Blue Moon

2016 May 21

Dominic Ford – originally published on

The Moon will reach full phase – a full moon which will also be a blue moon.

The term blue moon does not refer to any change in the Moon’s color, but is traditionally given to any full moon which is the third of four to fall within one of the Earth’s seasons – defined astronomically to start and end on the Earth’s solstices and equinoxes.

Normally, only three full moons occur in each of the Earth’s four seasons each year. However, there are on average 3.11 full moons within each season. This is because full moons occur on average once every 29.53 days which is slightly less than a third of the length of one of the Earth’s seasons – a quarter of the year. As a result, blue moons occur on average once every 2.7 years.

In past times because full moons were traditionally given names such as the harvest moon and the hunter’s moon, when ran in sequences of three names through each of the seasons. Since there were only three of these names for each season, a fourth full moon was left without a name.

In more modern usage, the term blue moon is often used alternatively to refer to any full moon which is the second to fall within a single calendar month. This usage is a twentieth century innovation which originally seems to have stemmed from a misprint in Sky & Telescope magazine in March 1946.

Coincidentally, however, blue moons also occur once every 2.7 years by that definition, since according to both definitions, a blue moon occurs whenever 13 full moons fall within a single year-long period.

As at any time when the Moon reaches full phase, it will be brighter than at any other time of the month, and will also be visible for much of the night on account of lying almost directly opposite the Sun in the sky.

Over the nights following 21 May, the Moon will rise a little under an hour later each day so as to become 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, around a week after full moon, it will rise at around midnight and set at around noon.

On this occasion the Moon will lie at a declination of -15°40' in the constellation Libra, and so will be appear highest in the southern hemisphere. It will be visible from all latitudes north of 64°N. Its distance from the Earth will be 402,000 km.

The exact position of the Moon at the time it reaches full phase will be:

Object Right Ascension Declination Constellation Angular Size
The Moon 15h59m30s -15°40' Libra 29'39"

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of