I spend a lot of time looking at the Night Sky. Often, someone will come by and wonder what I’m looking at. I will happily tell them, pointing out some common stars, or a planet or two, if one is up. Invariably, the question arises of how to tell the difference between a star and a planet.
Its easy, really.
Stars twinkle, planets don’t.
See, I told you it was easy.
Now, for what I mean by this. Planets are relatively close to us. While Neptune, Uranus and Pluto require a telescope to see, Mars, Jupiter, Saturn, Venus, and Mercury are all visible to the naked eye.
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These planets are within our solar system. And all are reflecting the Sun’s light. They do not emit, or generate, any light of their own. The stars, by comparison, are VERY far away. While they emit their own light, they are thousands of times further away than what our planets are. Consequently, the intensity of their light that reaches our atmosphere is not as great as the intensity of the light from one of our planets. Because the light from a star is not that strong, our atmosphere easily “scatters” it, creating the twinkling effect. Remember the nursery rhyme, “Twinkle, twinkle Little Star?” That’s what we’re getting at.
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Our atmosphere has a natural turbulence to it. This is why we have clouds and storms. Even on a calm day, the air is still moving. The light from the stars and planets passes through that turbulent atmosphere and Wa La!! we have twinkling stars!! Most people find them romantic; astronomers, however, are not fond of twinkling stars. We like ours very still, for good pictures and observing.