A blue moon will light up the night sky tonight.
The lunar occurrence, which has nothing to do with the coloring of the moon, means there are two full moons in the same calendar month, according to astronomy instructor Jonathan Braidman from the Chabot Space and Science Center in Oakland.
The phases of the moon line up to occur in the beginning and the end of the month -- which happens about every two to three years, Braidman explained.
The rare occurrence last happened in March 2010.
The next blue moon will be in July 2015, with full moons on July 2 and July 31.
The moon cycle takes only 29.5 days so over time the calendar day that the full moon falls on changes -- and is more likely to occur twice in longer months, such as August and July with 31 days.
Braidman noted folk roots of the "blue moon," which has come to be an expression for something rare.
The Farmers' Almanac mentions the blue moon as four full moons in one growing season, when usually there were only three.
Bob Jacobs, a NASA spokesman, tweeted Monday morning that a private memorial service would be held Friday in Cincinnati for astronaut Neil Armstrong -- coinciding with the rare lunar event.
Armstrong, an Ohio native, was the first man on the moon as part of the Apollo 11 space mission in 1969.
He passed away Aug. 25 at the age of 82.
The rare sighting, which will put the bright, full moon directly overhead at midnight Saturday, will give viewers an opportunity to see the spot where the Apollo 11 landed.
The full moon has often been anthropomorphized as a human face, Braidman said. The spot beneath what looks like the moon's right eye is approximately where the spacecraft landed and Armstrong touched down on the lunar surface.
Free lunar viewings at Chabot, located at 10000 Skyline Blvd. in Oakland, will be held Friday and Saturday night.
CONTACT: Chabot Space and Science Center (510) 336-7379