Tonight – October 24, 2017 – the moon is still near Saturn, and it’s this month’s apogee moon, that is, a moon at its most distant from Earth for this month.
Depending on where you live worldwide, the waxing crescent moon swings out to apogee on October 24 or 25. The exact time of this month’s apogee is October 25 at 2:25 UTC. At U.S. time zones, that means the moon reaches apogee on October 24 at 10:25 p.m. EDT, 9:25 p.m. CDT, 8:25 p.m. MDT and 7:25 p.m. PDT.
At this month’s lunar apogee, the centers of the moon and Earth are 251,751 miles (405,154 km) apart. Contrast this apogee distance to the moon’s distance at its next perigee – its closest point to Earth – on November 6, 2017, when the centers of moon and Earth will come to within 224,587 miles (361,438 km) of one another. That’s a difference of 27,164 miles (43,716 km) in less than two-weeks time.
October 25, 2017: Moon at apogee (251,751 miles or 405,154 km)
November 6, 2017: Moon at perigee (224,587 miles or 361,438 km)
Change in distance: 27,164 miles or 43,716 km
The reason for the moon’s varying distance is that the moon’s orbit around Earth isn’t perfectly circular. Rather, the moon’s orbit is an ellipse (“squashed” circle) with our planet Earth residing at one of the two foci of the ellipse.
However, the eccentricity of the moon’s orbit is not static. When the moon’s orbital eccentricity is closer to zero (circular), the perigee distance is farther than the mean perigee distance of 225,804 miles or 363,396 km. Also, the apogee distance comes closer to Earth than the mean apogee distance of 251,969 miles or 405,504 km.
On the other hand, when the moon’s orbital eccentricity increases to a maximum (most “flattened”), it’s the direct opposite. The perigee comes closer to Earth than the mean perigee; and the apogee resides farther away than mean apogee.
The moon’s mean eccentricity equals 0.055, but its actual value varies from 0.026 (least eccentric) to 0.077 (most eccentric). For the next couple of months, the moon’s orbital eccentricity will be increasing, resulting in more distant apogees and closer perigees:
November 21, 2017: Moon at apogee (252,359 miles or 406,132 km)
December 4, 2017: Moon at perigee (222,135 miles or 357,492 km)
Change in distance: 30,224 miles or 48,640 km
Then the greatest extreme is reached during the following lunar apogee and lunar perigee:
December 19, 2017: Moon at apogee (252,651 miles or 406,603 km)
January 1, 2018: Moon at perigee (221,559 miles or 356,565 km)
Change in distance: 31,092 miles or 50,038 km
The lunar apogee on December 19, 2017, will present the farthest apogee until March 24, 2020 (252,707 miles or 406,692 km); and the lunar perigee on January 1, 2018, will feature the closest perigee until November 25, 2034 (221,487 miles or 356,448 km).
Bottom line: Watch the moon this evening, on October 24, as the evening crescent pairs up with the planet Saturn on the stellar sphere. Depending on where you live worldwide, the moon sweeps to its most distant point from Earth for the month on October 24 or 25, 2017.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.