Why Does the Moon Have Phases?

Half of the Moon is always lit by the Sun — exactly like half of Earth is always in daytime. So why does the Moon seem to change shape? Because the Moon keeps moving around us, and from Earth we keep seeing that same lit half from a new angle. Drag the Moon around the orbit above and watch the shape on the right shift from sliver to full and back.

It is about angle, not shadow

This is the big idea, and it fools almost everyone: phases are not Earth's shadow on the Moon. Earth's shadow only reaches the Moon during a lunar eclipse, which happens just a couple of times a year. The rest of the time, the changing shape is purely about how much of the Moon's sunny side is pointed our way.

Walking around the orbit

Start with the Moon between Earth and the Sun. Its lit side faces the Sun, away from us, so we see darkness — a New Moon. A quarter of the way around, we catch it half-lit (First Quarter). Opposite the Sun, the whole face glows — Full Moon. One more quarter and we are heading back toward New. The lit half never changes; only our viewing angle does.

Why we only ever see one face

Here is a bonus oddity: the Moon always shows Earth the same side. It spins exactly once each time it circles us — a trick astronomers call being "tidally locked" — so the same hemisphere always points our way. That is why the phase shapes always look familiar: we are watching sunlight sweep across one fixed face, like a spotlight crossing a stage.

Then why isn't there an eclipse every month?

Fair question — if the Moon passes the Sun's side every single month, why no eclipse each time? Because the Moon's path is tilted about 5 degrees, so most months it slips a little above or below the perfect line-up and the shadows miss. Only when everything lines up just right do we get an eclipse.