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The Moon's orbit is not a perfect circle, but rather an ellipse - an oblong loop around our planet Earth. Hence, the Moon doesn't stay the same distance from the Earth throughout the month. The Moon comes closest to Earth for the month at perigee and swings farthest from the Earth for the month at apogee.
As for July 2011, the Moon reaches perigee on 2011 July 7, and some two weeks later, swings to apogee on 2011 July 21. At perigee on July 7, the moon will be 369,570 kilometers away, and at apogee on July 21, it'll be 404,355 kilometers distant.
Do Perigee and Apogee Distances Remain Constant?
I was onced asked if the perigee and apogee distances stay the same from month to month. Hardly! In fact, the lunar perigee falling on 2011 July 7 will give us the farthest perigee of the year. The closest perigee of the year came to pass on 2011 March 19, at which time the Moon was only 356,575 kilometers away. That is in contrast to 369,570 kilometers for the year's farthest perigee on 2011 July 7, nearly 13,000 kilometers farther than the year's closest perigee.
Closest and Farthest Apogees
The apogee distances also vary throughout the year. In 2011, the year's closest apogee took place on 2011 June 24 (404,271 km), some two weeks before the year's farthest perigee on 2011 July 7 (369,570 km). On the other hand, the year's farthest apogee occurred on 2011 April 2 (406,656 km), some two weeks after the year's closest perigee on 2011 March 19 (356, 575 km).
Lunar Perigee and Apogee Calculator
Note that there is a little less than a 2,400-kilometer gap between the year's most distant apogee (406,656 km) and closest apogee (404,271 km). That's in stark contrast to the nearly 13,000-kilometer gap between the year's most distant perigee (369,570 km) and closest perigee (356,575 km).
Perigee/Apogee Distances Related to Moon's Major Phases?
As we investigate things, we find that the year's extreme perigees and apogees take place on or near the same dates as the Moon's major phases. The year's closest perigee closely aligns with the Full Moon, farthest apogee with the New Moon, and the farthest perigee/closest apogee with the Quarter Moons. The table below helps to illustrate.
2011 Extreme Perigee/Apogees & Moon Phases
|Date of Extreme Perigee/Apogee||Date of Moon Phase|
|Closest Perigee - March 19||Full Moon - March 19|
|Farthest Apogee - April 2||New Moon - April 3|
|Closest Apogee - June 24||Last Quarter - June 23|
|Farthest Perigee - July 7||First Quarter - July 8|
What's The Explanation?
The Moon's perigee and apogee distances change throughout the year because the eccentricity of the Moon's orbit also changes throughout the year. Now for a few simple definitions. The line connecting perigee and apogee defines the Moon's major axis and is also called the line of apsides. The minor axis passes right through the center of the ecllipse, at a right angle to the major axis. See the illustration at top.
When the Moon's major axis points toward the Sun, or is aligned with the Earth-Sun line, the Moon's orbit reaches a maximum eccentricity. At maximum eccentricity, the orbit becomes more elongated, so the perigee point comes closer to Earth while the apogee point drifts farther away.
On the other hand, when the Moon's major axis is perpendicular to the Earth-Sun line, eccentricity reaches a minimum. At minimum eccentricity, the Moon's orbit comes closer to being circular, so perigee swings farther distant whereas apogee comes closer to Earth.
So That Explains the "SuperMoon" of March 19, 2011!
On 2011 March 19, the Moon's major axis pointed at the Sun at nearly the same hour that the Moon turned full. At this time, apogee was on the sunward side of Earth, whereas perigee was on the opposite side. Since the Full Moon - by definition - takes place when the Moon lies most directly opposite the Sun for the month, that means the Full Moon of 2011 March 19 almost exactly coincided with perigee, giving us both the closest Full Moon and the closest perigee for all of 2011.
Fourteen lunar months after the 2011 March 19 Full Moon, the Moon's major axis will again point at the Sun, with apogee again on the sunward side and perigee on the opposite side. A lunar (or synodic) month is the period of time between successive Full Moons (New Moons, First Quarter Moons, or Last Quarter Moons), a mean period of 29.5306 days. So 14 lunar months (some 413 days) after the 2011 March 19 Full Moon, the closest Full Moon and the closest perigee of the year will recur on 2012 May 6.
Seven Lunar Months Later?
Seven lunar months (or about 206 days) after the 2011 March 19 Full Moon, the Moon's major axis will again point at the Sun, though perigee will be on the sunward side of Earth while apogee will be on the opposite side. The Moon's major axis will be reversed, or flipped 180o relative to the Sun, to usher in the most distant Full Moon of the year on 2011 October 12. Fourteen lunar months thereafter, it'll be the farthest Full Moon all over again on 2012 November 28.
Moon's Major Axis Perpendicular to Earth-Sun Line
The Moon's major axis aligns with the Earth-Sun line every 206 or so days. Midway between these alignments, however, the Moon's major axis makes a right angle to the Earth-Sun line. When perpendicular to the Earth-Sun line, the eccentricity of the Moon's orbit shrinks, so perigee flies maximally outward while apogee falls maximally inward.
Because the Quarter Moons reside at a right angles to the Earth-Sun line, it should come as little surprise that the farthest perigees and closest apogees come in close proximity with the Quarter Moons!
copyright 2011 by Bruce McClure
Lunar Perigee and Apogee Calculator
Moon Phase Almanac
June 2011 Feature * August 2011 Feature