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07/26/2012 – Ephemeris – Why is a half illuminated moon called a quarter moon?
Ephemeris for Thursday, July 26th. The sun rises at 6:22. It’ll be up for 14 hours and 51 minutes, setting at 9:14. The moon, at first quarter today, will set at 12:56 tomorrow morning.
The moon was at the exact first quarter a few hours ago. By tonight the sunlit side of the moon will be a bit fuller, but might be at the half moon phase to the casual eye. So why do we call the half illuminated moon a quarter moon? The moon is a quarter the way around the earth from new moon. When it goes half way around the earth from new, we ave a fully illuminated moon from our vantage point, Another quarter the way around we have a half illuminated moon from our vantage point, but the moon is at third quarter position or last quarter. Another quarter way around and the moon is new again. The ancients unlike astronomers today called the first sighting of the moon’s crescent in the west after sunset new moon.
Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
Lunar phase diagram. From Wikimedia Commons.
The arrows in the diagram is light from the sun far off to the distant right.
Bob Moler's Ephemeris Blog 