Our Moon

The moon is the only object in the night sky on which you can make out details with the naked eye: bright and dark patches, a shape that changes night after night. On average it is 384400 km away, which sounds far, but cosmically speaking it is right on our doorstep. Twelve people have walked on it so far. Time for a close look at our constant companion.

Phases: illumination, not shadow

A stubborn misconception: the phases of the moon are not caused by the Earth's shadow. The moon does not shine by itself at all, it only reflects the light of the sun. Exactly one half of it is lit at any moment, just as one half of the Earth is always in the light. As the moon orbits the Earth, we look at this lit half from changing directions: at new moon it is turned away from us, at half moon we see it from the side, at full moon we look straight at it. From new moon to full moon the lit area grows every night, the moon is waxing, and afterwards it wanes again. From full moon to full moon about 29.5 days pass. If the Earth's shadow really does fall on the moon, you witness something much rarer: a lunar eclipse.

Always the same side

No matter when you look, you always see the same face of the moon. This is not because the moon does not rotate, quite the opposite: it rotates exactly once around its own axis while it circles the Earth once. Both motions take the same time, so the same side always faces us. This lockstep is called tidal locking, the Earth's tidal forces forced the moon into this rhythm over billions of years. Humanity first laid eyes on the far side in 1959, when the Soviet probe Luna 3 photographed it. And it is not dark, by the way, it gets just as much sun as the near side.

Craters and seas

The dark patches are called seas, in Latin maria, because early observers took them for water. In truth they are huge plains of solidified lava that rose from the moon's interior billions of years ago. The bright regions are older highlands, strewn with craters from countless impacts. Because the moon has no air and no weather, nothing erodes there: even the astronauts' footprints will survive for millions of years. Even binoculars show you craters, most impressively along the boundary between light and shadow, where the sun strikes at a flat angle and casts long shadows.

How far is far?

A distance of 384400 km, what does that mean in tangible terms? About 30 Earths would fit side by side between the Earth and the moon. Light, the fastest thing there is, needs around 1.3 seconds for the trip. The Apollo astronauts travelled for a good three days. And with a diameter of 3474 km the moon is about a quarter the size of the Earth, relative to its planet that makes it the largest moon in the solar system. Thanks to laser reflectors set up by the astronauts, we now know the distance to within centimetres, and we know this: the moon moves away from us by almost 4 cm per year.