Some nights the Moon is a full circle, some nights a thin sliver. Today you'll learn why — then you will predict the Moon's shape from where it sits in space.
Before we start — you've watched the Moon change shape from night to night — what do you think causes it?
The Moon is a giant ball of rock. We only see it because sunlight bounces off it. The half facing the Sun is always lit; the half facing away is always dark.
The Moon takes about a month to travel around Earth. As it moves, we look at its lit half from a new angle — so we see different amounts of the bright side. Those are the phases.
A common mix-up: phases are not Earth's shadow falling on the Moon. That's a lunar eclipse, which is rare. Phases happen every month simply because we see the lit half from the side. Eclipses are rare because the Moon's orbit is slightly tilted — so at most New and Full Moons the Moon passes a little above or below the Sun–Earth line, and the shadow misses.