The length of the new crescent Moon
Abstract
Danjon noticed that the length (cusp to cusp) of the new crescent moon was less than 180 degrees and suggested that the cause of the shortening is the shadows of the lunar mountains. McNally, however, attributed the crescent shortening to atmospheric seeing, while Schaefer suggests that length shortening is due to sharp falling off of the brightness towards the cusps.
We attribute length shortening to the Blackwell contrast threshold; we consider the thin crescent as a group of discs of varying angular size, and each has its equivalent Blackwell disc, the largest being at the centre of the crescent. The discs become smaller in the direction of the cusps, therefore the Blackwell thresholds become higher. According to this model, if we know the apparent diameter of the Moon and the width of the crescent, we can calculate the approximate visible length of the crescent.- Publication:
-
The Observatory
- Pub Date:
- August 2005
- Bibcode:
- 2005Obs...125..227S