How the Moon goes around the Earth

An explanation you can poke. Drag the sliders to move time, and drag the pictures to look from another side. Inspired by the explorable essays of Bartosz Ciechanowski.

The Moon circles the Earth once every 27.3 days

Here are the two bodies. The Earth sits in the middle and the Moon follows a nearly circular path around it. Press play, or move the slider yourself. Then drag the picture to turn the view.

With true sizes the Earth shrinks to a speck. The Moon is about 384,400 km away, roughly thirty Earth-widths. It covers that circle at about 1 kilometre per second, which is fast, and still takes 27.3 days.

Why does it keep curving round? Gravity pulls the Moon toward the Earth. Each second the Moon falls about 1.4 millimetres toward us. It also moves about a kilometre sideways, so it keeps missing. The Moon is perpetually falling around the Earth.

Phases are the Moon’s lit half, seen from changing angles

The Sun always lights half of the Moon, the half facing it. What changes is how much of that half we can see from Earth. In the first picture you look down on the system from above, with sunlight coming from the right. In the second you stand on Earth and look at the Moon.

Find the new Moon. The Moon stands between Earth and Sun, so its lit half faces away from us. At the full Moon it stands on the far side of Earth and we see the entire lit half. At the quarters we see half of the lit half, which is why a “half moon” is a quarter of the way round.

Earth’s shadow is not the cause. It only matters during a lunar eclipse, which is rare. Phases happen every month.

This picture holds the Sun’s direction fixed. That is a small cheat, and it is why one cycle here takes 29.5 days rather than 27.3. Two figures down, the cheat is removed.

The Moon turns once per lap, so one face always looks at us

We always see the same side of the Moon. That does not mean the Moon is not rotating. To see why, compare two Moons. The amber half is the side that faces Earth at the start. The red arrow points out of the middle of that side.

The real Moon turns so that the amber side keeps pointing at Earth. To manage that, it turns exactly once during each lap, and the angle it has turned always equals the angle it has travelled. The other Moon never turns at all: its arrow keeps pointing the same way in space, and after half a lap Earth looks at the amber side’s back. A non-rotating Moon would show us every side in turn.

This match, called tidal locking, is no coincidence. Earth’s gravity slowly dragged the Moon’s spin until it stopped drifting relative to the orbit. The Moon also nods and wobbles a little, so over time we glimpse about 59% of its surface.

A lap takes 27.3 days, but a phase cycle takes 29.5

Now zoom out and put the Sun back in the middle. The Earth also travels around it, by about one degree each day. The Moon starts at new Moon, lined up between Earth and Sun, and the gray line marks the direction it started in, which points toward a fixed distant star.

Jump to day 27.3. The Moon is back on the gray line, which means it has made one full lap relative to the stars. That lap is the sidereal month. But the Earth has moved on by about 27°, so the Sun has moved too, and the Moon still has to travel those extra degrees to line up between them again. That takes two more days and gives the synodic month of 29.5 days. Phases follow the second one.

The orbit is tilted, so most months have no eclipse

The Moon’s path does not lie in the plane of the Earth’s journey around the Sun (the pale disc). It is tilted by 5.1°. Where the two meet is the line of nodes. Look from the side, and watch the dashed line from the Moon to the plane.

At new Moon, the Moon is on the Sun’s side of Earth, but it passes about 34,000 km below the line between them, so its shadow misses us. Set the tilt to zero and every new Moon would eclipse the Sun, and every full Moon would be eclipsed. Real eclipses need a new or full Moon that happens near a node.

The Earth moves in a small circle too

Strictly, the Moon does not circle the Earth’s centre. Both orbit their shared balance point, the barycentre. The Earth is 81 times heavier, so that point is close to us, about 4,671 km from Earth’s centre. The Earth’s radius is 6,371 km, so the point is still inside the planet.

The red dot never moves. The Earth swings around it, always on the side opposite the Moon, in a circle about 9,300 km wide.

The Moon is slowly leaving

Laser pulses bounced off mirrors left by the Apollo astronauts show that the Moon drifts away from us by about 3.8 centimetres a year, roughly the speed fingernails grow. Tides on Earth are the cause: they pull a little energy from our spin and pass it to the Moon’s orbit. Every figure above is a snapshot of a motion that is very slowly changing.