One of the first things we notice about the stars is that they twinkle — their light doesn’t hold steady. That has nothing to do with the stars themselves. In fact, if you could watch the stars from out in space, you’d see they shine steadily. Instead, twinkling is a result of what happens to the starlight closer to home — in Earth’s atmosphere.

To the eye alone, a star is nothing more than a pinpoint of light. So as that light enters Earth’s atmosphere, it’s easily distorted.

In particular, layers of air of different density act as lenses. They bend the rays of starlight as they pass through the atmosphere, causing them to wiggle back and forth as they descend. To the eye, that makes a star look like it’s shifting position.

Another effect is that the light is sometimes bent away from you, so the star suddenly looks fainter.

And different wavelengths of light — different colors — are bent at different angles. Sometimes only one color makes it to your eye, so a star can appear to change color rapidly, flashing from red to blue to white.

To see all of these effects at work, look southeast this evening for Sirius, the brightest star in the night sky. It climbs into good view by about 8 or 9 o’clock. As it climbs away from the horizon, its light passes through an especially thick layer of air, so it twinkles fiercely. It flashes on and off, and goes through a rapid cycle of color changes — all in the twinkling of an eye.


Script by Damond Benningfield

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