Sunday, August 10, 2014

“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.

1 comment:

  1. 'Tis the day for blog post corrections! Even at the ol' Patio Boat we had to come correct today....