THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
<http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A>
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian Date: 2458862.16
2019-2020: LXXXIII
"Where the celestial things are."
THE DAILY ASTRONOMER
Monday, January 13, 2020
Explaining the Lunar Cycle
[image: download.jpg]
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."
[image: moon_phases_diagram.jpg]
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
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.
To subscribe or unsubscribe from the "Daily Astronomer"
http://lists.maine.edu/cgi/wa?A0=DAILY-ASTRONOMER