Just a short post about the Supermoon of 14 November, widely publicised by the media the last few weeks as a not to be missed, once in a lifetime event. For example on Facebook
It’s a hype.
Supermoons are not rare, they occur regularly, on average every 14 months. The last one was 28 September 2015, the next one will be 4 December 2017.
Full moons have different sizes because the orbit of the moon is slightly elliptical. The image shows the moon orbit, exaggerated. The average distance to Earth is 385.000 km, but the moon can come as close as 356.500 km (perigee) and as far as 406.700 km (apogee). The moon orbit also rotates itself with a period of 8.85 year
As a result of these two effects, a full moon can sometimes occur when the moon is in or near its perigee. An observer on Earth will then see this full moon brighter and larger, than when it occurs in its apogee. Dividing the apogee distance by the perigee distance, we find 406.700 / 356.500 = ~ 1.14, so the moon will look ~14 % brighter and ~ 30 % larger. This effect is easily observable, as you can see in the image below. By the way, the name Supermoon has been introduced by astrologers, the correct name is Perigee Full Moon.
So, why this sudden interest in this particular Perigee Full Moon of 14 November?
The values given for apogee and perigee are actually averages. Because of the influence of sun and planets they vary slightly in time. Here are the perigee distances during the Supermoons of 2015 and 2016 :
- 356.876 km in 2015
- 356.511 km in 2016
The perigee distance on 14 November is a little bit smaller! To be precise , 365 km smaller, ~0.1%. So the Supermoon of 14 November will be 0.1% brighter and 0.2% larger. Observable for the unaided eye? Not at all, believe me…:-)!
Why the hype? When you look at the Perigee Full Moons in the past and future, you have to go back to 1948 to find an even smaller full moon perigee: 356.462 km (49 km smaller). And from now on you have to wait until 2034 to find a smaller one: 356.447 km (64 km smaller). These Supermoons will be ~0.02 % brighter.
That’s why it is said: the brightest Supermoon in 86 year…:-). Technically correct, but…. a hype.
My suggestion, try to observe the moon tomorrow, when it is rising, just after sunset. The moon looks always larger when close to the horizon! This is an optical illusion, the Moon Illusion. Combined with the Perigee Full Moon it will be beautiful
And when you are not free tomorrow, it is not that critical. One or two days later you can still admire the Supermoon.