Our Moon completes one orbit of the Earth in approximately 29.5 days. Therefore, we should see a Full Moon about every 29.5 days, and there will usually be one Full Moon each month. Sometimes, if a Full Moon occurs at the very beginning of a month, a second Full Moon will be seen at the end. This second Full Moon is called a Blue Moon. Blue Moons actually occur every 2.5 years. In 1999, however, we had a situation which only takes place about four times a century: two Blue Moons occurring in the same year. Both January and March had Blue Moons. The first Full Moon appeared on the 2nd and the second Full Moon, or Blue Moon, appeared on the 31st of each month.
The last time this happened was in January and April of 1961, and it will not happen again until January and March of 2018. A double Blue Moon most commonly occurs in January/March but is also possible in January/April or January/May and only when there is no Full Moon at all in February. The next regular Blue Moons will be in November 2001, July 2004, and June 2007.
