Simulated image courtesy of Tom Ruen.

Full 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.

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.

The full moon of 20 May is the third of four full moons to fall between the March equinox and the June solstice, which means that it is a blue moon, by one definition of the term.

This use of the term was coined by the Maine Farmers' Almanac in the 1930s as part of its efforts to recreate the calendars of Native American peoples. Following historical practice, the almanac divided the year into four seasons, separated by the equinoxes and solstices, and used lists of three names for the full moons which fell within each season.

However, once every 2.8 years, one of these seasons would have four full moons rather than the usual three. The name blue moon was given to the third of these four. The three traditional names were applied to the first, second, and fourth full moon falling within the allotted period. This custom appears to have originated with in the 1930s, with no earlier precedent.

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 20 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
15 May 198915:1803:2072%
16 May 198916:1503:3980%
17 May 198917:1604:0087%
18 May 198918:2004:2893%
19 May 198918:2104:2497%
20 May 198919:2504:5799%
21 May 198920:2905:37100%
22 May 198921:3106:2498%
23 May 198922:3107:2195%
24 May 198923:2008:2689%

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°59'S in the constellation Libra . It will lie at a distance of 397,000 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 1989
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 15h45m50s 24°59'S Libra 30'01"

The coordinates above are given in J2000.0.

The sky on 18 September 2021
Twilight ends
Twilight begins

11-day old moon
Waxing Gibbous


11 days old

Rise Culm. Set
Mercury 08:54 14:17 19:39
Venus 10:18 15:24 20:31
Moon 18:20 23:30 04:52
Mars 07:09 13:11 19:14
Jupiter 17:37 22:47 03:57
Saturn 16:46 21:38 02:31
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

20 May 1989  –  Full Moon
28 May 1989  –  Moon at Last Quarter
03 Jun 1989  –  New Moon
11 Jun 1989  –  Moon at First Quarter

Image credit

Simulated image courtesy of Tom Ruen.






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