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Just about everyone knows that the moon orbits Earth once every month, which is the time required for one orbit. What we do not commonly know is that the moon’s orbit is not circular; it is elliptical.
At its closest approach, called perigee, the moon comes as close as roughly 222,000 miles. The farthest away it gets at apogee is roughly 253,000 miles from Earth.
When a full moon occurs near perigee, we call it a supermoon. Surprisingly, there are frequently four or more supermoons (out of a possible 12-13 full moons) each year. This year will see supermoons in May, June, July and August.
Last week’s supermoon was impressive because the full moon was eclipsed, a full lunar eclipse. Unfortunately, it was not visible in Hawaii. It ended just as the moon rose in the eastern sky.
Who would notice the difference in size? The comparison in size of the full moon at its smallest, which occurs when the moon is at apogee, and its largest at perigee is only about 12%, but the difference in brightness is about 30%.
If one could see the two side by side, the differences would be apparent. The
size difference between the smallest (apogee) and largest (perigee) moon is about the same as that of a soccer ball compared with a basketball or a tennis ball compared with a baseball.
The difference in brightness is slightly less than the difference between a 75-watt incandescent bulb and a 100-watt bulb. This difference would be noticeable in a room if the two switched on and off alternately, but probably not for a full moon unless we commonly spend the dark early evening hours outside.
One might expect that
the full moon occurs at the same point in the moon’s orbit every month. That would be the case if the moon’s
orbit behaved, but unlike many celestial objects, it is much more complicated than it appears to the naked eye.
Our common measure of the month is the time between one full moon and another. Actually, astronomers measure the time from one new moon to another. But since we cannot see the new moon, the full moon serves as an easier reference point.
The average time for the moon to return to the same position relative to the stars is the sidereal month of 27.322 days. This is the actual orbital period of the moon around Earth. The time period is shorter than the synodic or lunar month of 29.531 days between two full moons.
The difference is because Earth moves around the sun during the month, and so the moon must travel a little more than 360 degrees in
order to be in alignment with the sun again.
Another factor is the
precession of the moon’s perigee. Geometrically, precession is the rotation of the elliptical orbit around Earth due to the gravitational influence of the sun. Thus the “anomalistic month” measures successive returns to perigee, a mean period of 27.555 days.
The supermoon on
Nov. 14, 2016, was the closest a full moon has been to Earth since Jan. 26, 1948. The next time a full moon will be even closer to Earth will be on Nov. 25, 2034.
Additionally, the moon moves faster at perigee than at apogee, and Earth moves faster when it is near the sun and slower when farther away. Combine all of these variables and it is no wonder that we will not see
another supermoon as extreme as November 2016
until November 2034.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
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