The full meaning of “once in a blue moon” is to be demonstrated to star gazers on Saturday, Halloween night, according to Astronomy Ireland.
Not since 1974* has a full moon been visible from Ireland on the date of Halloween, nor will it happen again for two decades, Astronomy Ireland said.
In another rarity popularly known as a “blue moon”, Saturday’s full moon will be the second full moon in a calendar month.
According to Astronomy Ireland’s David Moore this is “an extremely rare event” and he urged “everyone to get out and look on the spookiest of nights”.
Those who observe the moon are invited to record their comments and photos in Astronomy Ireland magazine .
“Even camera phones can take great photos of the moon these days, either close up, or against a backdrop of buildings, statues or landscapes,” said Mr Moore, who edits Astronomy Ireland magazine.
In a further twist, Saturday is also the date that the distant planet Uranus is at opposition to the Sun. As a full moon is also when the moon is at opposition with the Sun, “so Uranus is close to the Moon that night also”, said Mr Moore.
“Uranus is 3.8 degrees above the moon that evening but at magnitude +5.7 and with a full moon so close you probably won’t even be able to see it with binoculars, and certainly not the naked eye - but still , it is nice to know Uranus is there to celebrate such a triple whammy!” he said.
*Correction: A previous version of the article stated that the last full moon visible from Ireland on Halloween was in 1955, not 1974