The Moon Squeezed Largest

This Monday November 14, the Moon

will reach its perigee at 11:30 by Universal Time, and its Full position less than two and a half hours later, at 13:53.  It will appear at its largest, 33.5 seconds of arc wide, and will seem to be even huger if you see it shortly after it has risen.

11:30 UT is toward midday in Britain, 6:30 AM in eastern North America and 3:30 on the west coast.  So it’s the Pacific that gets to see this great looming Moon at its moment of maximum at or before midnight.  But for the rest of us, either on Sunday or Monday night the Moon will be pretty near to that maximum size.

In the Moon’s roughly 30-day orbit, perigee is the point nearest to Earth, and Full is the point opposite to the Sun.  As we elaborate in the “Moon” section of Astronomical Calendar 2016, when perigee falls near to either syzygy (Full or New), the moments of Sun-Earth-Moon or Sun-Moon-Earth line-up, the Moon’s orbit is tidally squeezed so that the perigee is more extremely near.

Part of the “Moon’s distance” graph from Astronomical Calendar 2016.

The perigee distance this time – that is, the distance between the centers of Earth and Moon – is 55.9 Earth-radii, or 356,509 kilometers.  According to Jean Meeus’s Astronomical Tables, page 404, this is the nearest in the period 1990-2020.  1992 Jan. 29 was also very near, 356,550 km.  In the 20th century, the nearest he found was 356.375 km on 1912 Jan. 4.  And in the longer span 1500-2500, the nearest will be 356,371 km, on 2257 Jan. 1.  So that must be close to the theoretical minimum possible in our era.  (The Moon, over vaster spans of time, is gradually receding from us.)

Actually the close coincidence in time between perigee and syzygy does not entirely explain the smallness of the perigee distance, because there have been quite a lot of occasions when the time-difference was smaller than now, yet the perigee not so close; for instance 2015 September 28, only 0.9 hour from perigee to Full, distance 356,877 km.  The motion of the Moon is determined by many factors; there are, as I’ve probably mentioned before, more than 600 terms in the simpler versions of the equations for the Moon’s position.  Notice that several of the record-near perigees are in January, near to the perihelion, which happens each year between January 1 and 5; it is the nearest-in moment of the Earth-Moon system in its journey around the Sun, so that is obviously another factor.

The Moon is the primary cause of tides on the Earth, and the Sun is the secondary cause.  So when they are lined up, we get the ocean tides of greatest amplitude.  If they happen to coincide with coastal storms, the level of the water may be boosted yet more and there may be floods.

Last year, Donald Olson of Texas State University and colleagues found that the high tides resulting from the perigean Full Moon of 1912 Jan. 4 probably caused increased calving of icebergs from the fronts of glaciers in Greenland, and that one of these could have been the iceberg that struck and sank the Titanic on April 15 of that year.  According to Olson, the 1912 perigee was the closest in 1,400 years – presumably because the complexity of the calculation can end in slightly different decimals.  The Greenland glaciers have since then retreated so far, because of global warming, that their ends are dozens of miles back up the fjords, and the icebergs calving from them get stuck for a while on the moraines at the fjords’ mouths and thus take longer to float out into the open ocean.

The media will probably raise alarms about this “Supermoon,” as they have done for some relatively ordinary Full Moons in the past.  What would be interesting to know is whether another factor, the rising temperature and rising level of the world ocean, is becoming visible in the statistics of perigean-Moon floods.