Sep 2016


The Truth About "Supermoons"
John Hlynialuk

I may have been the only one in this area who saw the “mini-moon” (a full moon at apogee) last May 21. No media covered it, but give us a full moon at perigee (coming this Nov 14) and you will see a lot of print about “supermoons”.

The term was invented by an astrologer (name withheld to not give undeserved credit) who defined it as
“a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit”. Right away there are two problems: first, the moon’s orbit is not a fixed path that repeats exactly month to month so you are taking 90% of a moving target. Arguments then arise as to how many “supermoons” you can get in a year. Secondly, it is very difficult to actually see a new moon, -you risk eye damage since new moons are always close to the Sun. Even during solar eclipses when the moon is silhouetted against the Sun you need to use proper solar filters during the partial phases. “Danger, Will Robinson, Danger!”

Orbiting objects would always be the same size if their orbits were perfectly circular, but circular orbits are extremely rare. Our moon follows quite an elliptical path being father away at “apogee” and closer at “perigee”. Furthermore the actual difference between the two varies. For example, on Sep 18 at 1 pm, perigee distance is 361 896 km. Two weeks later (Oct 4 at 7 am) apogee distance is 406 096 km for a difference of 44 200 km. The next three occasions give a difference of 48 801 km in late Oct, 50 045 km in Nov and 47 409 km in Dec. The difference is never the same. The moon’s apparent size does change between the extremes but the 14% range in size is only an average.

So if the moon is closer to the Earth at perigee, shouldn’t it
look bigger? Yes, but they are never seen side-by-side. Most people, if they happen to see both, rely on memory to compare the two moon sizes. If you go to the trouble of taking photos, they need to be with exactly the same camera, the same lens, and the moon must be close to the same location on two separate dates. Since the moon’s path across the sky is different in subsequent months, and it’s phase is not synchronized exactly to its orbit, and the weather sometimes works against you, it is not an easy task. I have succeeded getting photos only a few times in many years of trying. (OK, it was not a big priority.)

Image of Perigean Full Moon Jun 2013 (left) and Apogean Full Moon (right) John H. Photo

At the risk of adding to the media hype, the best “supermoon” this year is Nov 14 with perigee less than 3 hours from full moon, -a small and fairly rare time difference. Although the “mini-moon” last May 21 was a media “non-event”, do (“or do not") check out the next “mini-moon” on Jun 8, 2017. Do (“or do not, there is no try") to take pictures of both, this is the only way to really see the difference. Clear skies!