Understanding moon phases
Photo: Dan Bush.
As the moon orbits Earth, it changes phase in an orderly way. Follow the links at the bottom of the page to understand the various phases of the moon.
Why the moon changes phase …
Why does the moon seem to change its shape every night? The moon is a world in space, just as Earth is. Like Earth, it’s always half illuminated by the sun. In other words, the round globe of the moon has a “day side” and a “night side.” From our earthly vantage point, as the moon orbits around Earth, we see varying fractions of its day and night sides. These are the changing phases of the moon.
One key to understanding moon phases is to think about the whereabouts of the sun. After all, it’s the sun that’s illuminating and creating the day side of the moon. Moon phases depend on the sun. They depend on where the moon is with respect to the sun in space.
Another key to understanding moon phases is to remember that, like the sun and all the planets and stars, the moon rises in the east and sets in the west each and every day. It has to. The rising and setting of all celestial objects is due to Earth’s continuous spin beneath the sky.
Finally, remember that the moon takes about a month (one “moonth”) to orbit the Earth. Although the moon rises in the east and sets in the west each day (due to Earth’s spin), it’s also moving on the sky’s dome each day due to its own motion in orbit around Earth. The moon’s orbital motion can be detected in front of the stars from one night to the next. It’s as though the moon is moving on the inside of a circle of 360 degrees. Thus the moon moves about 12 degrees each day.
The moon’s orbital motion is toward the east. Each day, as the moon moves another 12 degrees toward the east on the sky’s dome, Earth has to rotate a little longer to bring you around to where the moon is in space. Thus the moon rises, on average, about 50 minutes later each day.
