What planets are visible tonight?

A planet is a world that circles the Sun, as Earth does. It makes no light of its own. What you see is sunlight bouncing off its clouds or rock. To the eye it looks like a star, a single point of light.

Five planets are bright enough to see without a telescope: Mercury, Venus, Mars, Jupiter and Saturn. Astronomers measure brightness in magnitudes, and the scale runs backwards. The smaller the number, the brighter the light, and the brightest things have negative numbers. On a perfectly dark night, the eye can see stars down to a magnitude between 6 and 7. Under a city sky, only the brightest stars and planets show through.

Uranus sits right at the edge, at a magnitude between 5 and 6. Sharp eyes can catch it under a dark sky, if they know exactly where to look. Even so, nobody knew it was a planet until William Herschel found it with a telescope in 1781. Neptune, at between 8 and 8, is always too faint for the eye.

Three clues give a planet away. The first is brightness. At its brightest, Venus reaches magnitude -4.92, and after the Sun and the Moon nothing in the sky outshines it. Jupiter reaches -2.94, and Mars, at its very best, the same. Venus and Jupiter are always brighter than Sirius, the brightest star of the night. Mars beats it only around its close approaches. Saturn, with its rings, reaches -0.55, brighter than most of the stars you can see. Mercury can reach -2.43, but it is at its brightest when it is too close to the Sun to see.

The second clue is the twinkle. A star is so far away that its light arrives as a single point. Pockets of moving air bend that point back and forth, and it twinkles. A planet is a small disc, too small for the eye to see as one, but wide enough that the wobbles average out. So planets usually shine with a steady light.

The third clue is the place. Planets keep to one band across the sky, the path the Sun follows. A brilliant, steady light on that path is very likely a planet.

The light you see from a planet has made two journeys. It left the Sun, crossed to the planet, bounced off its clouds and crossed back to you. The sunlight reaching your eye from Jupiter tonight left the planet about 45 minutes earlier.

Everyone on the night side of Earth sees the same planets, strung along the same line. Some see them higher, some lower, some earlier in the evening and some later. The ancient Romans named the five they could see after their most important gods, and we still use those names.

There is one planet you will never find in the night sky, because you are standing on it. Earth carries you along the same flat plane as all the others, round the same Sun. When you look at Jupiter or Venus, you are looking across our shared neighbourhood at a fellow traveller.