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BAA Observing calendar

Second Full Moon in January - Blue Moon - 13:28

Wednesday, 2018, January 31 - 13:28

Dominic Ford, Editor

This will be the second occasion when the moon reaches full phase in January 2018. It is relatively rare for two full moons to fall within the same calendar month, since they occur at 29.53-day intervals.

It happens on average once every 2.7 years, when a full moon takes place within the first few hours of a month.

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.

Historically, the term blue moon has been used to describe the third of four full moons to fall within one of the Earth's seasons.

In this system, the sequence of full moons through the year are assigned names according to the seasons in which they fall. Usually only three names are needed for the full moons in each season. If a fourth fell within the same season, one of them was left without a name. This additional full moon was called a blue moon.

At the exact moment when the Moon reaches full phase, it will lie at a declination of +17°04' in the constellation Cancer , and so will appear highest in the northern hemisphere. It will be visible from all latitudes south of 62°S. Its distance from the Earth will be 360,000 km. ( 223,693.6292 miles)

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

Object Right Ascension Declination Constellation Angular Size
The Moon 08h54m50s +17°04' Cancer 33'09"

 The details of this observing event were provided courtesy of In-The-Sky.org