Full Moon at Perigee

2018 Jan 2

Dominic Ford, Editor

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.

This month’s full moon will take place unusually close to the time of month when the Moon also makes its closest approach to the Earth – called its perigee. This means that the moon, at a distance of 356,567 Kilometres (221,560.462 Miles) from earth,  will appear 14 per cent larger than when the moon is at apogee, and 30 per cent more luminous than the faintest Full Moon. Any difference is imperceptible to the unaided eye. Perigee full moons such as this occur roughly once every 13 months.

The sequence of full moons through the year are often assigned names according to the seasons in which they fall. This month’s will be the first to fall in winter 2018 – the Old Moon.

Over the nights following 2 January, 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 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 +20°04′ in the constellation Gemini , and so will appear highest in the northern hemisphere. It will be visible from all latitudes south of 59°S.

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

Object Right Ascension Declination Constellation Angular Size
The Moon 06h48m20s +20°04′ Gemini 33’29”

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