More Moon and Jupiter

StarDate: March 7, 2011

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The crescent Moon and the planet Jupiter line up low in the west as darkness falls this evening. Jupiter looks like a brilliant star below the Moon.

Jupiter is dropping lower in the sky each evening, and before month's end it'll be lost in the Sun's glare. It'll return to view in early April, in the morning sky.

Jupiter will disappear because it's passing behind the Sun as seen from Earth -- a point known as conjunction. It reaches this point every 13 months.

Most of Jupiter's motion across the night sky is actually caused by Earth's motion. Our planet follows a much smaller, faster orbit around the Sun than Jupiter does. As we circle the Sun, Jupiter and each of the other planets outside Earth's orbit first appears in the morning sky; then lines up opposite the Sun, so it's in view all night; and then stays in the evening sky until it once again disappears behind the Sun.

The length of each planet's cycle depends on how far away it is. Mars is the closest planet beyond Earth's orbit, so it moves across the sky at a good clip. That means we have to chase it a little longer before it returns to the same point relative to the Sun -- about two years. Jupiter and Saturn are much farther out, so it takes them only a little more than one year to complete a cycle around the sky.

As Jupiter drops from view over the next couple of weeks, the planet Mercury will climb past it. The two planets will be closest to each other next week.

Script by Damond Benningfield, Copyright 2011

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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