December 12, 2008: Closest Full Moon in 23 Years

The Full Moon of December 12 is not just this year's closest and largest Full Moon. The 2008 Full Long Night Moon presents the closest and largest Full Moon since March 8, 1993. We won't have a closer encounter with the Full Moon again until November 14, 2016!

Perigee & Apogee

Every month, the Moon in its eccentric - or oblong - orbit around our planet swings closest to Earth at perigee and farthest from Earth at apogee. At perigee, the Moon comes roughly 30,000 miles closer to Earth than at apogee. However, the distance to perigee (moon's closest point) and to apogee (moon's farthest point) do not remain static from month to month. In fact, this year's closest perigee and farthest apogee both occur in December.

Closest Perigees at Full Moon; Farthest perigees at Quarter Moon

The eccentricity of the Moon's orbit varies throughout the year, depending on which lunar phase coincides with perigee. When the Full Moon coincides with perigee, the eccentricity of the Moon's orbit increases most greatly. The increased eccentricity brings perigee closer to Earth, yet pushes apogee farther away. In 2008, the Full Moon and perigee both fall on December 12, to stage the closest and largest Full Moon of the year. (By the way, the farthest apogee of the year falls two weeks afterwards - on December 26, 2008.)

When the Quarter Moon aligns with perigee, the eccentricity decreases to a minimum and the Moon's orbit comes closest to being a circle. When the Quarter Moon and perigee coincide, perigee is pushed farther away from Earth whereas apogee is pulled closer in. In other words, perigee and apogee are closest together at a Quarter Moon perigee. In 2008, the Quarter Moon and perigee both fall on Valentine's Day - February 14 - bringing about the year's most distant perigee. I'll leave it to an astrologer to tell you the significance of a Quarter Moon perigee on Valentine's Day!

Full Moon Perigees Recur in Cycles

Full Moons recur in mean periods of 29.53059 days, whereas the Moon returns to perigee in mean periods of 27.55455 days. Amazingly, 14 returns to Full Moon almost exactly equal 15 returns to perigee:

14 x 29.53059 days = 413.428 days

15 x 27.55455 days = 413.318 days

The Full Moon realigns with perigee in a little over 413 days, this time period about equal to 1 year 1 month and 18 days. The last Full Moon perigee occurred on October 26, 2007, and the next Full Moon perigee will fall on January 30, 2010.

Closest of Close Moons

Extra close Moons depend on a convergence of three things. First, the moon has to be at perigee. Second, the Moon has to be full. Third, the Full Moon perigee has to fall appreciably close to perihelion - the Earth at its closest point to the Sun. Perihelion occurs annually in early January.

However, the Moon comes closer to Earth on March 8, 1993 and November 14, 2016, than it does on December 12, 2008. This seems to call into question the nearness of perihelion as a condition for an extra close perigee. Obviously, December 12 is closer to early Janaury than either March 8 or November 14. But hold on. I have an explanation.

According to John Walker's Perigee and Apogee Calculator, the Moon reaches perigee about 5 hours after Full Moon on December 12, 2008. However, on March 8, 1993, there was only about an hour difference between Full Moon and perigee. On November 14, 2016, the difference between Full Moon and perigee will amount to about 2.5 hours. Of these three extra-close perigees, the November 14, 2016 production rates the closest. In time, this nearby 2016 perigee will be surpassed by a closer perigee on November 25, 2034, and an even closer encounter on December 6, 2052.

Perigee Distances Smaller than 356,400 kilometers

For the Moon to come closer than 356,400 kilometers (221,457 miles) is quite a feat. In fact, this won't happen at all in the 21st century (2001-2100) century or the 22nd century (2101-2200). The last time the Full Moon perigee swung this close to Earth was on January 15, 1930 (356,397 km), and the next time won't be till January 1, 2257 (356,371 km).* By the way, the December 12, 2008 perigee will lie at a distance of 356,566 kilometers (221,600 miles).

High Tides, Earthquakes and Volcanoes?

Yes, the extra-close Full Moon will definitely bring forth extra-buoyant perigean spring tides along the ocean shorelines. It's even said that extra-close Moons can trigger earthquakes and volcanoes!

Happy 80th Birthday to Jean Meeus!

How fitting that Jean Meeus' 80th birthday happens to fall on December 12, 2008, to commemorate this year's extra-close Full Moon. Indeed, Jean Meeus' Astronomy Morsels have played a major role in shaping my understanding of the Moon and perigee.

copyright 2008 by Bruce McClure

* page 39, More Mathematical Astronomy Morsels by Jean Meeus

November 2008 Feature * January 2009 Feature