Tonight sees the second blue moon of 2018, the first time there have been two blue moons in a single year since 1999.
A blue moon is the second occurrence of a full moon in a calendar month.
Steve Sims, who used to write the Night Sky column in Scilly Now & Then magazine, explained: "We had two full moons in January, none in February and two in March. The moon's cycle is 28.5 days, so the only month that can skip a full moon is February and then only if the January full moon is on the 31st."
Two blue moons in a year will not occur again until January and March 2037. Meanwhile, there will not be another blue moon at all until 2020.
Today’s full moon (March 31st) is the one that dictates when Easter will fall, Easter Sunday being the first Sunday after the first full moon after the vernal equinox. Easter can therefore vary between March 22nd and April 25th, a range of 35 days.
The most recent extreme Easters were March 23rd in 2008 and April 23rd in 2003, with Gig Weekend the following week.
Steve said: "It’s difficult to find anyone who’s happy with Easter flitting about so much, particularly on Scilly where a very early Easter increases the length of the dead zone until late April and a late one means the season kicks in properly weeks later than the locals want. Most people like a nice sensible Easter around April 12th.
"There’s always pressure to make Easter a permanent fixture, say the closest Sunday to April 15th, but it’s a religious festival that is tied to the moon and it’s anybody’s guess whose jurisdiction (if indeed anyone’s) this would come under."
Next year Easter falls on April 21st.