Appropriately for the spookiest night of the year, there’ll be something unusual going on in the sky this Halloween – a blue Hunter’s Moon.
The moon will not (sadly) be blue, but it’s the second full moon in the month, which makes it a blue moon (by some definitions at least, see below).
Full moons are separated by 29 days, Nasa says, and most months are 30 or 31 days long, so a “blue moon” happens every two and a half years on average.
A Hunter’s Moon is the next full moon after the Harvest Moon – seen this year at the beginning of October.
Full moons occur when the moon appears as a full circle in the sky, when the whole side of the moon facing the Earth is lit up by the sun.
The moon will be full at 2.49pm on 31 October, according to Royal Museums Greenwich.
A blog post at Royal Museums Greenwich explains: “It may at first seem odd to think of a full Moon occurring during daylight hours.
“The reason this happens is that the time refers to the exact moment when the Sun and Moon are aligned on opposite sides of the Earth.
“This moment is known as the 'syzygy' of the Sun-Earth-Moon system, and can happen at any time day or night.”
Nasa expert Gordon Johnston says American names for full moons – such as Hunter’s Moon – are derived from Native American folklore.
Johnston writes: “The Maine Farmer's Almanac first published Native American names for the full Moons in the 1930s.
“Over time these names have become widely known and used.
“The Hunter's Moon is the full Moon after the Harvest Moon. According to the Farmer's Almanac, with the leaves falling and the deer fattened, it is time to hunt.
“Since the harvesters have reaped the fields, hunters can easily see the animals that have come out to glean (and the foxes that have come out to prey on them).
“The earliest use of the term ‘Hunter's Moon’ cited in the Oxford English Dictionary is from 1710.”
Read more: What is a supermoon?
Strictly speaking, the definition of a “blue moon” as the second full moon in a given month is wrong – but it is now widely used, says Royal Museums Greenwich.
Royal Museums Greenwich writes: “Traditionally the definition of a blue moon is the third full Moon in an astronomical season containing four full moons.
“The astronomical seasons begin and end at the equinoxes and solstices (e.g. the winter season begins at the winter solstice and ends at the spring equinox, the spring season begins at the spring equinox and ends at the summer solstice and so on).
“Another definition of the blue moon, perhaps the more commonly used due to its simplicity, is actually a mistake, made in the 1940s and perpetuated by radio shows and the Trivial Pursuit board game through the 1980s.
“This definition describes the blue moon as the second full Moon in any calendar month with two full moons.”
Watch: Nasa announces it has found water on the Moon