Blue Moon

Saturday 18th May 201920:13

The Moon will reach full phase. At this time in its monthly cycle of phases, the Moon lies almost directly opposite the Sun in the sky, placing it high above the horizon for much of the night.

The sequence of full moons through the year are often assigned names according to the seasons in which they fall. This month’s is assigned the unusual name of a blue moon.

Blue moons come about as a result of a quirk in the way that full moons are counted. Usually, only three fall within any each of the Earth’s season   which last an average of three months. A sequence of three traditional names are given in turn to these three full moons. However, occasionally, a fourth full moon can fall within a single season.

This happens because full moons recur every 29.53 days, and so the Moon’s phases cycle on average 12.37 times each year, or 3.11 times each season. As a result, there will be four full moons within a season once every 2.7 years.

When this happens, there are only three traditional names to go between four full moons, and so the third of them is called a blue moon instead.

Why the third of the four, not the last? The final full moon of winter is traditionally called the lenten moon, since it always falls within the 40 days of Lent. Easter Sunday itself is defined to be the first Sunday after the first full moon of spring, and so must be 29–37 days after this full moon.

This logic would break down if a blue moon were inserted between the lenten moon and Easter, and this is avoided assigning that title to the third (rather than the fourth) of four full moons.

Needless to say, the term blue moon does not refer to any change in the Moon’s color. It’s origin is disputed, but one theory is that it’s a corruption of betrayer moon, for a blue moon in winter leads to Easter being delayed by a lunar cycle.

Incorrect usage

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. It will lie almost directly opposite the Sun in the sky, placing it high above the horizon for much of the night.

Over the nights following 18 May, the Moon will rise a little under 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, around a week after full moon, it will rise at around midnight and set at around noon.

At the exact moment when the Moon reaches full phase, it will lie at a declination of -15°41′ 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 378,000 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 15h44m10s -15°41′ Libra 31’31”

The coordinates above are given in J2000.0.

This entry in the observing calendar was provided by