Slim Crescent Moon Hints at Tomorrows Conjunction

The attempt to see the earliest hint of a crescent Moon is a challenge some sky-watchers can't resist.  After all, it is the first chance to see the Moon after it has been lost in the sun's glare for the preceding week. 

The Moon takes about 1 month to go around the Earth.  Actually  the Moon orbits the Earth once every 27 days.  But since the Earth-Moon system orbit the sun, it takes to go from one phase around to that same phase (called the Synodic period) takes 29.5 days.  As the Moon orbits the Earth, the amount of the lit side (one half of the Moon is always lit) slowly increases.  A crescent Moon is when the Moon has moved from an angle of 1 degree (i.e. the angle from the sun) to 89 degrees.  At 90 degrees from the sun the Moon is at First Quarter.  Yet the range from 1 degree to 89 degrees shows very subtle variations in how the Moon looks in the sky.  At an angle of 3 or 4 degrees, the Moon appears as a very thin sliver, while at 45 degrees it is still a crescent but much fatter.  

Tonight may be your first chance to catch a glimpse of the thinnest crescent moon.  Look for it close to the horizon in a line from Saturn and through Venus.  You should be able to see the Moon in the fading sunset.

The thin crescent Moon in the sunset sky.  Created with Stellarium