The Moon and its motions

The Moon is the Earth's only natural satellite and the brightest object in the night sky. It orbits the Earth in roughly a month, and as it does so its appearance changes in a regular cycle of phases. Because these changes arise from the relative positions of the Sun, Earth and Moon, they are well suited to animation, which can show how the geometry of the orbit produces each phase and eclipse over time.

The phases occur because the Moon shines only by reflected sunlight, so at any moment one half of it is lit. As the Moon moves around the Earth, the angle between the Sun, Earth and Moon changes, and observers see varying portions of the sunlit half — from the fully dark new moon, through the growing crescent and half-lit quarters, to the fully lit full moon, and back again. The Moon also keeps the same face turned toward the Earth at all times, because it rotates on its axis in exactly the same time it takes to complete one orbit, a state known as synchronous rotation.

At certain alignments the Moon passes through the Earth's shadow, producing a lunar eclipse. This can happen only at full moon, when the Earth lies between the Sun and the Moon. During a total lunar eclipse the Moon often turns a dim red, because sunlight bent through the Earth's atmosphere still reaches it. Eclipses do not occur every month because the Moon's orbit is slightly tilted relative to the Earth's path around the Sun. The Moon's gravity also raises the ocean tides on Earth. Interactive models bring these linked phenomena — phases, the locked rotation and eclipses — together in a single moving picture.

Frequently asked questions

Why does the Moon have phases?

The Moon reflects sunlight, lighting one half at a time. As it orbits the Earth, we see different amounts of that lit half, which produces the cycle of phases.

Why do we always see the same side of the Moon?

The Moon rotates on its axis in the same time it takes to orbit the Earth, so the same face always points toward us, a condition called synchronous rotation.

When can a lunar eclipse happen?

Only at full moon, when the Earth passes directly between the Sun and the Moon and the Moon moves through the Earth's shadow.