


Moon Phases - Understanding Them

Just looking at the moon from the earth, it would be almost impossible to figure out what is happening. It appears that the moon is orbiting around the earth about once a day. Science has proven that the moon does orbit the earth, but not once a day but closer to once a month. What is really happening is our earth is spinning once a day which makes it appear that the moon is going around us once a day.
The moon goes around the earth approximately one time every 29 1/2 days. The moon always shows the same side toward the earth. We never see the side facing away from us.

The moon itself does not have any light. Its light is reflected from the sun. The sun always shines on the moon and half of it is always illuminated. What we see depends on where the moon is in relation to us and the sun. When the moon is directly between earth and the sun, it is considered the new (dark) moon. During this phase, the sun shines its light on the part of the moon we can't see, so we don't see any illumination. As the moon goes further around the earth, the sun shines its light on more of the moon that we can see. When the moon moves half way around the earth, the earth is between the sun and moon. This is when the half that we see is totally illuminated, thus the full moon.

One of the confusing things to me is the terminology of the quarters. The first quarter is really the first half moon and the third quarter is really the last half moon. It makes sense if you think about quarters being a fourth of the phase, but it is easy to try to associate quarter with amount of illumination instead of the actual phase. Another reason it is confusing is because the quarters start with zero (although you probably won't see it listed as a quarter). The first quarter starts at the half moon getting fuller (waxing) until the full moon. The second quarter starts at the full moon and the illumination gets smaller (waning) every day. The third quarter starts with the half moon waning and ends with the new moon. There is no "fourth quarter".
If you read the terminology in the previous article, I mentioned that the crescent is less than half illumination and gibbous is more than half illumination. We also saw that waxing means getting fuller and waning is just the opposite. Here is the complete phase with terminology:

  1. New moon (dark)
  2. Waxing crescent (less than half moon - right side illuminated)
  3. 1st quarter (half moon - right side illuminated)
  4. Waxing gibbous (more than half moon - right side illuminated)
  5. Full moon (2nd quarter)
  6. Waning gibbous (more than half moon - left side illuminated)
  7. 3rd quarter (half moon - left side illuminated)
  8. Waning crescent (less than half moon - left side illuminated)

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