Just like the Sun and the night time stars, the Moon’s apparent rising in the east and setting in the west each day is not from the Moon’s orbit around the Earth, it’s from the Earth spinning. If we first think about the Moon in relation to the Earth, we can explain how it appears to travel across the sky over the course of a night, and why it rises and falls at different times and in different locations. Most vividly, its entire appearance changes over the course of two weeks, morphing from a bright circle to a circle sliced in half and finally fading to nothing.Īll these changes seem complicated but each one can be explained by thinking about where the Earth, Sun and Moon are in relation to one another. And from night to night it rises and falls at different times and in different parts of the sky.

It moves across the sky rapidly over the course of a night. The way the Moon looks to us is continually changing. Our Moon must travel a little farther in its path to make up for the added distance and complete its phase cycle. Why? In that time, as our Moon moves around Earth, the Earth also moves around the Sun. Moon fact: The Moon’s phases repeat every 29.5 days, but it’s orbit around the Earth only takes 27. Knowing how this dance between the Moon, Earth and Sun plays out lets us understand the Moon’s constantly changing appearance. And it appears to have phases because the amount of lunar surface bathed in sunlight we can see from Earth depends on where we and the Sun are.

Sun and moom