Sky ArchiveTonight

Closest far-moon of 2017 on August 30

Tonight – August 30, 2017 – the moon is quite close to the planet Saturn on the sky’s dome, and it’s also at or near apogee – the moon’s farthest point from Earth in its monthly orbit. The apogee on August 30, 2017, has the distinction of being the closest apogee of 2017, making tonight’s moon the closest far-moon of 2017.

Wait … what? Closest far-moon. Yes, on August 30, 2017, at 11:25 UTC, the moon reaches apogee – its most distant point in its monthly orbit. But this apogee is the closest of the 13 lunar apogees that occur in 2017.

At this month’s closest apogee, the moon lies 404,308 km distant. Contrast this with the farthest apogee of the year on December 19, 2017, when the moon will lie 406,603 km away. The mean apogee distance is 405,504 km.

The moon is also near first quarter phase now. First quarter moon happened the day before this month’s lunar apogee, on August 29. It’s no coincidence that the first quarter moon – and closest far-moon – happen in close vicinity of one another.

In any year, it’s either the first quarter moon or last quarter moon that closely coincides with the year’s closest apogee. Next year, in 2018, the closest apogee of the year (404,144 km) will happen when the last quarter moon and lunar apogee fall within a few hours of each other on April 8, 2018.

Often – but not always – the year’s closest apogee will recur in a period of 14 lunar months (14 successive returns to the same phase). That’s because 14 lunar months is nearly commensurate to 15 returns to apogee.

14 lunar months x 29.53059 days = 413.428 days
15 returns to apogee x 27.55455 days = 413.318 days

This 413-day period is approximately equal to one year, month month and 18 days. And, guess what? Fourteen lunar months after 2018’s closest apogee on April 8, 2018 (404,144 km), the following year’s closest apogee will fall on May 26, 2019 (404,138 km). Then 14 lunar months after the year’s closest apogee on May 26, 2019 (404,138 km), it’ll be the year’s closest apogee on July 12, 2020 (404,199 km).

Today – August 30, 2017 – the lunar apogee distance of 404,308 km presents the closest apogee of the year.

Lunar perigee and apogee calculator

Moon at perigee and apogee: 2001 to 2100

Phases of the moon: 2001 to 2100

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Bottom line: The closest far-moon of 2017 happens on August 30. The year’s closest far-moon often takes place in the month that the quarter moon and apogee most closely align.

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August 30, 2017
Sky Archive

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Bruce McClure

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