One autumn evening a few years ago, a friend called me describing an object in the night sky to the northeast that caught his attention. This object was twinkling on and off, flashing in different colors and appeared to be shifting position.
The object that caught his attention was the star Capella in the constellation Auriga. By early evening in October, Auriga and its bright star Capella rise in the east and is just above the horizon. Bright stars low in the sky will behave just like my friend described. It’s all due our earth’s atmosphere.
Capella is the sixth brightest star in the night sky. The light from Capella has been traveling at the speed of light towards earth for about 43 years.
When that light enters the earth’s atmosphere it interacts with the earth’s dynamic atmosphere. The light hits pockets of air at different temperatures and densities. These pockets are like lenses and bend the stars light rays so the star appears to move about, twinkle and flicker.
Light from the star is made up of different colors and these colors are bent at different angles so the star appears to change colors from red to white. The closer the star is to the horizon, the thicker the atmosphere and the stronger the effect. As the star moves higher in the sky, its light will become steadier.
The whole effect on the observer is a wonderful natural event.
Planets generally don’t twinkle because they are much closer and therefore bigger than the distant stars. If the air is very turbulent planets can jump around in the eyepiece as well.
This fall we can see Jupiter setting to the west, Saturn due south.
So on the next clear night go outside and compare bright stars like Capella with Jupiter and Saturn to see the difference.