Astronomy for Beginners: How to Tell a Planet from a Star

For anyone new to stargazing there’s a simple way to tell a star from a planet in the night sky. If the object you are looking at appears to twinkle then it’s likely a star, if it’s light is very steady and consistent then it’s a planet.

So why do stars twinkle while planets don’t?

In reality it’s all a bit of an illusion caused by the Earths atmosphere. Despite stars generally being much bigger and brighter than planets, because they are so far away their light reaches us as a very narrow beam which gets easily distorted by our atmosphere.

The planets in our solar system on the other hand are much, much closer to us meaning the light they reflect from the sun is spread over a wider area. Rather than appearing as a tiny pinprick of light like stars do, most planets can be seen as sphere shaped.

This is much more noticeable when viewing through a telescope. Even if the light being reflected at us from one part of the planet gets distorted by Earths atmosphere, light coming from another area will most likely be unaffected. This results in a more constant appearance and brightness in the sky.

As with most things, there will sometimes be exceptions to this but in general it’s a good way for beginner astronomers to tell their planets from the stars. Generally the closer an object is to the horizon, the more prone it will be to twinkling. This is due to the light having to pass through more atmosphere before reaching your eyes.

Of course if your able to view the stars and planets from outside the Earths atmosphere you won’t notice and twinkling. This is one of the reasons why many of the top observatories here on Earth are located at very high altitude, so there’s less atmosphere to look through.

Plus this is one of the big advantages of using telescopes located in space such as NASA’s Hubble Space Telescope. With no atmosphere to contend with, Hubble allows scientists to get a clear, undistorted view of the universe around us.