The two brightest stars in the night sky line up in the south this evening, but only if you're south of about Birmingham or Los Angeles. The rest of the United States will see only the brighter of the two.
Look well up in the south around 10 o'clock for Sirius, the brightest star in the night sky. It's visible from the entire country. It twinkles colorfully, sometimes showing flashes of red or green or blue. The twinkling reveals something about the atmosphere above you.
Stars twinkle because their light passes through layers of air that are different temperatures and densities. These layers act like lenses, bending the starlight. Individual colors of light are bent at different angles, so sometimes more of the red wavelengths make it to our eyes, sometimes more of the blue. That makes the stars appear to shimmer and sparkle.
How much twinkling tells you how stable the atmosphere is above you. Astronomers prefer as little twinkling as possible, because a stable atmosphere provides sharper images of the stars.
From far-southern latitudes, look well below Sirius and a bit to its right for another bright sparkler -- Canopus. It's quite low in the sky, so you need a clear horizon to find it. No matter how steady the atmosphere, Canopus will twinkle quite a bit because you're seeing it through a thicker layer of air.
Canopus remains in view for only a few hours, but Sirius is on display for most of the night.
Script by Damond Benningfield, Copyright 2001, 2005