How to Tell the Difference Between Stars and Planets Just by Looking

Photo Credit: Utah State Parks

Telling planets and stars apart seems like it would be difficult – until, that is, you remember that classic lullaby that put us all to sleep.

Remember “Twinkle, twinkle little star”?

Yep. Stars twinkle, and planets don’t.

With the exception of our solar system’s sun, stars are all so far away that astronomers talk about their distance from Earth in terms of lightyears, or the distance light can go in one Earth year.

Considering light moves at about 186,000 miles per second, a lightyear is pretty freaking far.

The closest star to our sun is called the Alpha Centauri, and it’s 4 lightyears away from our planet. Because it takes a star’s light several lightyears to get to Earth, we see it as a small point in space.

But before we can see it from the surface of Earth, starlight is refracted. This refraction is influenced by every change in density and temperature in every media that the light passes through.

Photo Credit: Wikipedia

Planets are much closer to us than stars. A planet’s light (which is really sunlight reflected back to us) gets refracted to a much lesser degree because it travel a much shorter distance. Planets usually appear bigger (because they’re closer), and their light twinkles significantly less.

Actually, planetary light looks basically steady.

Of course, if you catch a planet a bit lower in the sky – maybe so you’re looking at it through the horizon – you’ll have a harder time telling it apart from a star. If light is traveling through the horizon, that means it’s going through a lot more atmosphere before it reaches your eyes than if it were directly overhead. That causes more light refraction, and, thus, more twinkling.

Photo Credit: Wikipedia

It’s easier to tell stars and planets apart later because there’s less light refraction when you’re looking directly up.

If this trick gives you trouble at the beginning, keep practicing and tou’ll be able to tell the planets and stars apart in no time.

Are you going to try this simple trick? Share your results with us in the comment area, if you’d like.