A rare event is called "once in a blue moon." But a real blue moon - not a reference to the moon's tint but designating its appearance a second time in a single calendar month - was visible Thursday night where there were no clouds - along with a partial lunar eclipse that could be sighted throughout the Middle East. The last time this unusual event occurred on the last night of the year was in 1999. The big full moon rose in the eastern sky for the second time in December. While 11 years have passed since a full moon appeared for the second time in a calendar month on the last day of a Gregorian calendar year, blue moons occur - on average - every 2.7 years, according to astronomers. The next one will be in August 2012. Blue moons occur when the solar and lunar calendars don't correspond, as a year in the Gregorian calendar has about 11 more days than it would if every month were a lunar month of 29.5 days. Although "once in a blue moon" is widely used today, it goes back to the time of Shakespeare. The moon can very rarely appear bluish any time in the year due to environmental influences. In 1883, it appeared blue after the eruption of the Krakatoa volcano. In 1950, it resulted from forest fires in Canada and Sweden.