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 the moon will appear slightly larger and brighter than at other times, though 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 2009 – the Old Moon.
Over the nights following 31 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 at around midnight and set at around noon.
This will be the second occasion when the moon reaches full phase in December 2009. 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 +24°02' in the constellation Gemini , and so will appear highest in the northern hemisphere. It will be visible from all latitudes south of 55°S. Its distance from the Earth will be 359,000 km.
The exact position of the Moon at the time it reaches full phase will be:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
|The sky on 28 October 2020|
12 days old
All times shown in PDT.
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.
|31 Dec 2009||– Full Moon|
|01 Jan 2010||– The Moon at perigee|
|07 Jan 2010||– Moon at Last Quarter|
|14 Jan 2010||– The Moon at perihelion|
Simulated image courtesy of Tom Ruen.