Follow CBSDFW.COM: Facebook | Twitter

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

READ MORE: Tax Refund Delays Grow As Filing Deadline Gets Closer

When the full moon 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.

READ MORE: CDC: Fully Vaccinated People No Longer Need To Wear Masks, Socially Distance In Many Indoor Situations

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 blue moon will be in January 2018.

MORE NEWS: Man Sentenced To Life For Murder Of Sara Hudson, Who Was Found In Burning SUV In Dallas

(© Copyright 2015 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed.)