Sun, Earth and Moon

In the morning the sun rises, in the evening it gets dark, and sometimes a round full moon hangs in the sky, sometimes just a thin crescent. It all feels ordinary, but it follows a magnificent clockwork of three celestial bodies: Sun, Earth and Moon. Let's look at how they move.

Day, night and year

The Earth spins around itself once every 24 hours. The side currently facing the sun has day, the side facing away has night. At the same time, the Earth travels along a huge path around the sun: one full lap takes a year, about 365 days. And the moon? It orbits the Earth and takes about 29.5 days from one full moon to the next.

The phases of the moon

The moon does not shine by itself: it only reflects sunlight. The sun always lights up exactly one half of the moon. How much of that bright half we see depends on where the moon currently is on its path around the Earth. When it sits in the same direction as the sun as seen from Earth, it turns almost only its dark side towards us: new moon. When it sits exactly opposite the sun, we look straight at the bright half: full moon. Crescent and half moon lie in between.

Eclipses

Sometimes all three line up exactly. When the moon slides between the sun and the Earth, it casts a shadow on the Earth: a solar eclipse. When the moon instead passes through the Earth's shadow, it turns dark and often reddish, because the Earth's atmosphere bends a little red sunlight into the shadow: a lunar eclipse. Both are rare because the moon's orbit is tilted by about 5 degrees against the Earth's orbit. Most of the time the moon therefore passes just above or below the line and nothing happens at all.