The moon, the ringed planet, and Spica

SkyCaramba astronomy blog for the week ending March 19, 2011

A full moon passes two bright dots in the sky the weekend of March 19. The first dot is a planet. The second is a star.

The planet is Saturn. It’s about 8.6 astronomical units from Earth at this time. An astronomical unit is the average distance between the Earth and sun. Saturn’s light needs about an hour and ten minutes to get to us from that far away.

If you want to see Saturn but aren’t sure which object it is, having the full moon nearby is a big help. Another big help now is that Saturn will be at opposition in a few weeks. That means it and the sun are on opposite sides of the great sphere of sky surrounding Earth. When the sun sets, Saturn rises. Where the sun would be overhead at high noon, Saturn will be close to the same position at “high midnight”. Or if you know where the sun would be at about 3 in the afternoon, you can look for Saturn there at 3 in the morning.

Saturn’s currently in the constellation Virgo. Virgo’s bright star is Spica. Saturn and Spica are close enough the moon passes Spica the night after it passes Saturn. Spica, of course, is a lot farther from us. Its light takes 262 years to get to us. One of the brightest stars in the sky and positioned where you can see it almost everywhere in the world, it’s been one navigators have relied on for centuries. And like so many other stars, Spica became even more interesting when modern astronomical observations became possible.

Spica is a binary (double) star. Spica and its companion are close–closer than our sun and Mercury! When astronomers noticed small brightness variations in Spica, they thought that happened because one star passed in front of the other. Logical, however, it’s now known that the variation is caused by the two stars being so close they distort each other with their gravity.

The main star in Spica is actually a variable by itself. It expands and contracts. Fundamental forces acting in different ways cause molecules to push away from each other in some circumstances and draw closer to each other in others. Much of the time the forces balance out. If they didn’t, we and the things around us couldn’t exist. Pulsating variable stars are believed to be what happens when gravity pulling star matter together and intermolecular repulsion pushing things apart reach a less than ideal balance.

The light variations from Spica are too subtle to notice with naked eye observations. But some other variable stars change dramatically. There’s one called Mira that goes so dim it disappears from naked eye view for months at a time. For now, get to know Spica. Saturn’s in the middle of a three-year visit to Virgo. When the ringed planet leaves for Libra late next year, Spica will be the only bright object left in the otherwise dim Virgo.