Telescope Reviews: Young Crescent Moon from Tempe

For those who may be interested in other observations of this crescent, a detailed report from Tucson by veteran crescent watchers Jim Stamm and John Polacheck may be read on the Islamic Crescents' Observation Project page at:

http://www.icoproject.org/icop/rat28.html

Stamm and Polacheck, observing with an 8-inch SCT, report first seeing a 5 degree long segment of the crescent at 6:58 pm local time (01:58 UT on April 18, 2007), when the percent illumination would have been 0.54 percent (elongation from Sun 8.4 degrees). After a short while, they say, they had the impression of this growing "instantaneously" into an 80 degree long arc, with the brighter sections becoming connected over the next 35-40 minutes.

Since the theoretical width of the crescent changed by no more than 1 arc-second during this interval, the rapidly changing appearances should probably be taken as an indication of the difficulties (at the start) of perceiving a faint target in a sky of nearly equal brightness, rather than as real changes in the lighting pattern on the Moon. Stamm is the same careful observer who reported that a record crescent he observed at an elongation of 7.7 degrees (in January 1996, also from Tucson Sky and Telescope Dec. 96, pp. 104-106)) appeared unbroken over its full 45 degree length, even though he had seen fatter crescents broken.

Stamm and Polacheck do not mention if, when first seen, the brightest parts of the crescent had a point-like appearance resembling the well-documented beads of light seen beyond the cusps of the terminator in later phases. However, given that the theoretical width of crescent was about 11 arc seconds (0.0054 of the lunar diameter) at its thickest point, the initial 5-degree long arc (0.044 of the lunar diameter) must have been at least 8 times as long as it was wide, probably more.

The ICOP website describes many additional sightings of this crescent, starting in Iran. It must, of course, have been seen by many observers at longitudes between those of Tucson and Esfahan, but we do not have their descriptions or photos.

On my monitor, the most bead-like of these photos is the one from Kuwait taken by Ali Demashq at an elongation of about 16 degrees; but it does not appear bead-like at all when viewed at its full size.

As for myself, from 117.89W/33.64N, I looked for, but was unable to see, the crescent (at 8.7 degrees elongation) using a 4-inch telescope in a less than favorable sky. However this is hardly surprising because, due to coastal clouds, it was possible to track the expected position of the Moon for only about 15 minutes after the Sun was blocked by nearby homes and trees, taking it barely to the theoretical time of sunset on a level horizon (Stamm and Polacheck spotted the crescent 7 minutes after the actual and 2 minutes before the theoretical sunset time). Clouds similarly prevented me from seeing the rising of the old crescent before sunrise the previous morning.

A storm system cleared out the clouds by the next evening, on which the crescent was easily visible; although, of course, by then, at an elongation of 23 degrees (4% illuminated) the central part of the limb, as it probably is in Ali's photo, was perfectly continuous. The final thin extensions near the cusps, at this phase, looked much more arc-like than point-like to me.

Perhaps I am misunderstanding what this group means by "Saber's Beads," but it remains unclear to me that there actually exists a distinct lunar phase in which the whole length of the visible crescent (as seen with adequate magnification under good conditions), or even a substantial part of it, breaks up into points resembling the dots of light commonly seen at the cusps.

-- Jim Mosher