Have you ever looked up into the night sky, seen a bright light, and wondered to yourself: “Is that a star or a planet? And if it’s a planet, which one is it?” If so, wonder no further! Ask Science has simple rules to help make you become an expert on the night sky.
Watching the starry sky is one of my favorite pastimes - especially when I'm in the countryside and there's not a ton of pollution blocking the view. Some of the most exciting things to watch in the night sky are the other planets in our solar system. But how do you know exactly what you're looking at? Is that shiny orb a planet? A star? a UFO?
I'm glad you asked.
There are several ways to tell whether that point of light you’re seeing is a planet or a star. The most common method is to see if the light appears to twinkle, that is change in color or brightness.
Is it a Planet or a Star?
Even though stars are enormous, they are so far away from the earth that they appear very, very small. (With the notable exception of our sun). The light from the star gets refracted, or bent, ever so slightly as it passes through the different layers of the Earth’s atmosphere. This bending causes the star to appear to twinkle.
The planets in our solar system are much closer to the Earth. The light we see from the planets is sunlight being reflected off of the planet. Since the planets are so much closer to the Earth than the stars, they appear larger to us. When their light gets bent by the Earth’s atmosphere, the amount the light bends is small compared to how big the planet looks, so there’s no twinkle.
Like all great rules however, this one has an exception. Sometimes the Earth’s atmosphere is moving around so much, or has such a large amount of pollution, that even the light from planets can appear to twinkle.
Caught in My Orbit
The way ancient astronomers were able to first distinguish between planets and stars is that while the stars do appear to change position in the night sky, they do so all together. Planets on the other hand move in specific orbits independent of the motion of the stars.
The planets also follow the general path of the ecliptic, the imaginary path that the sun follows across the sky from east to west.
So if you see a bright, non-twinkling light along that path, there’s a good chance that it’s a planet.
And the Planet Is...
So now that you can tell a star from a planet, how do you know which planet you’re looking at? Some of them are pretty easy to recognize. Here's how...