Opposing crescent moons are visible across eastern North America on the morning of August 18 and evening of August 19.
by Jeffrey L. Hunt
Opposing crescent moons are two observations of the moon on consecutive days around the New moon. The first step is observing the razor-thin waning crescent on the morning of August 18. The New moon follows later in the day. On the evening of August 19, the second step is observing the moon’s thin waxing crescent that appears above the western horizon after sunset.
This is a challenge that involves a little planning, exceptionally clear weather, and unobstructed horizons. Observers who try this often change locations to see each crescent.
Viewing this set of opposing crescents is region-specific. The moons are visible for eastern North America including southern Mexico. (They might be visible from the Denver, Colorado region. For the morning observation, the moon is less than 2° above the horizon for the morning observation in Utah.) It might be possible for those in northern South America to see this is well.
The planning: An astronomy program like Stellarium (stellarium.org) is great for locating the positions of stars, moon, and planets. The program’s features include the ability to set your location (longitude, latitude) and time, among many others. To locate the moon on August 18, locate the time of sunrise and set the program’s time for 30 minutes before sunrise. Where is the moon in height above the horizon and its azimuth coordinate? Find out the same for 30 minutes after sunset on August 19.
Locate observing spots with clear horizons in the east-northeast and west-northwest. Record the times of the last sighting of the morning crescent on August 18 and the first sighting of the evening crescent on August 19. The shortest interval between observations of the two crescents is 34 hours, 37 minutes by Robert C. Victor and Alexander Seidler on December 31, 2013 – January 1, 2014 from Palm Springs, CA. The current opportunity predicts an interval of 38 hours, 41 minutes, This is certainly not the shortest recorded span because of longer daylight intervals during August, but worth the challenge attempting unusual observations of the lunar crescents.
Here’s what to look for in the Chicago, Illinois, area:
August 18, 30 minutes before sunrise (5:34 a.m. CDT). The crescent moon (1% illuminated) is nearly 3.0° in altitude in the east-northeast (azimuth 67°).
August 19, 30 minutes after sunset (8:15 p.m. CDT. The waxing crescent moon (1% illuminated) is 3.1° in altitude in the west-northwest (azimuth 283°).
The conditions for observing these crescents occurs about every 22 months, so that the moon is north of the plane of the solar system (in the northern hemisphere); the lunar orb is near perigee (August 20); and the moon is appropriately spaced from the sun at sunrise and sunset.
So you know where and when to look. Show up at the locations an hour before sunrise for the morning view and at sunset for the evening attempt. Report your observations in the comments.
August 14, 2021: This evening the waxing moon is near Zubenelgenubi, the southern claw, that is a stellar double. Use a binocular to see both stars that are in a gravitation dance.
August 13, 2021: This evening the crescent moon appears between Evening Star Venus and Spica as the lunar slice dances eastward. Jupiter and Saturn are in the southeastern sky.
August 12, 2021: This evening the crescent moon appears between Venus and Spica as the lunar slice dances eastward.
August 11, 2021: The waxing crescent moon is to the upper left of Evening Star Venus this evening in the western sky.
August 10, 2021: The crescent moon is near Venus in the western sky after sunset.
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