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Today, we can time travel back to 1969 as the exact same lunar phase appears in the July 20 sky.
An exact match?! Yes, if you're not counting seconds: The Moon's phases repeat on the same calendar date every 19 years, called a Metonic cycle. Although known by the Babylonians, Greek astronomer Meton also realized the importance of this period around 430 b.c.e., registering it in Western classical texts. Now, we are precisely three Metonic cycles (a total of 57 years) after Apollo 11: With 29.530589 days from Full Moon to Full Moon, times 235 months, times 3 cycles, we get 20,819.065 days or 57.038356 years since July 20, 1969.
Other features of the lunar orbit (e.g., node crossing, perigee) don't quite line up, meaning that eclipses won't always repeat on a Metonic cycle. And some 19-year blocks have an extra leap day compared to others, which can throw the date off by one day. But if you remember when there was a Full Moon on your birthday, without blinking you know the next one is 19 years after that, and so on.
Check out Tranquillity Base at the eyepiece, located near the southwestern rim of the Sea of Tranquillity, with only 1.6 hours difference in lighting compared to that seen by Eagle on the exact same day of the year.
Related: Astronomy magazine July 2019 issue: Apollo 11 50th anniversary
Sunrise: 5:48 A.M.
Sunset: 8:24 P.M.
Moonrise: 12:52 P.M.
Moonset: 11:44 P.M.
Moon Phase: Waxing crescent (47%)
*The times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon's illumination is given at 10 P.M. local time from the same location.
This story is an excerpt from the July 2026 Sky This Month feature, where you can find upcoming events and star charts for the month’s best observing events.
Alister Ling, who lives in Edmonton, Alberta, is a longtime watcher of the skies.
The post Tonight: See the Moon as it looked the day Apollo 11 landed appeared first on Astronomy Magazine.
