Opposing Crescents (October 8-October 9): The morning of October 8 and the evening of October 9 present an opportunity to see very thin crescent moons, a waning crescent followed by a waxing crescent – opposing crescents. The two windows to see the two crescents are very narrow, weather dependent, and location critical. The western wild fires and dust from the fall harvest could influence whether the crescents are visible as well. Observing the crescents may require you to change locations. The shortest reported interval for viewing opposing crescents is 34.6 hours by Robert C. Victor, former staff astronomer at Abrams Planetarium, Michigan State University. This 2018 interval is about 36.3 hours. Not a record, but certainly a test of observing skills. Shortest intervals occur when the New moon occurs near perigee, hence moving its fastest between the two visible crescents. Because the moon’s crescent is less than 1% illuminated, knowing the moon’s altitude and azimuth are essential. When I asked Mr. Victor about the observing prospects, he made this analysis for my location:
So even for a very experienced sky watcher, some planning is necessary.
Using a planetarium program like Stellarium (stellarium.org) to calculate the crescents’ positions is helpful. Using Google Sky during your observing may help. Use the USNO’s online calculator (http://aa.usno.navy.mil/data/docs/RS_OneDay.php), use Form B, to calculate the moon’s rising and setting times, and the moon’s rising and setting azimuths for your specific latitude. On October 8, look east about 45 minutes before sunrise. The moon is 5° up. On the evening of the next day, about 20 minutes after sunset, look west. Mercury is 5.5° to the lower left of the very thin moon. To prepare for this, follow the moon in the morning sky as outlined in our daily notes that follow. As the moon approaches Regulus, this should give you some indication whether you have a clear horizon for the morning observation. On the evenings of October 7 and October 8, look for Mercury about 20 minutes after sunset. This, again, indicates whether you have a clear horizon for the evening crescent. For the morning observation, note the last time you see the crescent. For the second observation record the time you first see the emerging moon. If you view either crescent, please enter your observations here: https://tinyurl.com/opposing-crescents-2018