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70 Falmouth Street Portland, Maine 04103
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Altitude: 10 feet below sea level
Founded January 1970
Julian Date: 2458862.16
Julian Date: 2458862.16
2019-2020: LXXXIII
"Where the celestial things are."
THE DAILY ASTRONOMER
Monday, January 13, 2020
Explaining the Lunar Cycle
Even casual moon watchers will know that the moon requires nearly a month to complete an entire lunar phase sequence. In fact, the word month derives from the vowel-rich word "moonth," relating to the time separating successive full moons, a period approximately equal to 29.5 days. Many planetarium patrons have found the lunar phase cycle to be quite confusing because the moon requires about 27.5 days to complete one orbit around Earth. If the moon needs 27.5 days to revolve once around Earth, why are successive full moons 29.5 days apart?
As this issue arises quite frequently, we thought it would be worthwhile to explain this apparent discrepancy. First, we see the moon because it reflects sunlight back into space. At any given moment, the sun half illuminates the moon as it is a sphere in the vicinity of a single light source. (If life were complex and Earth revolved around two stars, more than half of the moon -and Earth- would be illuminated.) As the moon moves around our planet, we see different phases. The phase at any given time depends on the angle between the Sun, Earth and the moon. When the moon is on the opposite side of Earth relative to the Sun -called "opposition"- we see the entire lit half: a "full moon."
Now, because this little screen serves as our own play Universe and we can do what we want with it, let's suspend the laws of physics. We're going to pretend that Earth doesn't revolve around the Sun, but instead maintains the same position. While Earth rotates on its axis in this same place, the moon continues moving around its Earth-centered orbit. Consequently, 27.5 days after the full moon, the moon, Sun and Earth are in the same position relative to each other as they were during the previous opposition. The moon is full again. If we keep Earth in the same place, successive full moons would be separated by 27.5 days! The lunar phase cycle and the lunar orbit would be perfectly synchronized.
In the real world -or Universe- Earth doesn't remain in the same place. It revolves constantly around the Sun as the moon does the same around Earth. During the 27.5 days the moon requires to complete an orbit around Earth following a full moon, Earth has moved slightly less than 30 degrees along its sun-centered circuit. After 27.5 days, the moon, Earth and Sun won't be in precisely the same opposition configuration again. The moon needs about two more days of travel in order to occupy this position again.
We see evidence of this 29.5 day cycle in the following list of full moons during the first half of 2020:
January 10, 2020
February 9, 2020
March 9, 2020
April 7, 2020
May 7, 2020
June 5, 2020
April 7, 2020
May 7, 2020
June 5, 2020
Remember that these full moons don't occur at the same exact time during these days, so sometimes the full moon will occur in the same numbered day in two successive moons.
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