Tag Archives: waning crescent

Making Friends with the Night Sky: The Waning Crescent

The waning crescent appears on the left in the Northern hemisphere, and the right in the Southern hemisphere

The waning crescent appears on the left in the Northern hemisphere, and the on right in the Southern hemisphere

Maui astronomer Harriet Witt describes the “old” waning crescent Moon.

Listen here [3:30m]:

Download here [3.2 Mb]: ftp://space.mit.edu/pub/ajb/radiopio/astrofacts_090813_mfns-wanningcrescent.mp3

What’s the facts:

The waning crescent, or “old moon”, can be seen shortly before dawn, a thin sliver that rises ahead of the Sun.  You have a short period to catch it; after the Sun rises, the thin crescent is hard to see in the bright glare of day.  The waning crescent occurs toward the end of the new Moon to new Moon cycle, a siderial period of  27 1/2 days if you measure the Moon’s position relative to the stars, or a synodic period of 29 1/2 days if you measure relative to Sun.  The difference is due to the Earth’s motion around the Sun.  During a “moonth” the Earth has traveled about 1/13th of its yearly orbit (at a rate of 1.3 million miles per day).  So from our point of view, the Sun has moved to a different part of the sky relative to the stars – by about 28 degrees – over the lunar cycle.  Every month brings a new perspective!

Original air date 13 August 2009.