How to Tell a Planet From a Star

It's one of the most common questions in stargazing: that bright light up there — is it a star, or a planet? The good news is you don't need any equipment to tell, and once you know the tricks you'll spot the planets at a glance for the rest of your life.

The number-one giveaway: planets don't twinkle

This is the test that almost always works. Stars twinkle. Planets shine steadily.

A star is so far away that, no matter how big it really is, it reaches your eye as a single pinpoint of light. As that pinpoint passes through Earth's churning atmosphere, the air bends it around — and it flashes, flickers and sometimes even seems to change colour. That's twinkling.

A planet is enormously closer. Instead of a point, it shows a tiny disc — too small for your eye to resolve as a shape, but big enough that it's really many points of light side by side. The atmosphere still jostles each one, but the flickers average out, and what you see is a calm, constant glow.

So: if a bright "star" is sitting rock-steady while its neighbours sparkle, you're almost certainly looking at a planet.

Clue two: brightness

The two brightest planets outshine every actual star in the night sky.

• Venus is staggeringly bright — often the first "star" to appear after sunset or the last to fade at dawn. People regularly mistake it for an aircraft landing light or even a UFO. Nothing else in the night sky except the Moon comes close.
• Jupiter is the next brightest, a brilliant, commanding point that can dominate the sky all night long.

If something looks too bright to be a star, that instinct is usually right.

Clue three: colour

Colour helps you go from "that's a planet" to "that's which planet."

• Mars is unmistakably reddish-orange — the "Red Planet" really does look ruddy to the naked eye.
• Saturn glows a softer, steady golden colour.
• Venus and Jupiter read as brilliant white.

Clue four: where it is in the sky

Position is a powerful filter, because the planets keep different company:

• Venus and Mercury never wander far from the Sun. You'll only ever see them low in the west just after sunset, or low in the east just before sunrise — never high overhead at midnight. Mercury is the shy one, hugging the twilight glow and easy to miss.
• Mars, Jupiter and Saturn can appear anywhere along the ecliptic — the same arc the Sun and Moon follow — and can be up in the middle of the night. When one of them is opposite the Sun (at "opposition"), it rises at sunset and is up all night, at its biggest and brightest.

Clue five: they wander

Watch the same patch of sky over a few weeks and the planets give themselves away by moving against the fixed background of stars. The constellations keep their shapes for a lifetime; the planets slowly drift through them. That restless wandering is literally where the word "planet" comes from — the Greek planētēs, "wanderer."

Put it together

Spot a bright light and run the checklist: Is it steady (not twinkling)? How bright? What colour? Where in the sky? A steady, brilliant white light low in the west after sunset is Venus. A steady reddish light high at midnight is Mars. A steady golden light is Saturn.

To go from "I think that's a planet" to a confident name, check which planets are actually up from where you are tonight — the direction to face and how high to look — and the mystery light resolves into an old friend.