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Full Moons are not equal! The Full Hunter's Moon coming on October 26, 2007 wins the title of the year's largest Full Moon. Very simply, it's the largest because it's the Full Moon coming closest to Earth for the year. It'll be less than 222,000 miles away. Contrast that with the farthest (and smallest) Full Moon of the year last April. On April 2, the Full Moon was over 252,000 miles distant.
Largest Full Moons come in 14-Month Cycles
The time period between successive Full Moons is referred to as one lunation or one lunar month. Also called the synodic month, the mean period equals 29.5306 days (29 days 12 hours 44 minutes). Because of the complicated interplay between the Earth, Moon and Sun, the lunar month can vary from about 29.3 to 29.8 days in length, a difference of roughly 13 hours.
Closest Full Moons recur every 14 lunar months. Thus, after the closest Full Moon of 2007 lights the nighttime on October 26, the closest Full Moon in 2008 will fall 14 lunar months later, on December 12. 14 lunar months after that, the cycle will recur on January 30, 2010.
Why in 14-Month Cycles?
Closest Full Moons repeat in 14-year cycles, because of the Moon's eccentric (oblong) orbit around the Earth. In the period of one month (or more precisely: one anomalistic month), the Moon's distance from Earth varies by about 30,000 miles. When the Moon comes closest to Earth for the month, the Moon is said to be at perigee. In 2007, the Full Moon closely aligns with perigee on October 26.
However, the month as measured by successive perigees (anomalistic month) is about 2 days shorter than successive returns of Full Moon (lunar month). For this reason, Full Moons do not realign with perigee on a monthly basis. Amazingly enough, though, 14 lunar months equal almost exactly to 15 anomalistic months.
|14 x 29.53059 days = 413.428 days|
|15 x 27.55455 days = 413.318 days|
In a nutshell, that's why closest Full Moons recur every 14 lunar months!
Farthest Full Moon in 7 Lunar Months
Also, take a look at this: 7 lunar months = 7.5 anomalistic months. What this means is that the Full Moon will align with apogee - the Moon's greatest distance from Earth for the month - on May 20, 2008. Two weeks later, it'll be the New Moon that aligns with perigee on June 3, 2008!
Perigean Spring Tides
For the few days following the October 26 Full Moon, the ocean tides will be especially high at high tide and low at low tide. Twice a month - at Full Moon and New Moon - you have what are called spring tides. This is when the Moon, Earth and Sun make a straight line in space, causing higher than usual tides. But when a Full or New Moon aligns with perigee, you have extra high perigean spring tides.
copyright 2007 by Bruce McClure
September 2007 feature * November 2007 feature
For your information:
|Moon Phase Almanac|
|Moon's Perigee and Apogee Dates|
|Ocean Tide Calculator|
|Another October Adventure: Arcturus, Ghost of Summer Sun!|