Observations

Trick or Treat – A Blue Moon for Halloween?

Moon.GeoffWyatt
The Full Moon shows it true colours. Image: Geoff Wyatt.

Is there a Blue Moon on Halloween in 2020?

Many parts of the world will see a Blue Moon on Halloween this year – but eastern & central Australia will not.

These days a Blue Moon is commonly defined to be the second full Moon in a calendar month. The whole world will see a full Moon at the beginning of October, on either the 1st or the 2nd depending on the time zone.

But at the end of October the situation is more complicated. Europe, Africa, the Americas and most of Asia see a second full Moon, and therefore a Blue Moon, on October 31. However, in Australia only Western Australia will see a Blue Moon in October.

  • For the NSW, ACT, Victoria & Tasmania the same full Moon occurs at 01:49am AEDT on November 1st – and therefore it is not ‘blue’.
  • Queensland is not on daylight saving time but its full Moon occurs at 0:49am AEST on November 1st and so it is not ‘blue’.
  • The Northern Territory is also not on daylight saving time and its full Moon occurs at 0:19am ACST on November 1st and so it is not ‘blue’ either, but only just.
  • South Australia also has a full Moon on November 1st, at 01:19 ACDT, and so it is not ‘blue’.
  • In Western Australia, however, the full Moon occurs on October 31 at 10:49pm AWST so this will be an October Blue Moon.

But will the Moon look full on October 31?

Yes, although a full Moon occurs at a specific time, across Australia it will look full throughout the night of Halloween – so we can all enjoy a socially distanced trick-or-treat under bright moonlight.

When do the eastern states see a Blue  Moon?

The eastern and central states & territories of Australia all experience a full Moon on November 01, as noted above, and again on November 30. The November 30 full Moon is the second in the month and therefore it is a Blue Moon.

What other definitions are there for a Blue Moon?

Usually there are twelve full Moons in a year, three per season. Each is given a name, such as Harvest Moon or Egg Moon, according to old ecclesiastical rules which suits events occurring at that time of year. But sometimes a 13th full Moon squeezes itself into the year, and one season will have four full Moons. If the names are to make any sense the third of the four full Moons in the season is deemed to be the odd one out. A publication called the “Maine Farmers Almanac” termed this third full Moon ‘blue’ – although why they called it that remains a mystery. The modern definition dates from 1946 when the popular US astronomy magazine “Sky and Telescopemisinterpreted the “Maine Farmers Almanac”, this misinterpreted definition was popularised by the game Trivial Pursuit in the 1980s, and it stuck.

By either definition a Blue Moon occurs about once every 2.7 years. The Moon itself never looks blue, unless an unusual dust storm was to block the red light coming from it, leaving only blue to reach our eyes. Highly saturated digital images of the Moon do show a metallic blue in many of the Moon’s oceans, or Maria, indicating the presence of titanium. The image above shows a hint of this colouring.

What else is interesting about the full Moon on November 01 (and October 31st)?

The full Moon on November 1st is also the most distant full Moon for 2020 at 406,167km. That’s about 20,000km further than average. Some would call this a mini Moon, Astronomers might call it a Full Moon apogee syzygy.

What else is interesting about the full Moon on November 30?

The full Moon on November 30 is not only blue (for a few of us) but it also undergoes an eclipse – although it is such a shallow one it will be difficult, if not impossible, to detect. This is a penumbral lunar eclipse in which the Moon passes through only the fainter outer or ‘penumbral’ shadow of the Earth. It is dimmed only slightly.

For Sydney this eclipse begins before the Moon rises, is at its maximum at 8:43pm AEDT and ends at 10:56pm AEDT. For central parts of the country and Western Australia the eclipse is approaching its end as the moon rises.

Whether its blue or not, Sydney Observatory wishes you a Happy Halloween for 2020!

For more, follow the changing phases of the Moon with our monthly Moon Phase Calendar.

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