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Farthest lunar perigee on September 13

Moon image via US Naval Observatory

The moon sweeps to perigee – its closest point to Earth in its monthly orbit – on September 13, 2017. Yet, at a distance of 369,860 km, this particular perigee counts as the most distant of this year’s 13 perigees. That’s in contrast to the year’s closest perigee of 357,207 km on May 26, 2017.

The moon swings to perigee and reaches its last quarter phase on September 13, 2017. One fortnight (two weeks) previous to the year’s farthest perigee, the moon had swung out to its closest apogee – farthest point from Earth in its monthly orbit – on August 30. It’s no accident that the year’s farthest perigee (close moon) and year’s closest apogee (far moon) happen in close vicinity of the quarter moons.

Lunar perigee and apogee calculator

Like everything else in nature, the moon’s orbit is always in flux. Its shape, and its orientation relative to the Earth and sun, change all the time. The complexities of the lunar orbit all combine to bring about today’s most distant lunar perigee of the year at 16:04 UTC (11:04 a.m. CDT; translate to your time zone).

If you’re game, we’ll share a secret with you. We’ll tell you why a quarter moon at perigee is farther than the mean perigee of 363,396 km, and why a quarter moon at apogee is closer than the mean apogee distance of 405,504 km. We’ll also explain why a full moon or new moon at perigee is closer than the mean perigee, yet why a full moon or new moon at apogee is farther than the mean apogee. It all has to do with the varying eccentricity of the moon’s orbit.

The moon’s eccentric orbit

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.

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.

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).

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 (B in below diagram), 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 2017, the moon’s minimal eccentricity ushers in the closest apogee of the year on August 30 (404,308 km), and the year’s farthest perigee one fortnight (two weeks) later, on September 13 (369,860 km). (See B in above diagram.)

Read more: Close and far moons in 2017

Some 103 days after the minor axis points sunward (B in above diagram), it’s then the moon’s major axis that points in the sun’s direction (C in above diagram). 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-far from Earth at lunar apogee – yet extra-close to Earth at lunar perigee.

Therefore, the new moon of December 18, 2017, will closely align with the year’s most distant apogee (406,603 km). One fortnight (two weeks) after the farthest perigee in 2017, the full moon on January 2, 2018, will present the closest perigee of 2018 (356,565 km). (See C in the above diagram.)

Also, some 103 days before the minor axis points sunward (B in above diagram), the moon’s major axis also points in the sun’s direction (A in above diagram). Again, this causes the eccentricity of the moon’s orbit to increase, to bring about a closer perigee yet farther apogee.

So, in 2017, the new supermoon of May 25, 2017 (A in above diagram) closely coincided with the year’s nearest perigee (357,207 km). Then 7 lunar months (some 206 days) thereafter (C in above diagram), the new moon of December 18, 2017, will closely align with the year’s most distant apogee (406,603 km).

And 7 lunar months (206 days) after the year’s farthest full moon (micro moon) on June 9, 2017, it’ll be the closest full moon of 2018 on January 2, 2018.

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.

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

Resources:

Lunar perigee and apogee calculator

Moon at perigee and apogee: 2001 to 2100

Phases of the moon: 2001 to 2100

Bottom line: In 2017, the moon swings to its most distant perigee on September 13, 2017.

Bruce McClure

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