Just about everyone knows that the moon orbits Earth once every month. 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 225,623 miles. The farthest away it gets — at apogee — is 252,088 miles from Earth.
When a full moon occurs near perigee (when it is closest to Earth), we call it a super moon. Surprisingly, there are frequently four or more super moons (out of a possible 12-13 full moons) each year.
Last month’s super moon was impressive because the full moon occurred about as close to perigee as it ever gets.
Who would notice? 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 percent, but the difference in brightness is about 30 percent.
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 the full moon to occur 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. Because 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. This 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, 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. Geometrical 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.
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 super moon as extreme as November’s until November 2034.
