Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

If two Full Moons in one calendar month are called a Blue Moon, then what, if anything, are two New Moons called, as occurred in September?

Ronald R. Greene, Kingman, Arizona
ASYSK1216_01REcopy
No matter how many Full Moons appear in any given timespan, its color will not actually change to blue.
John Chumack

The length of time between any two occurrences of the Moon’s phase is always 29.5 days, but thanks to the varying length of calendar months, this means we can occasionally get two Full Moons in one month (except for February); this is indeed called a blue Moon. A blue Moon can also (and more traditionally) refer to the third of four Full Moons in one season. In fact, the former — and much more well known — usage arose from confusion over applying the seasonal rule.

This means that exactly which lunar apparition should be the “blue” one is sometimes up for debate. One can imagine a scenario in which two Full Moons rise in June, but the third Full Moon of summer doesn’t appear until July. Which one should count as the blue Moon?

Astronomically speaking, it doesn’t matter in the slightest, but it makes for some delightful barroom trivia, and exemplifies how language evolves with use.

The New Moon equivalent is usually referred to as a black Moon, and, depending on your nomenclature, one occurred on October 30, following a New Moon on October 1 (both with regard to Universal Time), just in time for an extra-spooky Halloween.

Alternatively, a black Moon can also refer to a month with no Full Moon at all. A simple comparison of the length of the lunar period and the length of calendar months reveals that February is the only month that can host such darkness. This phenomenon next occurs in February 2018.

Korey Haynes  
Associate Editor  


0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT
FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
ObservingforBeginners_MedRec
Observing the night sky is a fun and easy activity that anyone can do, but getting started can be daunting for beginners.
Find us on Facebook