Supermoon

About three or four times a year, the new or full moon coincides closely in time with the perigee of the moon—the point when the moon is closest to the Earth. These occurrences are often called ‘perigean spring tides.’ The difference between ‘perigean spring tide’ and normal tidal ranges for all areas of the coast is small. In most cases, the difference is only a couple of inches above normal spring tides. Image and caption via NOAA.

Supermoon is  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. There are 4-6 supermoons a year on average.

By Nolle’s definition, the new moon or full moon has to come within 361,863 kilometers (224,851 miles) of our planet, as measured from the centers of the moon and Earth, in order to be considered a supermoon.

By that definition, the year 2014 has a total of five supermoons. The first supermoon, for 2014, came on New Year’s Day, with the January 1 new moon. The second came with the new moon of January 30, 2014. The full moons of July, August and September will also be supermoons, by Nolle’s definition. Thus, the next supermoons – aka perigee full moons – in 2014 are:

Full moon of July 12 at 11:25 UTC

Full moon of August 10 at 18:09 UTC

Full moon of September 9 at 1:38 UTC

The full moon on August 10, 2014, will present the closest supermoon of the year (356,896 kilometers or 221,765 miles).

However, the new moons on January 1 and January 30 were not far behind, featuring the year’s second-closest and third-closest supermoons, respectively. On January 1, the moon turned new less than 10 hours before reaching lunar perigee – the moon’s closest point to Earth in its orbit. On January 30, the moon turned new nearly 12 hours after reaching lunar perigee.

Want more detail? Okay. In 2014, the moon comes closest to Earth on August 10 (356,896 kilometers), and swings farthest away some two weeks before, on July 28 (406,567 kilometers). That’s a difference of 49,671 kilometers (406,567 – 356,896 = 49,671). Ninety percent of this 49,671-figure equals 44,703.9 kilometers (0.9 x 49,671 = 44,703.9). Presumably, any new or full moon coming closer than 361,863.1 kilometers (406,567 – 44,703.9 = 361,863.1) would be “at or near (within 90% of) its closest approach to Earth.”

Dates of closest full supermoons in past and future years. More often than not, the one day of the year that the full moon and perigee align also brings about the year’s closest perigee (also called proxigee). Because the moon has recurring cycles, we can count on the full moon and perigee to come in concert in periods of about one year, one month and 18 days.

Therefore, the full moon and perigee realign in periods of about one year and 48 days. So we can figure the dates of the closest full moons in recent and future years as:

March 19, 2011

May 6, 2012

June 23, 2013

August 10, 2014

September 28, 2015

November 14, 2016

January 2, 2018.

There won’t be a perigee full moon in 2017 because the full moon and perigee won’t realign again (after November 14, 2016) until January 2, 2018.

By the way, some astronomers will call all the full moons listed above proxigee full moons.

But, like you, we’ll have fun just calling ‘em supermoons.

#collected