THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
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70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458758.5
2019-2020: XXIII
"If knees bent the other way, what would a chair look like?"
-Gallagher
THE DAILY ASTRONOMER
Thursday, October 3, 2019
High Moon
October already and we've hardly spent any time at Pandora's Jar! Time
passes far too rapidly and the day contains fewer hours that we need to
complete all the tasks at hand. My apologies, again, for those who still
await answers to their queries. Today we address one question about
the moon's positions in different seasons.
*"Why is the full moon higher in the summer than the winter? I know
that the Sun's position changes throughout the year, but how can that
change affect the moon?" -R. Hill, Schenectady. NY*
Greetings!
Before we attend to the question, let's take a moment to determine if the
full moon's altitude in winter is higher than in summer. We regard the
full moon rise and set times (in Portland, ME) in June and December 2019
Full moon: 17 June 2019
Rise: 8:49 p.m.
Set: 6:03 a.m. (18 June 2019)
Duration: * 9 hours, 14 minutes*
Full moon: 12 December 2019
Rise: 4:39 p.m.
Set: 8:23 a.m. (13 December 2019)
Duration: *15 hours, 44 minutes*
Well, we can certainly conclude that the full moon remains in the sky far
longer around the winter solstice than the summer solstice. While the
times are specific to Portland, the winter full moon would be higher in the
sky than the summer full moon at other locations, as well.
To explain why the seasons affect the full moon's altitude, we should first
explain that throughout the year, the Sun follows an undulating path called
"the ecliptic." The graphic below shows how the ecliptic curves
alternately above and then below the celestial equator, the projection of
Earth's equator onto the sky.
[image: bsfig4.jpg]
The ecliptic describes that curve because Earth is tilted on its axis by an
angle of 23.5 degrees, equal to the angle of both the ecliptic's apex and
nadir. (See image below). If Earth's axis weren't tilted, the Sun's
path would be aligned with the celestial equator.
[image: SEASONS.GIF]
*Earth's angle (obliquity) is 23.5 degrees. *As Earth moves around the
Sun, the Sun's altitude changes. On the summer solstice, the Sun attains
its greatest altitude; on the winter solstice, its lowest.
We should now explain that the moon travels along a band centered on the
ecliptic because the moon's orbit is inclined by about five degrees
relative to Earth's orbit around the Sun. While the moon won't move
precisely on the ecliptic, it will always be close to it.
As the moon travels around Earth, its apparent position relative to the Sun
changes continuously. For instance, when the moon is new, it occupies the
same region of the sky as the Sun and the angular distance separating them
is 0 degrees. When the moon is full, the angular separation between the
Sun and moon is 180 degrees. So, around the first day of winter, the
Sun is very low on the ecliptic (see image above). The full moon that
occurs around that time will be 180 degrees away from the Sun, around the
point of the summer solstice. Since the moon is always close to the
ecliptic, the moon will be much higher in the sky at that time.
So, Earth's position relative to the Sun does affect the moon's position
relative to us.
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