Simulated image courtesy of Tom Ruen.

Blue Moon

Dominic Ford, Editor
From the Moon feed

Objects: The Moon
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The Moon will reach full phase. At this time of the month, it is visible for much of the night, rising at around dusk and setting at around dawn.

A Blue Moon

This will be the second full moon of December 1990, making it a blue moon – a term used to describe any full moon which is the second to fall within a single month. This use of the term first appeared in the March 1946 issue of Sky & Telescope magazine, where it was incorrectly stated that this was an established tradition. In fact, it was an entirely new usage of the term, although it had previously been used by the Farmers' Almanac with a different definition. However, the Sky & Telescope article became widely cited, and the term has now entered common usage.

It is possible for two full moons to fall within the same calendar month since the Moon's phases cycle, on average, 12.37 times each year. As a result, once every 2.8 years, a single year contains 13 full moons rather than the usual 12, and in that a year, one of the months must have two full moons.

Put another way, the Moon's phases cycle once every 29.53 days, and so if a full moon occurs on the first or second day of the month, it is possible that that next full moon will occur within the same month.

December 1990: a supermoon?

This month, the moon reaches full phase at around the same time that its elliptical orbit also makes its closest approach to the Earth – called its perigee. In recent times it has become fashionable to describe such full moons as "supermoons" – a term which originated among astrologers in the late 1970s.

The Moon's distance from the Earth varies because its orbit is not perfectly circular – it is slightly oval-shaped, tracing out a path called an ellipse.

As the Moon traverses this elliptical path around the Earth each month, its distance varies by 14%, between 356,500 km at perigee (closest approach to the Earth) and 406,700 km at apogee (furthest from the Earth).

Its angular size also varies by the same factor, between 29.4 arcmin and 33.5 arcmin. When full moon coincides with perigee, it does appear very slightly brighter than at other times, but the difference is so small as to be imperceptible to the untrained eye.

The chart below shows the size of this month's full moon in comparison to the largest (perigee) and smallest (apogee) possible apparent size of a full moon, drawn to scale.

The Moon
Full Moon
at perigee
The Moon
December 1990
Full Moon
The Moon
Full Moon
at apogee

In 1990, two full moons occur close enough to perigee to qualify as "supermoons" by the most common definition of the term: those on 02 December and 31 December.

To emphasise the subtlety of the changes in the Moon's size, the plot below shows the variation in the apparent size of the Moon at each of its full moons in 1990.

The Moon illusion

Although the angular size of the Moon only changes by a very modest amount in reality, a very common optical illusion is that the Moon appears very much larger than it really is when it is close to the horizon. This is called the Moon illusion – and is nothing more than an optical illusion. Any photograph will reveal that the Moon is exactly the same size regardless of whether it appears on the horizon or directly overhead.

The reason why we perceive this optical illusion is hotly debated. However, it may explain why some people are convinced that the Moon appears larger on some nights than others, despite the actual changes in its true size being so small.

The Cold Moon

The sequence of full moons that fall through the year are sometimes assigned names such as the "Cold Moon", according to the months and seasons in which they fall. This practice has been popularised in recent decades by the Farmers' Almanac in the United States. The names used by that almanac claim to have ancient origins from Native American tribes. This claim has been examined in detail by Patricia Haddock's book Mysteries of the Moon (1992) and is partially true, but the selection of names is largely arbitrary.

Throughout history a great variety of different names have been given to the sequence of lunar cycles through the year, and modern lists of such names, such as those popularised by the Farmers' Almanac, tend to inevitably be a medley of names taken from many different cultures.

According to the Venerable Bede's De temporum ratione (The Reckoning of Time; 725 AD) – an authoritative account of the calendar used in Saxon England – the lunar month containing the first full moon after the December solstice (within winter) was called the "month after Yule (Æfterra Gēola)".

The biography of Charlemagne (circa 817–833 AD), written a few years after his death, gives a name of the "winter month (Wintar-mánód)" for the same lunar month.

However, in the scheme followed by the Farmers' Almanac, which has become rather widely quoted, any full moon in the month of December is called the "Cold" Moon.

Observing the Moon in coming days

Over the nights following 31 December, the Moon will rise around an hour later each day, becoming prominent later in the night. Within a few days, it will only be visible in the pre-dawn and early-morning sky. By the time it reaches last quarter, a week after full moon, it will rise in the middle of the night and set at around noon.

The table below lists the rising and setting times of the moon in the days around full moon:

Date Moonrise Moonset Phase
26 Dec 199011:5702:3559%
27 Dec 199012:2903:5270%
28 Dec 199013:1205:0880%
29 Dec 199014:0406:2189%
30 Dec 199015:0907:2195%
31 Dec 199015:0807:2199%
01 Jan 199116:2308:10100%
02 Jan 199117:4308:4898%
03 Jan 199118:5909:2093%
04 Jan 199120:1509:4886%

The exact moment of full moon

The exact moment of full moon is defined as the time when the Moon's ecliptic longitude is exactly 180° away from the Sun's ecliptic longitude, as observed from the center of the Earth. However, the Moon does not appear in any way special at this instant in time, and a full moon can be observed at any time of night.

At the moment it reaches full phase, the Moon will lie at a declination of 24°42'N in the constellation Gemini . It will lie at a distance of 358,000 km from the Earth.

The celestial coordinates of the Moon at the time it reaches full phase will be:

Object Right Ascension Declination Constellation Angular Size
The Moon 06h43m50s 24°42'N Gemini 33'19"

The coordinates above are given in J2000.0.

The sky on 23 September 2021
Sunrise
06:42
Sunset
18:50
Twilight ends
20:22
Twilight begins
05:10

16-day old moon
Waning Gibbous

92%

16 days old

Planets
Rise Culm. Set
Mercury 08:51 14:08 19:26
Venus 10:30 15:28 20:27
Moon 19:57 02:29 09:09
Mars 07:05 13:03 19:01
Jupiter 17:15 22:24 03:33
Saturn 16:29 21:21 02:13
All times shown in EDT.

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

31 Dec 1990  –  Blue Moon
07 Jan 1991  –  Moon at Last Quarter
15 Jan 1991  –  New Moon
23 Jan 1991  –  Moon at First Quarter

Image credit

Simulated image courtesy of Tom Ruen.

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