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The phases of the Moon

The Moon is always half lit. What changes is how much of that half we can see from here. Move the slider and watch the two views together.

From abovedrag to turn the view

Seen from here

Full MoonYou see 100% of it lit

Day 14.8 of the lunar month, out of 29.5

How far it is, in the drawing

In this drawing the Moon is much closer than it really is, as in almost every book: it helps everything fit on the page. The real one is 60 Earths away, lined up in a row. Try the other button.

Why the Moon changes shape

It does not change shape at all: it is a ball, and the Sun always lights exactly half of it. What changes is the side we look at it from. When it is on our side of the Sun we mostly see its dark face, and we call it a New Moon; when it is on the far side we see its whole lit face, and it is a Full Moon. In the view from above, the bright half always points toward the Sun, however you move the slider.

The Moon goes all the way around Earth in 27.3 days, but the phases repeat every 29.5. The difference is that meanwhile Earth has also moved along its own orbit, and to line up with the Sun in the same way again the Moon has to go a little farther. The days we count by watching the sky are the second number.

If the Moon’s orbit were exactly in the same plane as Earth’s, there would be an eclipse every month. Instead it is tilted by 5.1 degrees, and almost always the Moon passes a little above or a little below. Turn the view from above until you see it edge-on: you can see the tilt.

Open the card on the phases of the Moon

The numbers come from the NASA Planetary Fact Sheet, line by line.