What Determines the Moon Phases?

The position of the Moon and the Sun during Each of the Moon’s phases and the Moon as it appears from Earth during each phase. Credit: NASA/JPL-Caltech

We have a different view of the Moon every night. We often informally describe how the Moon seems like with the eight Moon shapes or phases:

πŸŒ• Full
πŸŒ– Waning Gibbous
πŸŒ— Third Quarter
🌘 Waning Crescent
πŸŒ‘ New
πŸŒ’ Waxing Crescent
πŸŒ“ First Quarter
πŸŒ” Waxing Gibbous

The Moon displays these eight stages one after the other as it moves through its cycle all month long. It takes around 27 days for the Moon to completely orbit Earth. That signifies that the Moon’s cycle is 27 days long.

If you have gazed into the sky during the nighttime, you may have observed the Moon seems to transform shape each night. Some nights, the Moon may look like a crescent. On other beautiful nights, the Moon might look like a golden circle. And on other nighttime, you might not be able to observe the Moon at all. The different Moon shapes that we observe at different times of the month are officially called the Moon’s phases.

Why is the Moon so different every night? The shape of the Moon isn’t shifting throughout the month. However, our viewpoint of the Moon does shift.

The Moon does not create its own light. There is only one light source in our known solar system, and that is our mighty Sun. Without the Sun, our Moon would be entirely dark. What you may have learned referred to as “moonlight” is actually just sunlight bouncing off of the Moon’s surface.

The Sun’s light emerges from one direction, and it always lights up or illuminates one half of the Moon – the side of the Moon facing the Sun. And I guess you guessed it, the other side of the Moon is completely dark.

On Earth, our view of the brightened part of the Moon varies each night, depending on where the Moon is in its path, or orbit, around Earth. When we have a full view of the illuminated side of the Moon, that phase is called a full moon, and it has religious and spiritual significance in numerous cultures.

But after the night of each full moon, as the Moon moves around Earth, we begin to see less of the Moonlit by our mighty Sun. Ultimately, the Moon reaches a point in its orbit when we don’t notice any of the Moon illuminated. At that point, the far side of the Moon is facing the Sun. This state is called a new moon. During the new moon, the side facing Earth is, as you guessed it, dark.

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