How to spot planets without a telescope

Five of the planets in our solar system are bright enough to see without any equipment. Mercury, Venus, Mars, Jupiter, and Saturn. Most people, if they think about it for a moment, will realize they have probably seen at least Venus and Jupiter without knowing what they were. The bright “evening star” that hangs around in the west after sunset is Venus, almost without exception. The unfairly-bright “star” that doesn’t twinkle in the southern sky around midnight is usually Jupiter. Mars is rarer but unmistakable when it’s around, because of its color.

Knowing which is which doesn’t require an app, though an app helps the first few times. It requires two pieces of information, both of which you can learn in one evening.

The twinkle test, again

Stars twinkle. Planets don’t, or twinkle far less. The mechanism is the same one explained in our Jupiter guide — a star is essentially a point of light and the air shakes it, while a planet is a tiny disc whose shakes average out. The practical upshot is that if you see an unusually bright thing in the sky that’s holding still while everything around it flickers, you’re looking at a planet.

This breaks down at very low altitudes. Within ten degrees of the horizon, even planets seem to twinkle because the air is thicker and the turbulence stronger. Don’t trust the test for things near the ground.

The ecliptic

Every planet visible to the naked eye stays close to a specific line across the sky called the ecliptic. This is the path the Sun draws across the daytime sky, and the Moon follows it too, give or take a few degrees. If you’ve ever noticed where the Sun rises and sets through the year, and traced an arc between those points, you’ve drawn the ecliptic.

Planets do not wander into the deep northern or southern sky. They don’t approach Polaris. They sit on or near the ecliptic, marching slowly along it from week to week. If you’ve spotted the full moon recently, every planet visible that night is somewhere along the same arc the Moon is on.

The five, in order of how easy they are

Venus is the easiest by far. It’s so bright (magnitude minus four at its best) that people regularly mistake it for an aircraft landing light or a hovering UFO. It only ever appears within about three hours of sunrise or sunset, because Venus orbits inside Earth’s orbit and is therefore always relatively close to the Sun in our sky. The “evening star” is Venus low in the west after sunset, the “morning star” is Venus low in the east before sunrise. The same planet, six months apart in its cycle.

Jupiter is next. At its best it shines at magnitude minus two, which is brighter than any star. Cream-colored, steady, doesn’t twinkle. It’s visible for about eleven months out of every thirteen — briefly hidden only when it’s behind the Sun. If you see something obviously bright high overhead at midnight that’s not the Moon, it’s almost always Jupiter.

Mars is the trickiest of the easy planets, because its brightness varies wildly. At opposition (the night Earth is between Mars and the Sun, which happens every 26 months) Mars can outshine Jupiter, with a sharp orange-red color. The rest of the time, especially when Mars is on the far side of its orbit, it’s a faint reddish “star” that’s easy to miss. The color, though, is the giveaway. No real star is that orange. Antares and Aldebaran come closest, but neither has Mars’s saturated red-orange.

Saturn shines at magnitude zero or one — moderate, less obviously bright than Jupiter. It has a faintly yellow tint that you can spot if you look carefully and have something to compare it to. The rings are not visible without optical aid. Even a small telescope reveals them, and the moment you see them for the first time is one of the more memorable experiences in casual astronomy.

Mercury is the hardest. It’s always close to the Sun, so it appears only briefly in twilight, low on the horizon, for about a week at a time. You’ll spot it maybe six times a year if you’re paying attention, low in the west after sunset or low in the east before sunrise, depending on which side of its orbit it’s currently on. It has a pinkish tint when you can see it well, but mostly you just see a faint twinkly dot that wasn’t there a week ago.

How SkyMinute handles this

We tell you only the planets that are actually findable from your location tonight. The threshold is ten degrees of altitude — anything below that is usually blocked by trees, buildings, or general low-altitude haze. We list them in order of brightness, which is also approximately the order of how easy they are to spot. If no planets are above ten degrees on a given night, you’ll see “Just the stars tonight” instead, which happens every few months as the planets cycle through their orbits.