The Moon will reach full phase – the
fourth full moon of summer 2013,
traditionally called a blue moon.
The term blue moon does not refer to any change in the Moon's color,
but is traditionally given to any full moon which is the
fourth to fall within the same season – defined astronomically to be the
periods between the Earth's solstices and equinoxes.
The term was used in that context in past times because full moons were given
traditional names such as the harvest moon and the hunter's
moon, that ran in sequences through the seasons. Since there were only
three traditional names given to the full moons which fell within the span of
each season, the fourth full moon was left without a name.
There are, on average, 3.11 full moons within each of the Earth's
seasons, since full moons occur on average once every 29.53 days and the
Earth's seasons last on average a quarter of the year. This means that blue
moons occur on average once every 2.7 years.
In modern usage, the term blue moon is often used alternatively to
refer to any full moon which is the second to fall within a single calendar
month. This usage is a twentieth century innovation which originally seems to
have stemmed from a misprint in Sky & Telescope magazine in March
Coincidentally, however, blue moons also occur once every 2.7 years by
that definition, since according to both definitions, a blue moon occurs
whenever 13 full moons fall within a single year-long period.
As at any time when the Moon reaches full phase, it will be
brighter than at any other time of the month, and will also be visible for
much of the night on account of lying almost directly opposite the Sun in the
Over the nights following 19 September, the Moon will rise a little
under 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, around a week after full moon, it
will rise at around midnight and set at around noon.
On this occasion the Moon will lie at a declination of +01°45' in the constellation Pisces, and so will be appear high in the sky at all but the most extreme latitudes. It will be visible at all latitudes between 81°N and 78°S. Its distance from the Earth will be 374,000 km.
The detailed circumstances of this event are:
The coordinates above are given in J2000.0.
The sky on Thu, 19 September 2013
The circumstances of this event were computed from the DE405 ephemeris published by the Jet Propulsion Laboratory (JPL).A survey of the historical usage of the term blue moon was provided by
Sinnot et. al
in Sky & Telescope in 1999.