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Founded January 1970
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"Ceaseless and insensible."
THE DAILY ASTRONOMER
Thursday, November 10, 2016
Monday's Supermoon
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Well....um, I, ah....well, um....yes, actually....
That was in answer to your question:
"Idiot, did you lose sleep because of the election coverage and
that's why you didn't send a DA on Wednesday because you
were too tired to function?"
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You have heard the term "super moon." Though coined in the 1970's, the word has recently come into vogue through social media and now everybody and his second cousin anxiously awaits the arrival of the next super moon. The main stream media hypes it, the astronomers explain it, the earnest sky watchers admire it, and the austere, dyspeptic types sniff disdainfully at it.
But, what, exactly is it and why does it command our attention?
We'll address this issue by first explaining that the super moon is a full moon that occurs when the moon is both full and at or near perigee (its closest distance to Earth.) Though many people logically think the moon is closest to us when it is full, it actually isn't, most of the time.
The moon is full every 29.5 days. The time span separating successive full moons is called the moon's "synodic period." Synodic relates to the time when a celestial body occupies the same position relative to an Earth-bound observer. See the graphic below.
The moon's phase cycle. The moon is full when it is on the far side of Earth relative to the Sun. At this time, we observe all of the moon's illuminated facade. The time period between successive full moons, called the Synodic Period, is approximately 29.5 days. Image from Laurentlizard.com
The moon's orbit is also elliptical and therefore its distance from Earth constantly changes from perigee, its point of least distance, to apogee, its point of greatest distance. The period of time separating successive perigees -called an anomalistic month- is 27.55 days. There is nearly a two day duration difference between the synodic month and anomalistic month. Consequently, the two periods will be out of sync.
Perigee-Apogee. This image shows the moon's orbit. The elongation of the ellipse is highly exaggerated for purposes of clarity. (The lunar orbit elongation is 5.49%, higher than most planetary orbits, but not as large as depicted in this graphic.) The moon's closest distance to Earth is perigee and its point of greatest distance is called apogee.
Only when the moon is full around the time of perigee will we experience a super moon. To make matters more deliciously complicated, the moon's perigee distances vary each orbit. Therefore, Orwell, some supermoons are more equal than others. Monday's super moon will be the brightest in 69 years, because the perigee occurs only 2.4 hours before the full moon.* (Perigee at 6:21 a.m. and full moon at 8:52 a.m.) It is the closest perigee in 30 years! It will be only 55.9 Earth radii from Earth, (221,531 miles.)
Perigee moon and apogee moon. The perigee full moon (super moon) is 14% larger and 30% brighter than the apogee full moon. While these differences are significant, it would be difficult to judge this difference because one cannot observe the apogee and perigee full moons in the sky simultaneously. Image: NASA
Monday night we will all behold an uber-super moon: a full moon that is brighter than it has been in 69 years!
*The "full moon" is an instantaneous event. A moon is full when its center is 180 degrees away from the Sun's center. As the moon is moving about 2280 miles per hour, this central point moves away from the 180 degree mark rapidly. The moon appears full for about three nights, but a full moon is over in an instant.