Why you should embrace the 14 November "supermoon" - and why not

On November 14 a slightly unusual sky event awaits us: a perigee full moon (i.e. a near-coincidence of full moon and the closest point of the Moon in its elliptical orbit around the Earth) which is the closest between 1948 and 2034 - though beating those full moons only by a few hundred kilometers i.e. a fraction of one percent of the distance. Here's what that means and what it doesn't. In a nutshell:

• The full moon of November 14 is the largest in the sky of the year, and it is possible to notice with the unaided eye that its angular diameter and especially area are larger that at other times, by up to 14 and 30 percent, respectively, relative to full moons at apogee i.e. when farthest from the Earth.
  
  
• Typically 3 or 4 subsequent full moons each year occur pretty close to perigee (as this diagram clarifies) and thus look indistinguishable to the eye; all of them are colloquially known as 'supermoons' these days, a decidedly non-astronomical term reluctantly picked up in astronomy outreach in recent years.
  
  
• While the November 2016 full moon holds a proximity record for several decades in both directions, it is totally indistinguishable for the eye from the perigee moons of any other year (and there are many 'close calls' much nearer than the above-mentioned record years: for example next year already).

So: enjoy the show but be aware that something looking exactly the same happens several times each year. Now for the hard numbers (disclaimer: I'm taking the 1948/2034 claim reported in many places for granted and haven't checked it year by year; also distances and times listed and calculated in different places deviate by tiny amounts) ...

• On 2016 November 14 full moon occurs at 13:54 UTC, when the distance between the centers of Moon and Earth is 356,520 km.
  
  
• On 1948 January 26 full moon occured at 7:12 UTC, when said distance was 356,490 km: in 2016 it thus stays only 0.008% farther away.
• On 2034 November 25 full moon occurs at 22:34 UTC, when the distance will be 356,446 km: in 2016 it stays 0.02% farther away, still not a difference 'to write home about.'
  
  
• In 2015 at the closest full moon (which coincided with a total eclipse) it stayed 0.1% farther away than in 2016 and looked exactly the same when out of eclipse.
• In 2017 at the closest full moon the distance will be 356,605 km or only 0.02% farther away than in 2016 (i.e. by same factor by which 2034 will be closer).
  
  
• In October 2016 the full moon was only 0.55% farther away than it will be on 14 November 2016.
• In December 2016 the full moon will be 0.82% farther away than on 14 November 2016; even that won't be evident in any way: thus three supermoons in a row.

In contrast in April 2016 the full moon was 13.9% more distant than it will be in November when it will thus have a 29.7% larger disk area and brightness. Claims vary how evident this is to casual but attentive observers and all too often "experts" just write it off - but those who actually look, including this blogger, can find it quite evident. So don't let the Moon orbit ellipticity visibility deniers distract you from giving it a try!

Finally some more math, inspired by a message received after posting a draft of this analysis: remember that all numbers above refer to the distance between the centers of Earth and Moon – while most observers (minus the residents of ISS and Tiangong-2) reside on the surface of the former which rotates quite rapidly, namely once per day. We are some 6370 km from the center and typically at an angle to the line connecting the centers of both. Which means a lot.

Take 14 November 2016: For the center of the Earth the distance to the Moon changes little during the day, beginning at 356,472 km at 0:00 UTC, reaching the minimum (the excitement is all about) at about 11:30 UTC and rising again to 356,788 km at 24:00 UTC – the distance shrinks by ~230 km and rises again by ~280 km during that (UTC) day.

But now go to Hawaii, well placed for actually seeing the Moon at perigee: At 0:00 UTC it is 361,133 km away, at local(!) perigee at 10:00 UTC 350,175 km and at 24:00 UTC 361,995 km – the distance first shrinks by 11,000 km, then increases again by 12,000 km! This diurnal effect just dwarfs the tiny differences between the various “supermoons” over the years and centuries where we are talking of a few dozen kilometers.

Oh, and at which latitude you sit also makes a difference. For example in Tahiti at 17°S the 14 November Moon culminates (58° high) at 9:35 UTC and is 351,043 km away. In the same longitude it culminates in the zenith at 12°N and 350,141 km distant – but at 50°S it culminates only at 26° and 353,669 km distant, i.e. 3500 km farther away at the same time. Once again a difference two orders of magnitude more than the differences between the various supermoons …

So to conclude: forget the “largest full moon in decades” meme – it’s mathematically correct but dwarfed in magnitude by effects of your place on the planet and the time on perihelion day. But embrace the fact that a few full moons each(!) year are significantly closer than the others. They are not exactly “super” but perhaps a bit ‘superior’ to the others and can be a bit more impressive than other full moons. That’s all, folks …

Main sources: basic information on ‘supermoons’ (date in 1948 off), distances of many full moons and many calculations performed on JPL’s HORIZONS. A selection of stories on the 2016 supermoons (often in denial of the visibility of the perigee effect and sometimes with funny astronomical misunderstandings): here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here. Stories - by this blogger - on the actual visibility of the perigee effect: here, here, here, here (bottom) and here.