Phases of the Moon
The Moon is the brightest object in the night sky when its whole face is showing. This only happens at full Moon. Why isn't it always full Moon?
The Moon makes a complete orbit of the Earth every 27.3 days. We only see the part of the Moon that is lit up by the Sun. The other part remains dark. The Moon receives the Sun's light from different angles as it orbits the Earth. This makes it look like the Moon is changing shape. We call these changes the phases of the Moon. The 'Moon Phases Diagram' shows the different lunar phases we see from Earth.
It takes 29.5 days to go from one full Moon to the next. We call this a lunar month. Have you noticed this is slightly longer than the time it takes to orbit the Earth? This is because the Earth is moving around the Sun. The Moon has to travel a bit further to get back to the same position.