• Stephen Strum

Are all these super moons unusual?

Updated: Aug 1, 2019

#supermoon

Comparing the moon's apparent size when at perigee, average, and when at apogee.
Full moons vary in size by about 14% and brightness by about 30% during the course of an average year.

Each of the first three months of 2019 will see a so-called "super moon," with the full moon being larger than average in apparent size. The astrologer Richard Nolle coined the term "super moon" back in 1979 to denote a full or new moon that occurs when the moon is at its closest point to the Earth in its elliptical orbit known as perigee. At perigee, the moon can be within 225,000 miles (about 360,000 km) of the Earth while at apogee, when the moon is farthest from the Earth, it can be more than 250,000 miles (about 405,000 km) away. While full moon occurs at perigee, it can appear about 14% larger than when it is at apogee. Since you can't see two moons in the sky side-by-side, that 14% difference is hard to notice. However, in terms of brightness, the difference is around 30%, and that is enough to be noticeable. The image above compares the relative size of the moon when it is at perigee compared to an average size and the apparent size when the moon is at apogee, a "micro-moon." The plot below compares half of the perigee moon to half of the apogee moon to better illustrate the apparent size difference.

Moon size comparison between super moon and micro moon.
This image compares the relative size of full moons at perigee (left) and apogee (right).

While the occurrence of a "super moon" may suddenly seem to be unusually frequent, it is not a new phenomenon and normally occurs 1-3 times each year. They generally occur in two or three consecutive full moons, followed by a 13-14 month interval before another set of two to three consecutive "super moons." So, depending on the timing, some years may fall entirely within that 13-14 month gap and not see a super moon. However, while we only see a set of full moons occur near perigee ever year or so, the moon is always at perigee once every month. So, there is still a point in time each month where the apparent size of the moon will be larger, but sometimes that occurs at quarter moon or new moon, or somewhere in between.


The graphic below shows how the moon is sometimes closest to the Earth at new moon, and other times at full moon, but more than half the time it reaches perigee somewhere in between new and full moon. If perigee occurs when the moon is either full or new, then tides will be a couple of inches higher than the average spring tides since the closer distance to the moon results in a stronger gravitational tug. The tides are larger than average anytime the moon is at perigee, but the tides are always largest near the time of new moon and full moon since the force of gravity from the sun and the moon are working along the same axis.

tides and the moon

The table below lists the days and times for all full moons that occur near perigee "super moons" from 2019 through 2050 in both GMT and CST times. So, while the idea of "super moons" has become more popular in the media the last couple years, it is a very regular occurrence and almost every year will see at least one or two "super moons" that will brighten up the night sky a little more than average. While you might think a "super moon" is a better time to look at the moon through a telescope, you are usually always better off studying the moon somewhere in between the full moon and new moon. When the moon is full, it is the same as high noon on earth, a time when shadows are small and limited. That makes details on the moon much harder to pick out. When the moon is halfway in between new and full the resulting shadows, particularly near the terminator which is the line between day and night on the moon, make the lunar terrain far more interesting.




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