Rarely Seen Event: Opposing Thin Crescent Moons on Consecutive Days
-- Robert C. Victor, (formerly of) Abrams Planetarium, Michigan State University
Author Fred Schaaf describes a very thin crescent Moon in his book “The 50 Best Sights in Astronomy and How to See Them” (published by John Wiley & Sons, 2007). Recounting an early sighting of a very young Moon, he wrote that the Moon was “thin as a single snippet of pale gold hair falling, falling gently through dusk to the repose of the low forest horizon” and that it was “a slight but dreaming and luminous simile scarcely touching the face of that twilight sky, a face whose tender tones and shades were the only things which could possibly be delicate enough to hold the moon.” He wrote that he also had the feeling that this crescent was “an indelible mark, a slenderest sliver of pure celestial beauty that eternity keeps forever from harm or slightest alteration.” He adds, “Last but not least, I felt that my observing site, the most familiar and mundane places in my neighborhood, was not so ordinary after all if it could hold this wonder in its sky.”
Here in the Coachella Valley of southern California, there will be a rare chance to see not just one, but two unusually thin crescent Moons about 36 hours apart on two consecutive days: at dawn on Friday, February 28, and at dusk on Saturday, March 1. Binoculars will be a great help on both occasions. New Moon, invisible as it passes a few degrees north of the Sun, occurs between the two sightings, at 12:00 a.m. (midnight) at the beginning of March 1.
Illustrations of the old crescent at dawn on Feb. 28, and of the young crescent at dusk on March 1, appear on the February 2014 Sky Calendar, available online at www.pa.msu.edu/abrams/skycalendar/ Also included on the calendar are illustrations of the Moon passing four planets on the mornings leading up to the opposing crescents: Mars on Feb. 19 and 20, Saturn on Feb. 21 and 22, and Venus and Mercury during Feb. 25-28.
Valley residents can try for the old crescent on Friday, Feb. 28, between 5:46 a.m. and 5:56 a.m. PST, when the very thin crescent will appear 11 to 12 degrees south of due east and rising slowly, from 2 degrees to 4 degrees above the horizon. That “viewing window” spans only from 30 minutes to 20 minutes before sunrise, so binoculars are recommended for finding the delicate thin crescent in brightening twilight.
Planets might be helpful for locating the Moon that morning. Venus will then be very prominent some 22 degrees up in the southeast, with fainter Mercury 22 to 23 degrees lower left of Venus. The Moon can be found 12 degrees lower left of Mercury.
The young Moon on Saturday, March 1 will best be seen in the Coachella Valley between 6:03 p.m. and 6:13 p.m. PST, with the hairline thread of a crescent appearing from 4 degrees to 3 degrees south of due west and sinking slowly, from 5 degrees to 3 degrees above the horizon. This viewing window spans from 20 to 30 minutes after sunset, so binoculars are again recommended for spotting the young crescent, which will be even thinner and closer to the Sun than the old crescent of the morning before, even though it will be higher.
Seeing either one of these two crescent Moons will require clear weather, and careful advance planning to make sure the line of sight from observer to Moon would not be obstructed by local topography -- there are plenty of mountains around here which often block the view of events such as these, close to the horizon! From the Coachella Valley different sites may well be required for the two sightings, the first to view the old Moon in the eastern sky before sunrise on Feb. 28, and the second to see the young Moon in the western sky after sunset on Mar. 1. You can increase your chances by checking out your viewing site in advance, by observing the Moon at a date and time when it will have nearly the same position in the sky as the Moon at your target date and time.
Here are directions for accurately predicting the positions of both the old and the young crescent Moons at the middle time of each recommended 10-minute window.
Previewing the location of the Fri. February 28 old Moon (at dawn, 5:51 a.m.)
Observe the waning gibbous Moon on the evening of Mon. Feb. 17 at 8:38:15 p.m. (It will be 91 percent full and easy to observe if not hidden by clouds or local topography). You will find it at azimuth 97.3 degrees and altitude 3 degrees. Then when you look for the Moon on Feb. 28 at 5:51 a.m., you’ll find it 4.3 degrees to the right of the spot where the Moon appeared on Feb. 17 at 8:38:15 p.m.
For an even closer forecast of position, observe the 84 percent Moon the next evening, Tues. Feb. 18, at 9:35:30 p.m., and you’ll find it at azimuth 102.3 degrees, altitude 3 degrees. Then when you look for the Moon on Feb. 28 at 5:51 a.m., you’ll find it just 0.7 degree to the left of your Feb. 18 sighting.
Previewing the location of the March 1 young Moon (at dusk, 6:08 p.m.)
Observe the 98 percent waning gibbous Moon on the morning of Sun. Feb. 16 at 6:46:29 a.m., in the daytime not long after sunrise. Not long before moonset, it will be at azimuth 269.7 degrees, altitude 4 degrees. Then, to find the young Moon on March 1 at 6:08 p.m., look just 3.3 degrees to the left of where you saw the Moon on Feb. 16.
An even better predictor would be the 94 percent Moon on the morning of Tues. Feb. 17 at 7:18:40 a.m. You’ll find it then at azimuth 264.7 degrees, altitude 4 degrees. Then, to find the young Moon on March 1, look just 1.7 degrees to the right of where you spotted the Moon on Feb. 17.
What to look for when you observe a thin crescent Moon
Can you see any illumination of the Moon’s dark side? It’s called earthshine for a good reason: It is the Moon being illuminated by sunlight reflected by the Earth. Usually earthshine is not seen when the lunar crescent is very close to the Sun, because the Moon is immersed in the foreground bright twilight sky. For your best chance to detect earthshine on a very thin crescent, look soon after moonrise, before the sky gets too bright, or not long before moonset, when the sky has darkened sufficiently. But if the Moon is very low, then Earth’s atmosphere will dim the light.
When you observe a very thin lunar crescent, note how much of the Moon’s circumference you can see. It will help to visualize the Moon’s disk as a clock face. When the crescent isn’t too close to the Sun, an arc of 180 degrees is typical. But the arc of a crescent Moon very close to the Sun is noticeably shorter.
For example, on the morning of Feb. 28, if you perceive the Moon’s circumference illuminated from 4 o’clock to 10 o’clock, then the sunlit arc would extend halfway around the clock face, and its length would be 180 degrees. But if you observe the sunlit arc only from 4 o’clock to 9 o’clock, then its length would be only 150 degrees.
Are there any breaks in the crescent, or any thin spots where only a thread of light makes the crescent continuous?
Note the times of your first and last observation of the crescent. Calculate the time interval between your sightings and New Moon, which occurs at 12:00 a.m. at the beginning of March 1.
Alexander Seidler and I spotted opposing crescent Moons on consecutive days, on December 31, 2013 in morning twilight (an easy sighting for binoculars and unaided eye within 21 hours before New, and on January 1, 2014 very early in evening twilight (a very difficult sighting, within 14 hours after New). Alex had not previously observed a crescent Moon within 24 hours of New, and I had never before spotted opposing crescents on consecutive days. (See reports of our sightings.)
From the Coachella Valley, the opposing pair of Feb. 28-Mar. 1 will be more balanced, with both crescents just over 18 hours away from New. The morning crescent, though farther from the Sun, will be less favorable for viewing, rising only 42 min. before the Sun, compared to the evening crescent setting 48 minutes after the Sun. The times of observation recommended in this article are 20-30 minutes before sunrise Feb. 28, and 20-30 minutes after sunset Mar. 1.
Here are reports of our sightings of opposing crescent Moons of Dec. 31-Jan. 1:
Sighting of Opposing Crescent Moons on Dec. 31, 2013-Jan. 1, 2014
Robert C. Victor
On the very clear morning of Tuesday, Dec. 31, two of us, Alexander Seidler and myself, drove to a high spot in a residential area in the northwest corner of Palm Springs, CA. From that site, we had an excellent view of the Coachella Valley below, and spotted the old crescent Moon soon after its rising in ESE shortly before 6:00 a.m. Pacific Standard Time. We saw it first with 8x42 binoculars, then immediately with unaided eye, and then enjoyed the view through an Orion SkyQuest XT-4.5 Dobsonian Reflector at 45x. Earthshine was visible, even with unaided eye. Alex was able to hold the Moon in view with unaided eye until 6:34 a.m., when it was 20 hours and 40 minutes before New. We remained at the site until sunrise and observed a spectacular green flash.
That day, I examined some topographical maps to help me select a different site to provide us with an unobstructed view of the young Moon on the evening of Jan. 1. I picked a site in a residential area on the upper northern part of Desert Hot Springs with a clear view toward Banning Pass in the WSW. I visited the site on the evening of Dec. 31 to make timed observations of Venus. Timing the setting of Venus from that site that evening helped me predict, within narrow limits, where the very thin young Moon would appear at earlier stages of twilight on Jan. 1. We returned to the Desert Hot Springs site on Jan. 1, equipped with two pairs of 8x42 binoculars, a pair of 15x70’s, and two Orion SkyQuest 4.5-inch Dobsonian reflectors, one at 36x and the other at 45x. As we looked through some thin streaks of cirrus cloud, Alex was the first to spot the extremely thin crescent, through the 4.5-inch at 36x, at 5:11 p.m. PST, when its age was 13 hours 57 minutes. Another observer in our party of four, Andrew Smith, spotted it through the same scope within a few minutes, and I finally caught a brief glimpse using the other telescope at 45x. A fourth member of our evening party, Misti Rausch, never did see the Moon that evening. It may have been because she lost her eyeglasses and had difficulty adjusting the telescope to the appropriate focus. When I tried to refocus my telescope with my eyeglasses on immediately after I saw the crescent to enable others to have a look, I was unable to see the Moon and never did recover it. None of us observed the Moon with the binoculars or with unaided eye that evening.
The time interval between Alex's last observation of the waning Moon on Dec. 31 and his first observation of the waxing Moon on Jan. 1 was 34 hours and 37 minutes. As far as I know, this may be a new record for the shortest time interval between sightings of opposing crescents. Stephen J. O’Meara spotted opposing crescents 35.7 hours apart exactly 19 years earlier, on Dec. 31 and Jan. 1, 1994-95 (see his account in the May 1995 issue of Sky and Telescope, page 105), but he was observing from the Big Island of Hawaii, where winter days are longer than in southern California, and so it would not have been possible for him to see the opposing Moons as close together in time as we did.
We are eagerly awaiting our next opportunity to see opposing Moons on consecutive days, on Feb. 28 and Mar. 1, 2014. At civil twilight here in the Coachella Valley, the crescent at dawn on Feb. 28 and at dusk on Mar. 1 will both be just over 18 hours from New Moon, which happens to occur at 12:00 a.m. PST on March 1.
Sighting of Opposing Crescent Moons, by Alex Seidler
On the evening of December 30th, 2013, my good friend Robert Victor called me up to remind me that the following morning would be an excellent opportunity to catch a glimpse of a very old and very thin moon. Bob and I met bright and early the morning of December 31st and promptly drove to a predetermined vantage point in Palm Springs, California. We arrived not long before 6:00am. Almost immediately we began scanning a patch of the eastern horizon for a moon thinner than I had ever seen. Bob first spotted it and gave me concise directions to find it myself. When I first laid eyes the moon that morning it was sitting right on the horizon like a ball on the distant ground. After seeing the beautifully thin crescent for the first time, I couldn't look away. It was easily visible with the naked eye and I could even see Earthshine on the dark side of the moon. I stood there in awe and excellent company watching the moon rise into a brightening sky for over half an hour. The last glimpse I had of the moon that morning was at 6:34am. We stayed a little longer to watch the green flash occur at the beginning of sunrise, a wonderful bonus and another first for me. 34 hours and 20 minutes later we were at the top of a hill in the Desert Hot Springs over looking west into a narrow pass between the San Jacinto and San Gorgonio mountains. Two friends joined our party for this viewing, Andrew Smith and Misti Rausch. Bob had two 4.5” Dobsonian reflector telescopes pointing west, and we all had a pair to binoculars hanging from our necks. Bob knew exactly when and where to look for a moon less than 14 hours young. He predicted it would first become visible around 5:11pm. The view of the western horizon was littered with thin clouds which appeared to be stacked four layers high. At first I thought the clouds would diminish our chances of seeing the moon, but that wasn’t going to stop us from trying. While panning ever-so-slowly with the telescope I saw Luna pop out of a moving cloud at 5:11pm a mere 34 hours and 37 minutes after our last sighting of the old Moon. At first sight it looked like part of the cloud but it quickly became evident that the cloud was moving away from this gorgeous silver sliver. The ultra-thin moon I had seen the day before was suddenly the second thinnest moon I have ever seen. The moon of December 31st was a smile in the east and the young moon of New Years day was merely a smirk in the west. It was so thin and short that I could hardly extrapolate the moon's diameter. Bob was also able to find the moon shortly thereafter. The young moon sighting didn’t last long; I was only able to see it twice, once when I first spotted it and again after Andrew had taken a look. Bob told us that any effort to see the moon past 5:21pm would have been in vain. Again I was treated to a bonus after the moon sighting when we watch the crescent Venus setting.