Blue moon? Friday's celestial event comes with a catch

July 30, 2015
Blue moon? Friday's celestial event comes with a catch
This Saturday, Jan. 30, 2010 file photo shows the full moon from Nairobi, Kenya; as the second one of the month, it is also called a "blue moon." A blue moon is expected to appear at 6:43 a.m. EDT Friday, July 31, 2015. (AP Photo/Sayyid Azim)

This is an astronomy story that comes up only once in a blue moon, like on Friday. Maybe.

When the appears at 6:43 a.m. EDT Friday, it will become the second full moon of July. Geoff Chester of the U.S. Naval Observatory said the traditional definition of a blue moon is two full moons in a month.

It happens about every 2½ years and has nothing to do with color.

But there's a catch that may disqualify this as a blue moon.

Chester said an older definition from a Maine almanac calls a blue moon the fourth full moon in an astronomical season. This is only the second full moon of summer, so by that definition no blue moon.

The next will be in January 2018.

Blue moon? Friday's celestial event comes with a catch
Light from the setting sun reflects off the One World Trade Center tower as the moon rises in its waxing stage seen from The Heights neighborhood of Jersey City, N.J., Wednesday, July 29, 2015. The moon will reach its full stage Friday in a phenomenon known as the blue moon. (AP Photo/Julio Cortez)

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