The Moon will reach full phase. At this time of the month, it is visible for much of the night, rising at around dusk and setting at around dawn.

The Cold Moon

The sequence of full moons that fall through the year are sometimes assigned names such as the "Cold Moon", according to the months and seasons in which they fall. This practice has been popularised in recent decades by the Farmers' Almanac in the United States. The names used by that almanac claim to have ancient origins from Native American tribes. This claim has been examined in detail by Patricia Haddock's book Mysteries of the Moon (1992) and is partially true, but the selection of names is largely arbitrary.

Throughout history a great variety of different names have been given to the sequence of lunar cycles through the year, and modern lists of such names, such as those popularised by the Farmers' Almanac, tend to inevitably be a medley of names taken from many different cultures.

According to the Venerable Bede's De temporum ratione (The Reckoning of Time; 725 AD) – an authoritative account of the calendar used in Saxon England – the lunar month containing the third full moon after the September equinox (within autumn) was called the "month before Yule (Ærra Gēola)".

The biography of Charlemagne (circa 817–833 AD), written a few years after his death, gives a name of the "holy month (Hailag-mānod)" for the same lunar month.

However, in the scheme followed by the Farmers' Almanac, which has become rather widely quoted, any full moon in the month of December is called the "Cold" Moon.

Observing the Moon in coming days

Over the nights following 18 December, 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 in the middle of the night and set at around noon.

The table below lists the rising and setting times of the moon in the days around full moon:

The exact moment of full moon

The exact moment of full moon is defined as the time when the Moon's ecliptic longitude is exactly 180° away from the Sun's ecliptic longitude, as observed from the center of the Earth. However, the Moon does not appear in any way special at this instant in time, and a full moon can be observed at any time of night.

At the moment it reaches full phase, the Moon will lie at a declination of 25°41'N in the constellation Taurus . It will lie at a distance of 405,000 km from the Earth. The chart below shows the size of this month's full moon in comparison to the largest (perigee) and smallest (apogee) possible apparent size of a full moon, drawn to scale.

Full Moon
at perigee

December 2021
Full Moon

Full Moon
at apogee

The celestial coordinates of the Moon at the time it reaches full phase will be:

The coordinates above are given in J2000.0.

The sky on 18 Dec 2021

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

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Image credit

Simulated image courtesy of Tom Ruen.

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