Where does the phrase "once in a blue moon" come from?

The phrase does not refer to color. There once was a "blue" moon - when
the Krakatoa volcano in Sumatra erupted in 1883, sending particulate
matter into the atmosphere and making the moon appear blue.

This expression refers to the times when a full moon occurs twice in a
month. It occurred most recently on January 31, 1999. Another blue moon
will be arrive on the night of March 31.

Two blue moons in one year happens every 20 to 40 years with the last
occurrence in 1961. Astronomers predict the next occurrence of two blue
moons in one year will be in 2018, on Jan. 31 and March 31, and again on
the same dates in 2037.

Subscriber Steven Chostler comments:

Philip Hiscock challenges the "astronomical" view of "once in a blue
moon" in this recent article from "Sky and Telescope" magazine.

Of course, here in Denver, the "blue moon" will for a long time bring
to mind the Broncos' Superbowl victory to sports fans.

Follow this link:

http://www.skypub.com/sights/moonplanets/mar99bluemoon.html