the nearby presence of the waxing moon, this difference may be hard to appreciate. As bright as Jupiter seems, Venus is positively brilliant, it glows about 6 times brighter than Jove.
By looking at the arrangement of Jupiter, Saturn, Mars, Venus, and Mercury, you can get a sense that they lie on a similar plane. Notice too that the Moon is offset a bit by the line formed when connecting the planets. The plane of the Moon's orbit about Earth is within a few degrees of the orbital plane of the planets. This line up of the Moon and the planets is perfectly timed for this year's National Astronomy Day (April 20).
The alignment of the 5 visible planets on the same side of the sky is unusual, but it certainly isn't a freak occurrence. Just 2 years ago, a somewhat similar grouping occurred, but the last time a close-knitted grouping occurred was in 1940. Every 20 years or so, when the plodding Jupiter and Saturn appear close together from Earth's perspective, a similar alignment can occur.
What's interesting this time is that the planetary grouping is getting tighter and tighter, and they'll continue to clump together for the next several weeks. By the end of next week, Mercury, Venus, Mars, and Saturn will all be in the constellation of Taurus the Bull, and thus, they'll all be within about 20 degrees of each other. By about May 5, Venus, Mars and Saturn will form an equilateral triangle, and by May 10, Mars and Venus will appear to be almost touching each other. Jupiter is currently the odd man out, but it's catching up and will approach the rest of the group in May.
If you view the western sky every few days during the next couple of weeks, you'll observe that those planets lower in the sky are jockeying about the most. Mercury, Venus, and Mars, are closest to the horizon and are generally are more acrobatic than Jupiter and Saturn, which are higher up in the sky. This is because the lower planets are nearer to the Sun and move around old Sol at a faster clip. However, only the planets outside of Earth's orbit display retrograde motion, whereby on occasion, they appear to move backwards over the course of several nights. Since we're moving more expeditiously around the Sun than Mars, Jupiter or Saturn, as we overtake these outer planets, our changing viewing geometry makes it seem that they're moving in a backward or retrograde manner.
The current particular configuration of the planets won't be repeated until 2072, and since for most of us, a 6 foot layer of dirt will prevent our view of the sky then, we should take advantage of the present opportunity. In 2040 and again in 2060, the five planets will also be in the same part of the sky, but their juxtaposition won't be as spectacular.
By the way, you might be able to catch a glimpse of Comet Ikeya-Zhang in the pre dawn hours. It'll be in the northeastern part of the sky a couple of hours before daybreak - the Little Dipper is midway between the comet and the Big Dipper. You'll probably need binoculars to observe it.
For
more about this see the Abrams Planetarium Web Site:
http://www.pa.msu.edu/abrams/SkyWatchersDiary/Diary.html
and the Sky and Telescope site at http://skyandtelescope.com/