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How to Calculate the Phases of the Moon in a Month

The sun shines on both the earth and the moon. When we look at the moon, it is half in sunlight and half in dark. Because our viewing angle changes, we sometimes see the moon as half-light and half-dark, or as mostly light or mostly dark. With a little practice, you can look at the moon and tell what phase is coming next and estimate when the next phases will come for the next month.

Instructions

- 1
  Look at the moon, noting especially which side of the lighted area is curved. When the moon is less than half-full, it is called a crescent moon. When the moon is more than half-full, it is called gibbous. When the lighted area is growing, the moon is said to be waxing. When the lighted area is shrinking, the moon is said to be waning. The four major phases of the moon are: new, with no light; first quarter, half-lit with the curve on the right; full, which is completely lit; and third quarter, which is half-lit with the curve on the left. After that, the next phase is new again.
- 2
  Use the mnemonic "DOC" to remember the sequence of the phases. A "D" has the curve on the right. After the D comes the full moon -- symbolized by "O." After the O comes "C" -- with the curve on the left. The acronymn DOC helps you keep the sequence straight so you will know what is coming up next. The sequence of specific phases is: new; waxing crescent; first quarter (half-full with the curve on the right); waxing gibbous; full; waning gibbous; third quarter (half-full with the curve on the left); waning crescent; and new. In general, if the curve is on the right, the moon is getting fuller. If the curve is on the left, it is getting smaller.
- 3
  Estimate the date of the next phase based on the total time for the cycle. The entire cycle -- from new to new -- is 29.5 days. The time between phases is 29.5 divided by 4, which equals 7.375 -- just over 7 days. Look at the moon and estimate how far you are from the last phase to the next phase and compare this to 7. For example, if the moon is at waxing crescent and is a little more than halfway between new and first phase, the first phase will be in about 3 days -- 3 is about half of 7. That means that the full moon is about 10 days away, because 10 equals 7 plus 3.

Astronomy