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Farthest lunar perigee on July 27

The moon sweeps to perigee – its closest point to Earth in its monthly orbit – on July 27, 2016. It reaches this point only about half a day after it reaches the last quarter phase. Although the moon swings to perigee 13 times in 2016, this particular perigee counts as the most distant for the year. In other words, if there were a total eclipse of the sun today (which there isn’t), the moon would be unable to cover the sun completely as seen from Earth.

Lunar perigee and apogee calculator

Like everything else in nature, the moon’s orbit is always changing. Its shape, and its orientation relative to the Earth and sun, changes all the time. The complexities of the lunar orbit all combine to bring about today’s most distant lunar perigee of the year at 11:25 UTC (6:25 a.m. CDT; translate to your time zone).

The moon’s orbit around Earth, like the Earth’s orbit around the sun, is not a perfect circle. It’s a slightly oblong ellipse. That’s why, every month, the moon reaches a nearest point to Earth at perigee and a farthest point at apogee.

However, the moon’s orbit is not highly eccentric (oblong), but nearly circular, as shown on the illustration below.

The moon's orbit around Earth is not a perfect circle.  But it is very nearly circular, as the above diagram shows.  Diagram by Brian Koberlein.

The moon’s orbit around Earth is not a perfect circle. But it is very nearly circular, as the above diagram shows. Diagram by Brian Koberlein.

The illustrations above and below label perigee (moon’s closest point to Earth) and apogee (moon’s farthest point from Earth). A line drawn from perigee to apogee defines the major axis, or the longest diameter, of the moon’s elliptical orbit. In the parlance of astronomers, the perigee-to-apogee line is called the line of apsides. The center of the line of apsides to either the perigee point or apogee point is called the semi-major axis.

Earth does not lie at the center of the line of apsides. Instead, the Earth is offset from the center of the major axis, or line of apsides, toward the lunar perigee point. To be more precise, the Earth resides at one of the two foci of the ellipse.

Keep in mind, also, that the moon’s major axis (longest diameter of an ellipse) always makes a right angle to the moon’s minor axis (shortest diameter of an ellipse).

Image credit: NASA. The moon's orbit is closer to being a circle than the diagram suggests, but the exaggeration helps to clarify. The moon is closest to Earth in its orbit at perigee and farthest away at apogee.

Image credit: NASA. The moon’s orbit is closer to being a circle than the diagram suggests, but the exaggeration helps to clarify. The moon is closest to Earth in its orbit at perigee and farthest away at apogee.

Varying eccentricity of the moon’s orbit

When the moon’s major axis, or line of apsides, makes a right angle to the sun-Earth line, the moon’s eccentricity decreases to a minimum. In other words, the moon’s orbit is closest to being circular when the moon’s minor axis points toward the sun. Although the moon still swings closest to Earth at perigee and farthest from Earth at apogee, the perigee distance increases and the apogee distance decreases whenever the moon’s eccentricity lessens, or more closely approaches a circle in shape.

Therefore, in 2016, the moon’s minimal eccentricity ushers in the farthest perigee of the year on July 27 (369,662 kilometers or 229,697 miles), and the year’s closest apogee some two weeks later, on August 10 (404,262 kilometers or 251,197 miles).

Read more: Close and far moons in 2016

Top: When the moon's major axis (perigee-apogee line) points sunward, with perigee residing between the Earth and sun, the result is a new moon at perigee. Bottom: Some 206 days later, the moon's major axis again aligns with the Earth and sun, but this time around, perigee is opposite the sun in Earth's sky, giving rise to a full moon at perigee. Image and caption via NOAA.

Top: When the moon’s major axis (perigee-apogee line) points sunward, with perigee residing between the Earth and sun, the result is a new moon at perigee. Bottom: Some 206 days later, the moon’s major axis again aligns with the Earth and sun, but this time around, perigee is opposite the sun in Earth’s sky, giving rise to a full moon at perigee. Image and caption via NOAA.

Some 103 days after the minor axis points sunward, it’s then the moon’s major axis that points in the sun’s direction. When the major axis, or line of apsides, aligns with the sun-Earth line, the eccentricity of the moon’s orbit increases to a maximum, and its orbit becomes maximally oblong. That causes the moon to swing extra-close to Earth at lunar perigee – yet extra-far away from Earth at lunar apogee.

So, in 2016, the full supermoon of November 14, 2016, will present the year’s closest perigee (356,509 km or 221,524 miles); and two weeks previous to the year’s closest perigee, it’ll be the year’s farthest apogee on October 31, 2016 (406,662 km or 252,688 miles).

Want to know more? Eclipses and the moon’s orbit

Bottom line: In 2016, the moon swings to its most distant perigee on July 27, 2016.

Bruce McClure

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