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March 2013: Year's Farthest Lunar Perigee & Closest Lunar Apogee



The Moon's orbit isn't perfectly circular. It's a somewhat elongated ellipse, so the Moon's distance from Earth varies throughout the month. The closest point to Earth in the Moon's orbit is called perigee and its most distant point is called apogee.

But the eccentricity of the Moon's orbit isn't constant either, so the Moon's distance at apogee and perigee varies throughout the year as well. As a matter of fact, the year's farthest perigee comes on March 5, 2013, and the year's nearest apogee occurs two weeks later, on March 19, 2013.

At the year's most distant perigee on March 5, 2013, the Moon is 369,957 (229,881 miles) kilometers away. In contrast, the year's closest perigee brings the Moon within 356,991 kilometers (221,824 miles) of Earth on June 23, 2013. These distances, by the way, refer to the measurement between the centers of the Moon and Earth.

As for lunar apogee, the year's closest apogee falls on March 19, 2013, at a distance of 404,261 kilometers (251,196 miles). In contrast, the Moon on July 7, 2013, swings farthest from Earth for the year as it reaches an apogee distance of 406,490 kilometers (252,581 miles). Note that the farthest perigee on July 7, 2013, comes two weeks after the nearest perigee on June 23, 2013.


Changing Eccentricity of the Moon's Orbit


The changing eccentricity of the Moon's orbit means varying distances for apogee and perigee throughout the year. Now for some terminology: the line connecting perigee with apogee is called the Moon's major axis or line of apsides.

In the illustration at right, courtesy of Wikimedia Commons, the X1-X2 line depicts the Moon's major axis. The eccentricity of the Moon's orbit is greatest when the major axis (or line of apsides) points toward the Sun.

Yet when the Moon's major axis is perpendicular to the Sun-Earth line, then the Moon's eccentricity shrinks to a minimum. That means the Moon's orbit is closer to being a circle, causing lunar perigee to move away from Earth and lunar apogee to move inward. This is what happens in March 2013.

However, some 103 days after the Moon's major axis is perpendicular to the Earth-Sun line in March 2013, it it shifts a full 90o relative to the Sun. At this juncture, the Moon's major axis aligns with the Earth-Sun line. This causes the Moon's orbit to increase in eccentricity, bringing perigee inward yet pushing apogee outward. This explains the extreme perigee and extreme apogee on June 23 and July 7, 2013.

Then in another 103 days or so after that, the Moon's major axis finds itself perpendicular to the Earth-Sun line again. This reduces the the Moon's orbital eccentricity once again, ushering in the year's second-closest lunar apogee on September 27, 2013, and two weeks later, the second-farthest lunar perigee on October 10, 2013.


copyright 2013 by Bruce McClure


Moon at Perigee and Apogee: 2001-2100


February 2013 Feature * April 2013 Feature