“Supermoon” Corrected

More accurately, my own calculations corrected. Last year (here) I gave some data on the recurrence of supermoons, better known, certainly in past times, as Perigree Moons. Peri means near; a Perigree Moon is therefore a full moon at a time when the moon is closest to the earth in its ever-so-slightly-elliptic path. I said at the time that Supermoons recur at 14 months intervals—and added “thus roughly every 378 days.” Now that number is clearly wrong.

The last Supermoon came on June 23, 2013. Today is the date of another. The distance in time between those two dates is 413 days. What I did wrong was to multiply 14 by 27, which equals 378. I was using the number of days the moon orbits the earth, but rounded. The actual time for that is 27.3 days. StarDate reports, however, as follows (link):

The Moon takes 27.3 days to orbit Earth, but the lunar phase cycle (from new Moon to new Moon) is 29.5 days. The Moon spends the extra 2.2 days “catching up” because Earth travels about 45 million miles around the Sun during the time the Moon completes one orbit around Earth.

That “catching up” phrase is perhaps a way of avoiding the task of explaining that the 27.3 day measurement is a measurement relative to the stars (called sidereal time) whereas the 29.5 day measurement is a measurement relative to the sun (called synodic time). Synodic time is slightly greater than sidereal time due to small changes in the earth’s and moon’s rotation over what might be called absolute time.

In any case if we multiply 14 months by 29.5 days, then we get a 413 day interval between Supermoons.

Supermoon

Last night was a supermoon, thus the coincidence of a full moon taking place very close to the time when the moon is closest to the earth. In matters of astronomy, someone like me, who tends to be geometrically challenged, can never remember exactly what this sort of things means. Hence I don’t apologize in repeatedly revisiting such subjects: at least for a week or two I can remember the relationships of these mysterious bodies hanging in the sky.

The moon’s orbit around the earth is ever so slightly elliptical. Hence at one point in its travels it is closest to the earth (perigee), at the other farthest (apogee). The image I show comes courtesy of the Physics Department of Utah State University (link). The words are from Greek, peri meaning near, apo meaning away, and the ge is derived from the Greek for earth; we get Gaia from that. The official name for this point in the lunar cycle is called Perigee Moon. “Supermoon” was a very recent coinage by the astrologer Richard Nolle in 1979 (per Wikipedia).

The graphic, as noted, exaggerates the ellipse. The inset, also from Wikipedia, shows the difference in perceived size of the moon at its last perigee before yesterday (on March 19, 2011) and its more or less average size (on December 20, 2010). The difference between perigee and apogee according to NASA is 14 percent, but The Huffington Post shows 13 percent, which pleases this tiny subset of humanity here. Supermoons occur once every 14 full moon cycles, thus  roughly every 378 days—just long enough to be news every time, weather permitting, of course, while the memory of why the moon should now seem so much brighter and larger will, each time, have been once more forgotten. That’s when blogs come in handy to have a private reference… How can those monstrously big jets stay in the air? (one of Brigitte’s favorite questions). How can that moon stay up there rather than falling down into the pit of eternity?
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See the correction a year later here.