2018, October 8-9: Opposing Crescents #astronomy #moon #opposingcrescents

Opposing Crescents: A thin waning crescent in the morning, followed by a waxing crescent evening of the next day.


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:

The October pair is worth a try, because the timing of the New Moon for U.S. is good, and the Moon passes well north of the Sun at New. Where will you be on Oct. 8 and 9? If at your hometown, then at morning civil twilight on Oct. 8 at 06:28 CDT, the Moon will be at azimuth 91.1 deg, alt. 3.2 deg (corrected for refraction), and it will have a topocentric elongation of 9.1 degrees and be 16 hrs 19 min before New. You can preview the Moon’s location of that time by observing the Sun from the same site on Sept. 20 at 6:57 a.m.  You can start looking for the Sun earlier that morning, and where/whenever you see the Sun just above the eastern horizon relative to your horizon landscape on Sept. 20, the Moon on Oct. 8 will appear in very nearly the same place (azimuth and altitude) 29 minutes earlier. For example, if you see the Sun at 6:57 a.m. CDT on Sept. 20, then the Moon will appear in almost exactly the same place at 6:28 a.m. on Oct. 8. I used Xephem software to calculate all that.
From your location, the Moon at evening civil twilight on Tuesday, October 9, at 6:49 p.m. CDT, will be at azimuth 259.2 deg., altitude 2.5 deg., with a topocentric elongation of 11.2 degrees and an age of 20 hr 02 min. Just by an unusual coincidence, the Moon’s declination at the time it can be viewed that evening is almost precisely the same as the Sun’s. So you can preview the Moon’s location simply by noting the Sun’s location 45 minutes earlier. For example, note the Sun’s location at 6:04 p.m. (17 minutes before sunset), and the Moon will be there 45 minutes later, at 6:49 p.m.

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

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