Planets in the sky

Some of the brightest lights in the night sky are not stars at all. They are planets, our neighbours in the solar system, reflecting sunlight back to us. Mercury, Venus, Mars, Jupiter and Saturn are visible to the naked eye, and people have been watching them for thousands of years. Their very name gives away their secret: the Greek word planetes means wanderer.

Wanderers in front of the stars

The constellations barely change within a human lifetime, the stars are far too distant for that. Planets, however, are close and orbit the sun just like we do. That is why they visibly shift against the starry background from week to week, sometimes faster, sometimes slower, and the outer ones even seem to run a loop backwards when the Earth overtakes them on its inner, faster orbit. You never have to search the whole sky for them: planets always travel along the same narrow band as the sun and the moon, the zodiac. It was exactly this wandering that made people in antiquity recognise them, long before anyone knew what a planet even is.

Why planets do not twinkle

The best quick test in the evening sky: does the light twinkle or does it stand steady? Stars are mere points even in a telescope, so the restless air makes their light flicker. A planet, however, is a tiny disc in the sky, even if your eye cannot resolve it. Its light therefore comes from many points side by side that do not flicker in unison. The individual wobbles cancel each other out, so it shines calmly and steadily. So if you see a strikingly bright, serenely shining point, chances are good you have found a planet.

Venus, the morning and evening star

The brightest starlike object in the sky is Venus, outshining every real star many times over. Its dense cloud cover reflects sunlight like a mirror. Because its orbit lies inside the Earth's orbit, as seen from us it never strays further than about 47° from the sun. That is why you never see it high in the sky at midnight, but only for some hours after sunset in the west or before sunrise in the east. Antiquity long took the morning star and the evening star for two different objects, yet it is always the same Venus.

From Mercury to Saturn

Each of the five naked eye planets has its own character. Mercury, the innermost planet, never strays further than about 28° from the sun as seen from us, so it always stays close to the hazy horizon and is the hardest to catch. Venus dazzles as the morning or evening star. Mars stands out with its reddish colour, which is rust, iron oxide, on its surface. Jupiter shines brighter than any true star and takes around 12 years for one trip around the sun, so it moves on by roughly one zodiac constellation per year. Saturn, finally, glows yellowish and takes its time: it needs a full 29.5 years for one lap around the sun. Uranus and Neptune, on the other hand, are practically invisible without binoculars or a telescope.