You are here

Moon and Companions

November 3, 2010

The fingernail-thin crescent Moon has a couple of companions at first light tomorrow: the star Spica, which is just to the lower left of the Moon, and the planet Saturn, which is farther to the upper left of the Moon.

These two pinpoints of light are equally bright right now. Or they look equally bright in our sky. The truth is, there’s no real comparison between them.

Saturn shines by reflecting sunlight. It looks bright in our sky because it’s big and its surface is fairly reflective. But put it out at the distance of Spica -- about 250 light-years -- and it’s not even a speck: It would appear less than one–trillionth as bright as it does now.

As faint as that is, it’s actually within the reach of big research telescopes. But there’s a complication. A planet orbits a star, which can be millions or billions of times brighter than the planet. And as seen from Earth, the separation between the star and planet is practically zero. So like trying to see a firefly that’s sitting a wing’s-breadth away from a searchlight, it’s darn tough to see a planet through the glare of the star.

It can be done, though. Astronomers can place a tiny disk in their telescopes to block out a star’s light, allowing planets to shine through. The technique works best from space, outside Earth’s blurring atmosphere.

With this technique, astronomers have seen and even photographed several planets -- tiny fireflies shining across the cosmic gulf.


Script by Damond Benningfield, Copyright 2010


Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.