Posted by: T. Boyd | February 12, 2011

Why stars twinkle (and planets do not)

Long ago God spoke many times and in many ways to our ancestors through the prophets.  And now in these final days, he has spoken to us through his Son. God promised everything to the Son as an inheritance, and through the Son he created the universe.  The Son radiates God’s own glory and expresses the very character of God, and he sustains everything by the mighty power of his command. (Hebrews 1:1-3 New Living Translation)

Why do stars twinkle while the light from planets looks steady?  The first part of the question is easier to answer than the second. Here’s a hint – the stars do not twinkle when viewed from the space station or from anywhere above the earth’s atmosphere. You guessed it: the twinkle is caused by turbulence in the air as the light beam passes through.

Then why do not planets also twinkle? As an example, if you look at Jupiter (in the western sky in the early months of 2011 after sunset) and compare it to a bright star, like Sirius which is the brightest object high in the eastern sky at the same time, you can tell that Sirius has a beautiful sparkling, wavering appearance that you do not see with Jupiter.

One way to explain this is that Jupiter’s diameter is very large in our view compared to Sirius (in a telescope, you can see that Jupiter is a round ball with surface features, while Sirius stays as a single point of light – even in the most powerful earth-based telescopes). Because of the size of the planet, there are many paths of light between the planet and us on earth. So even though each path of light is wavering because of the atmosphere, the average between all the paths to reach the eye looks steady. The eye blends them together into a sight that looks unwavering; therefore a planet doesn’t appear to twinkle.

But, you might point out, surely Sirius, the brightest star in our night sky, is much, much larger than Jupiter. In fact, its diameter, estimated to be 90 billion miles is more than a million times Jupiter’s diameter of 83 thousand miles.  One the other hand, because Jupiter is so much closer, the apparent diameter (as it looks to an earth based observer) of Jupiter is about 8000 times that of SIrius.

This, in turn, means that the apparent size in area of Jupiter is about 64 million times as big. So if we think of Sirius as having only one path of light hitting our eye, then Jupiter would have about 64 million paths to send its light rays to our eye. And the result is an averaging of all those paths to our eye to give the appearance of a steady light from the planet.  In other words, the wavering is smoothed out.

Whew! That’s a lot of words to explain a simple idea, but I hope it helps you understand what is happening when you see the steady light of  Venus, Mars, Jupiter, or Saturn  compared to the surrounding stars. Note: you rarely see Mercury since it stays very close to the Sun, and the other planets, Uranus, Neptune, and Pluto are too faint to be seen easily.

The other lesson learned here is how very far away are even the nearest stars  (Sirius is in the family of  about a dozen nearest stars – those that are less than 10 light years away.   The closest, Proxima Centauri, is about 4 light years away). Most of space is empty of stars and planets.

The universe is unimaginably immense, and yet our God who is its creator and sustainer is much greater.   The bible says that it was created through Jesus Christ, and that is sustained by the word of His power. (Hebrews 1:1-3)  God is incomprehensively powerful. majestic, and glorious, and yet He loves each one of His creatures.  Our names are written on His hand (Isaiah 49:16). Do you know that He loves you and me with His whole heart?


  1. […] This post was mentioned on Twitter by Boyd Moore, Boyd Moore. Boyd Moore said: Why stars twinkle (and planets do not) […]

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