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What the planets do

Planets wander. They line up with each other. They come round to their closest and brightest. And now and then one of them seems to stop and go the other way for weeks or months. Every one of these can be worked out years ahead.

A conjunction two things side by side

Naked eye · lovely in binoculars

Two planets, or a planet and the Moon, appear right next to each other. Sometimes close enough to hide behind a fingertip, held at arm’s length.

They are not close. Not remotely. When Jupiter and Venus sit side by side they are still more than six hundred million kilometres apart. You are just looking along a line that passes near both. It is like two lamp posts, one down the road and one far away. They seem to touch because of where you happen to be standing.

A bright pair, a fingertip apart, in a deepening blue twilight, is one of the prettiest things the sky does.

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An opposition closest and brightest

Naked eye · a telescope shows the most detail of the year

Mars, Jupiter and Saturn orbit further out than us. Opposition is when Earth passes right between one of them and the Sun. We are on the inside track. We overtake them, the way a faster runner laps a slower one.

Three good things happen at once. The planet is at its closest, so it looks biggest. It is fully lit from our point of view, so it looks brightest. And it is opposite the Sun. So it rises as the Sun sets and stays up all night. If you are only going to look at Mars once, look near opposition.

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A backwards loop the planet that changes its mind

Naked eye · but only if you look on more than one night

Watch Mars against the stars behind it, week after week, and it creeps slowly eastwards. Then it stops, and for two months or more it goes backwards. It stops again, and carries on the way it was going. It draws a little loop in the sky. The old name for that is retrograde motion, and it is the strangest thing a planet does.

It is the same overtaking as an opposition, seen from the inside lane. Think of passing a slower car on a motorway. While you are drawing level, the other car seems to slide backwards against the fields behind it. It is still going forwards. Earth is the faster car. Mars is the slower one. Nothing about Mars changes at all — the loop is us moving.

For two thousand years these loops were the hardest thing in the sky to explain. If everything went round the Earth, nothing had any reason to go backwards. Astronomers built wheel-upon-wheel machinery to account for it. Then somebody put the Sun in the middle instead. Earth became one of the runners. And the loops needed no machinery at all. They are simply what overtaking looks like. The strangest thing in the sky turned out to be the best clue to where we really are.

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Greatest elongation Mercury and Venus at their best

Naked eye, low down · needs a clear horizon

Mercury and Venus orbit closer to the Sun than we do. So from here they can never wander far from it. They are stuck near the glare. They rise just before dawn or set just after dusk. You never see them in the middle of the night.

Greatest elongation is the moment one of them reaches the furthest from the Sun it can get. The top of its swing. That is your best chance to catch it. It is highest above the horizon, in the darkest sky it will manage. Plenty of people have never knowingly seen Mercury. These are the few weeks a year when you can.

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Jupiter’s moons, all night a small telescope is enough

Binoculars show the moons · a telescope shows this

Point anything at Jupiter and you see four little dots in a line. Those are the four big moons. They are going round fast. Io, the closest one, gets all the way round in 42.46 hours. So they move while you watch.

Four things can happen when a moon comes round the front or the back:

The last one is an eclipse. It is exactly like the Moon going dark in Earth’s shadow.

These are not rare. Jupiter has something like a thousand of them a year. So on most clear nights when Jupiter is up, something is happening.

See what Jupiter’s moons do tonight See what Saturn’s moons do tonight

Sometimes you get two at once. A moon on the face, and another one’s shadow beside it. And there are seasons. The moons orbit almost exactly in Jupiter’s equator. Jupiter is tipped by only 3.13°, so we always see that plane nearly edge-on. Io, Europa and Ganymede cross the face every time round; Callisto, the outermost, misses for years at a time when Jupiter is tipped towards us, and passes above or below. Saturn does all of this too, with two differences. Its plane is tipped much further, so the crossings come in a run every 14.72 years instead of every week. And of its moons only Titan is a fair target for a small telescope. The others are faint, and they sit right beside a very bright planet.

A transit a planet crossing the Sun

Never look at the Sun without a filter made for looking at the Sun. Not sunglasses. Same rules as an eclipse.

Now and then Mercury or Venus passes exactly between us and the Sun. Then you can watch a tiny black dot creep slowly across the Sun’s face. Perfectly round, over several hours.

These are properly rare. Mercury manages it about thirteen or fourteen times a century. Venus is far rarer. They come in pairs eight years apart, then nothing for over a century. The last pair was 2004 and 2012, and the next transit of Venus is in 2117. The trick is to be born at the right moment. Some of the people who will see it are alive already.

They matter more than they look. Time a transit of Venus from two far-apart places on Earth. From that you can work out the real size of the solar system. That is how it was first done well. The whole story: the distance to the Sun and planets.

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Jupiter’s moons rearrange themselves overnight, and a game asks you where they will be: Where will Jupiter’s moons be tomorrow? → A minute, no score, and the real answer afterwards.