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BAA Observing calendar

Full Moon

Saturday, 2018, March 31 - 13:38

Dominic Ford – originally published on

The Moon will reach full phase – making it visible for much of the night, lying almost directly opposite the Sun in the sky.

Full Moons are traditionally given names according to the season in which they fall, and this will be the first full moon of spring 2018, traditionally called the Egg Moon.

Over the nights following 31 March, the Moon will rise around 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, a week after full moon, it will rise at around midnight and set at around noon.

This will be the second full moon to fall within March 2018 – a phenomenon that only happens every three years, when a full moon takes place within the first few hours of a month and the next full moon also takes place within the same month, 29.5 days later.

In recent time, full moons such as this have commonly been called blue moons. This usage of the term is a twentieth century innovation, however, which originally stemmed from a misprint in Sky & Telescope magazine, which appeared in March 1946.

More traditionally, the term blue moon is used to describe the third of four full moons to fall within one of the Earth's seasons – defined astronomically to be the periods between the Earth's solstices and equinoxes. The term was useful in the past because of the system of traditional names given to full moons. There were only three traditional names given to the full moons which fell within each of the four seasons. If a fourth fell within the same season, one of them was left without a name.

However, the traditional meaning of the term blue moon has now been largely forgotten, since the traditional names given to the sequence of full moons through the year are now rarely used.

At the moment when the Moon reaches full phase, it will lie at a declination of -00°12' in the constellation Virgo, and so will appear high in the sky at all but the most extreme latitudes. It will be visible at all latitudes between 79°N and 80°S. 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 12h45m10s -00°12' Virgo 31'33"

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of