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Sky watch: Saturn's 'double star' in June

Last week, I wrote about a "double star" in Ursa Major; this week, I'd like to tell you about one in the constellation Virgo, on the opposite side of the sky. But unlike Mizar and Alcor, this double star is not permanent. In fact, it's not even a double star, but it sure looks like one.

Just after dark this week, cast your gaze midway up the southern sky. There you will find the bright bluish-white star Spica in the constellation Virgo; above and to its right lies Saturn.

You'll notice that Saturn doesn't appear to be alone, but seems to have a fainter companion just above it. This is the star Porrima, known as Gamma Virginis, also in Virgo.

If you've watched Saturn over the past month, you might not have noticed it lying so close to this star because Saturn is a planet and appears to drift through stars as it orbits the sun. This is true for all planets.

Mercury, being the nearest to the sun, appears to move the most quickly through the stars, while Saturn, being quite far from the sun, appears to move much more slowly.Last week, Saturn and Porrima did not appear this close; next week, they won't, either. This week, they nuzzle up on Thursday, with the first quarter moon not far west. On that night, Saturn and Porrima will lie a quarter of a degree apart, and if you aim a small telescope in their direction, you'll see the two in the same field of view. But don't be surprised if the pair appear inverted in the telescope from how they appear to the eye alone.

While Saturn and Porrima now appear next to each other in the sky, this is an optical illusion caused by the two lying nearly along the same line of sight from Earth. The two are really far apart. On Thursday night, Saturn will lie about 855,480,000 miles from Earth, but Porrima, which is a star not unlike the sun, will lie more than a quarter of a million times farther -- about 234 trillion miles, or 39 light years, away.

Keep an eye on this pair for the next few months, and you'll be able to see planetary motion in action. As the Earth continues its orbit around the sun and we face outward in slightly different directions from month to month, the pair will slowly appear to descend in the west at dusk. But Saturn will drift eastward through the stars, and by mid-July, we'll see Saturn a full degree east of Porrima.

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