Friday, November 16, 2012

The Thin Crescent Moon—A Postscript




On the morning of August 13, 1931, French astronomer André Danjon observed a Moon only 16h 12m before new with a 3-inch refractor. Much to his surprise, the thin crescent appeared to extend only 75° to 80° along the Moon’s limb — considerably less than the expected 180° (halfway around). When Danjon compiled many other observations of this “deficiency” effect, he came to a remarkable conclusion: Whenever the Moon is 7° or less from the Sun, there can be no visible crescent at all!

Danjon believed that mountains and other roughness along the lunar limb must be blocking some of the sunlit surface that would otherwise be seen, thereby clipping off the ends of the crescent. Bradley E. Schaefer (Louisiana State University, Baton Rouge) has modeled the crescent's perceived length by including physiological factors and atmospheric extinction. In any event, Danjon’s 7° limit should actually be revised to 7.5°, according to a 1998 study by Louay J. Fatoohi and his colleagues at the University of Durham.


from “Seeking Thin Crescent Moons”
by Roger W. Sinnott
at Sky & Telescope




Yesterday’s post about the young Moon—Old Ghosts Haunting A Garden Shop Moon—reminded me that I’ve been writing about thin crescent Moons since even before I bought a digital camera. I posted this ink drawing four years ago in The Almost New Moon In Black And White. I still have that little drawing. I just took it out and looked at it. It’s tiny, just an ink sketch on an index card with no color at all, but when I look at it I remember standing in my back yard and looking at the Moon on the horizon, low, just above the houses and trees. I wish I had done some kind of watercolor wash, but I’m glad I did something.

I wanted to include that quote from the Sky & Telescope article because it contains a few really interesting (well, interesting to me) little references.

First of all, this enjoyment of early and late Moons is really a global pursuit. The quote above is about a French astronomer. The article also talks about an Iranian observer who holds the record of seeing the earliest young Moon, with optical aid, of just about twelve hours. And the earliest young Moon seen with the naked eye was an American observer who observed a young Moon just under sixteen hours old.

Secondly, it was interesting to me that it was as recently as 1931 that an astronomer made the remarkable discovery that there exists a threshold for what crescent size will be visible.

And that French astronomer was using only a three inch telescope when he made the observation that started him thinking about a threshold for the visible crescent.

Simple binoculars, or even the combination of binoculars and a modern computer-guided telescope, are all anyone needs to get serious about tracking down very thin crescents. Or even pursuing a record. The article from Sky and Telescope has a list of hints and techniques for tracking down the thinnest crescent possible. This is a great application for the kind of telescope I posted about in Limits Of A Gadget: A Love Story.

But even “plain” naked eye observing is very, very fun and rewarding.

And photography and drawing or painting adds to the fun.

And I find it really thought-provoking in many ways.

There is the straightforward beauty of the Moon against the sky. Sometimes there is Earth-shine within the crescent. And then there is the combination of the Moon set against the landscape. Since the Moon’s orbit changes the Moon’s inclination, the landscape under the Moon always will be a little different from month to month, not to mention from season to season.

And then there is the larger issue of images in general. Photographs can certainly capture something of the magic of the view of a thin crescent, and it’s fun and challenging to get a camera to record the scene. But so can even an almost trivial drawing. This is one of the most interesting topics in art to me—how a seemingly low resolution, sometimes even “clumsy” sketch, can nonetheless capture something, even a tiny bit, of an almost indescribably beautiful scene. In the context of learning to draw, British artist Quentin Blake described the same issue, and I quoted him in my post Jeanne Hébuterne — Art As A Grail.

One final note about observing the thin crescent Moon is that it can be, so to speak, character-building. You get two or three days before the new Moon to look for a thin crescent old Moon. Then you get two or three days after the new Moon to look for a thin crescent young Moon. And now and then clouds are going to get in the way. It’s not unusual, of course, for a cloudy spell to last a week or more. And it can be very frustrating to look up at the sky and know the beautiful Moon is back there, just behind the clouds, but there is absolutely nothing you can do but wait and hope the clouds clear up. It happens. All astronomers, professional and amateurs, know the feeling. You sigh. But such is life. It builds character. Or so they say.

But the rewards are extraordinary. No matter how depressed I might be, and even if I’m feeling kind of ill like I’ve been feeling this week, I find it impossible to look up at the sky and see a shining crescent Moon and not have the beauty of the scene make all earthly troubles seem totally insignificant.

The sky is larger than anything that can happen here.

And the beauty is infinite.


























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