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This April presents the farthest Full Moon of the year on April 2, and the closest New Moon of the year on April 17. The April Full Moon will be about 50,000 kilometers (30,000 miles) more distant than the April New Moon. No April Fools!
As the Moon revolves around the Earth, its orbit doesn't follow a perfect circle. The orbit is a somewhat oblong ellipse. In the course of a month, the Moon's distance from the Earth varies, with the Moon lodging farthest from the Earth at a point called apogee, and coming closest at perigee. The Moon is roughly 30,000 miles farther from the Earth at apogee than at perigee.
It just so happens that the Full Moon this month closely aligns with apogee, and that the New Moon coincides with perigee. As a general rule, Full Moon and New Moon do NOT align with apogee and perigee. The period of time from Full Moon to Full Moon (or New Moon to New Moon) represents a mean period of 29.53059 days, whereas the mean time period between successive returns of apogee (or perigee) is 27.55455 days. The former is called a lunar month, whereas the latter is an anomalistic month.
Full Moon/New Moon & Apogee/Perigee Cycles
Even though there is nearly a 2-day disparity between the lunar month and the anomalistic month, the lunar and anomalistic months realign with one another in predictable cycles. It just so happens that 14 lunar months equal almost exactly 15 anomalistic months:
|14 x 29.53059 days = 413.428 days|
|15 x 27.55455 days = 413.318 days|
Therefore, on the 14th Full Moon after the April 2, 2007 Full Moon, the Full Moon and apogee will realign again on May 20, 2008; similarly, the 14th New Moon after the April 17, 2007 New Moon will feature the New Moon at perigee on June 3, 2008. This period of time (14 lunar months) is approximately one year one month and 18 days long.
7-Lunar Month Cycle
There's another way to look at it: 7 lunar months = 7.5 anomalistic months. So the 7th Full Moon after the April 2, 2007 Full Moon will fall on October 26, 2007. At this juncture, the Full Moon will be at perigee (not apogee), so it will be the closest Full Moon of 2007. Seven lunar months after that - on May 20, 2008 - the Full Moon will be at apogee again.
Perigean Spring Tides
When a New or Full Moon aligns with perigee, beware of large perigean spring tides. The year's closest New Moon on April 17 will usher forth a perigean spring tide, as will the closest Full Moon on October 26. For an article on perigean spring tides, see my February 2006 feature.
copyright 2007 by Bruce McClure
March 2007 Feature * May 2007 Feature
|Distance of Full Moon|