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  The Predictable Phases of the Moon:  
 

 

In the course of 29.5 days, the moon goes through eight different phases – from New Moon to Waning Crescent. What causes the moon to look so different during the month? The answer is “angles” – how the moon, sun, and Earth are lined up. What does this alignment mean to how we perceive the moon’s illumination?

The sun is always illuminating the moon; however, depending on where the moon and Earth are in their respective orbits (relative to the sun), the sunlit and shadowed portions of the moon will look different. And, since the Earth and moon orbit at known rates, the phases of the moon can be predicted, charted, and followed.

The Eight Phases of the Moon
The moon begins each new cycle as a New Moon. This is when the moon is between the Earth and sun (approximately aligned), with the sun illuminating the back of the moon. From Earth’s perspective, the moon looks to be in shadow as the illuminated part is hidden from view.

The other major phase of course is the Full Moon. This is when the moon falls in approximate line with the Earth and sun again, but this time, the moon is on the other side of the Earth. Thus, the illuminated part of the moon is facing us and appears to be completely round. Full.

Most of us are familiar with the crescent shape, which is curved and appears to be less than half of a moon. This is a good way to remember the crescent phases – there are two. Paired with either “waxing” (which means “expanding”) or “waning” (which means “shrinking”), the crescent is the associated shape for the 2nd and 8th phases.

The other term to remember is “gibbous” which is the opposite of “crescent” for the moon cycle. Gibbous simply describes the moon when more than half of it is illuminated. There are two phases with this name, again paired with “waxing” and “waning” to properly describe where in the cycle the phase is.

Thus, in order, the eight phases of the moon are:

 

1. New Moon | 2. Waxing Crescent | 3. First Quarter | 4. Waxing Gibbous

5. Full Moon | 6. Waning Gibbous | 7. Last Quarter | 8. Waning Crescent

 
 

 
  Did you know:
  The moon’s “synodic period” takes approximately 29.5 days. This means that, for an observer on Earth, it takes the moon this long to return to the same position (relative to the sun).

However, did you know – it actually only takes the moon 27 days to complete its orbit. Only, because the Earth is also moving along its own orbit, two days are added on to account for Earth’s ongoing revolution.