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Moon and Companions

December 18, 2011

The Moon and two bright companions line up across the southeast before dawn tomorrow. The closer of the Moon’s companions is Spica, the leading light of the constellation Virgo. It’s to the left or lower left of the Moon. The other is the planet Saturn, which is to the left of Spica.

Saturn is the brighter of the two, and it’s growing a little brighter night by night. By the middle of April, it’ll shine about twice as brightly as Spica.

The reason for the change in Saturn’s brightness is the changing distance between Saturn and Earth.

Right now, we’re separated by close to 950 million miles. But Earth follows a much smaller, faster orbit around the Sun, so we’re quickly catching up to Saturn. We’ll pass it in mid-April, when we’ll be about 130 million miles closer. At that range, Saturn will shine half again as bright as it is now.

Of course, the distance between Earth and Spica changes by that much, too. But the star is about 250 light-years away. At such a great range, the difference of a hundred million miles or so is insignificant. So Spica’s light holds steady all year long. And when Saturn is at its farthest from Earth, Spica outshines it.

Even as we move past Saturn, though, it’ll remain quite close to Spica in our sky. In fact, they will remain quite close until late next year, when Saturn will finally begin to pull away from its bright stellar companion.

More about this bright lineup tomorrow.


Script by Damond Benningfield, Copyright 2011


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