Rare full moon on Christmas Day 2015

For the first time in 38 years a full moon will appear in the skies on Christmas Day, 25 December 2015.

The last time there was a full moon on 25 December was 1977, and there won’t be another one until 2034 – so unless you want a long wait to catch a sight of this rare astronomical occurence, it’s a good idea to head outside and look up on Christmas.

The full moon, which is the last of the year, is called the Full Cold Moon because it occurs at the start of winter in the northern hemisphere.

The reason this is such a rare event dates back to what the Greek astronomer, Meton of Athens, first discovered in the 5th Century BC:

The time it takes the moon to complete 235 revolutions around the Earth amounts to 6,940 days, which is almost exactly how long it takes Earth to complete 19 revolutions around the sun — the two are just a few hours apart.

This means that the moon’s phases will recur on the same calendar date every 19 years. The pattern follows what astronomers call the Metonic cycle, after its founder.

Because these two cycles are a few hours apart, we just missed a full moon on Christmas day in 1996. Instead it happened on Christmas Eve.