Blue Moon Visible Tonight, August 21, 2013, On US East Coast? Rare Full Moon Yesterday Provides Optimal Lighting For Photographers During ‘Blue Hour’ [PHOTOS]

The blue moon occurred 20 August, 2013, on Tuesday night. But tonight, August 21, another apparent-full moon will be visible for enthusiasts on the East Coast of the United States. The blue moon yesterday was a rare sighting, and best viewed during twilight, providing optimal lighting for photographers hoping to snap a photo of the full moon, though it's not actually blue.

According to Yahoo! News, the moon looks full one day before and one day after the full moon, though technically there is only one moment when the moon is full in the night sky. Tonight, the moon turns full at 9:45 p.m. EDT (0145 Aug. 21 GMT), when it will be visible for people along the eastern coast of the United States, weather and light permitting.

But the blue moon that occurred yesterday was a rare sighting. Another blue moon will not occur again until 2015, hence the idiom "once in a Blue Moon," Yahoo! News reports.

So what is a blue moon?

UPI reports that a August 1937 page in the Maine Farmers' almanac defines a blue moon is when "one of the four seasons would contain four full moons instead of the usual three."

Though not actually blue, it perhaps gets its name from the twilight hour in which is appears most vibrantly, during the "blue hour" when the sky transitions from light blue to black, The Washington Post reports. This occurred Tuesday night between 7:27 pm and at 7:56 pm in Washington, DC.  The "blue hour" is not technically a full hour, but closer to 30 mins of a blue sky.

This "blue hour" is an optimal time for photography.  Ian Livingston, in a past CWG post, stated, "the blue hour provides a photographer the ideal combination of natural and man-made light for giving an image warmth and depth. These qualities are much harder to obtain once the sky goes black," The Washington Post reports.

Tonight in Washington, the moonrise is at 8:03 pm, and this is when The Washington Times suggests photographers will have the best opportunity again to take photos of the blue moon when the sky turns a light shade of periwinkle.