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Author Fred Schaaf describes a very thin crescent moon in his book The 50 Best Sights in Astronomy and How to See Them

Recounting an early sighting of a very young moon, he writes 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 smile 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. … An indelible mark, a slenderest sliver of pure celestial beauty that eternity keeps forever from harm or slightest alteration.”

Here in the Coachella Valley and elsewhere in the U.S., there will be yet another 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, Feb. 28, and at dusk on Saturday, March 1. Binoculars will be a great help on both occasions. The astronomical new moon, invisible as it passes a few degrees north of the sun, occurs almost midway between the two sightings, at 12 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.

Valley residents can try for the old crescent on Friday, Feb. 28, between 5:46 a.m. and 5:56 a.m., when the very thin crescent will appear 11 to 12 degrees south of due east, 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 in the brightening twilight.

Planets might be helpful for locating the moon that morning. Venus will be very prominent, some 22 degrees up in the southeast, with fainter Mercury 22 to 23 degrees to the 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., 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.

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 will not be obstructed by local topography; after all, there are plenty of mountains around here which often block the view of events such as these!

From the Coachella Valley, different sites may well be required for the two sightings. 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 the 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.

The old moon (at dawn, 5:51 a.m., Friday, Feb. 28): Observe the waning gibbous moon on the evening of Monday, 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 (7.3 degrees south of due east) and altitude 3 degrees. 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, Tuesday, Feb. 18, at 9:35:30 p.m., and you’ll find it at azimuth 102.3 degrees (12.3 degrees south of east), and 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 of a degree to the left of your Feb. 18 sighting.

The young moon (at dusk, 6:08 p.m., Saturday, March 1): Observe the 94 percent moon on the morning of Tuesday, Feb. 17, at 7:18:40 a.m. You’ll find it then at azimuth 264.7 degrees (5.3 degrees south of west), 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 just 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 a.m. at the beginning of March 1.

Alexander Seidler and I spotted opposing crescent moons on consecutive days, on Dec. 31, 2013, in morning twilight (an easy sighting for binoculars and unaided eye within 21 hours before new), and on Jan. 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. Read more about our sightings here.

From the Coachella Valley, the opposing pair of Feb. 28 and March 1 will be more balanced, with both crescents just more than 18 hours away from new. The morning crescent, though farther from the sun, will be less favorable for viewing, rising only 42 minutes before the sun, compared to the evening crescent setting 48 minutes after the sun.

Published in Astronomy