peter303 wrote:So rare, the Blue Moon repeats in March. About once in every three years on average.
The second
Blue Moon in a year is a
Double Blue Moon and happens about every 25 years on average, commonly when February (28/29d) has no Full Moon and thus bookended by Jan+Mar (31d)
BMs due to the 29.5-day lunar cycle.
peter303 wrote:I have heard of the term Black Moon for no full moon in a month, which happens about every 20 years in Feb on average. Feb 2018 is a Black Moon.
A
Black Moon generally refers to a second New Moon in a month -- every
BM month has a full moon.
BMs and
Double Black Moons have the same frequency as
Blues (~2.4 years) and
DBMs (~4x/century) and as it's just the flipside of the lunar cycle. Using
BM to describe a month without a full/new moon allows for three consecutive
"Black Moon" months like in 2014: Jan (2nd New), Feb (no New, but a Full), Mar (2nd New, a
DBM).
Blue Moons and
Black Moons for 2001-2020 (
Doubles, note Jan+
Mar)
2001 Nov . . . 2000 Jul
2004 Jul . . . 2003 May
2007 Jun . . . 2005 Dec
2009 Dec . . . 2008 Aug
2012 Aug . . . 2011 Jul
2015 Jul . . . 2014 Jan
2018 Jan . . .
2014 Mar
2018 Mar . . . 2016 Oct
2020 Oct . . . - 2019 Aug
http://www.obliquity.com/cgi-bin/bluemoon.cgi
) and check cloud cover ahead of time.
