The waxing crescent is the first phase in the new Moon to new Moon lunar cycle. You can catch it by looking toward the western sky early in the evening; there you will see a bowl-shaped sliver pointing toward the setting Sun, following it down to the horizon. The opposite side is darker but not completely dark – it is faintly lit up by sunlight reflected from the Earth’s surface, called Earthshine. The origin of Earthshine was first figured out by Leonardo Da Vinci in the 1500s; scientists now use Earthshine to track global cloud coverage and variations in the Earth’s climate. The waxing and waning crescent phases are the best time to observe Earthshine, so enjoy our spotlight on the Moon!
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!