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 May 2113, 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.

The Flower Moon

The sequence of full moons that fall through the year are sometimes assigned names such as the "Flower 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 third full moon after the March equinox (within spring) was called the "month before midsummer (Ærra Līþa)".

The biography of Charlemagne (circa 817–833 AD), written a few years after his death, gives a name of the "fallow month (Brāh-mānod)" 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 May is called the "Flower" Moon.

Observing the Moon in coming days

Over the nights following 30 May, 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
25 May 211314:2102:4573%
26 May 211315:2003:1081%
27 May 211316:2103:3588%
28 May 211317:2104:0393%
29 May 211318:2004:3497%
30 May 211318:2104:35100%
31 May 211319:2205:13100%
01 Jun 211320:2505:5698%
02 Jun 211321:2406:4895%
03 Jun 211322:1607:4989%

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 19°42'S in the constellation Ophiuchus . It will lie at a distance of 396,000 km from the Earth. 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
May 2113
Full Moon
The Moon
Full Moon
at apogee

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

Object Right Ascension Declination Constellation Angular Size
The Moon 16h24m40s 19°42'S Ophiuchus 30'06"

The coordinates above are given in J2000.0.

The sky on 17 September 2021
Twilight ends
Twilight begins

10-day old moon
Waxing Gibbous


10 days old

Rise Culm. Set
Mercury 08:55 14:19 19:43
Venus 10:19 15:27 20:36
Moon 17:44 22:42 03:44
Mars 07:11 13:15 19:18
Jupiter 17:42 22:52 04:02
Saturn 16:51 21:43 02:35
All times shown in EDT.


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

30 May 2113  –  Blue Moon
06 Jun 2113  –  Moon at Last Quarter
13 Jun 2113  –  New Moon
21 Jun 2113  –  Moon at First Quarter

Image credit

Simulated image courtesy of Tom Ruen.






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