The Moon will reach the closest point along its orbit to the
Earth, and as a result will appear slightly larger than at other times.
The Moon makes close approaches to the Earth (perigees) roughly once a month
since its orbit is not perfectly circular, but instead slightly oval-shaped
– tracing out a path called an ellipse.
As the Moon traverses this elliptical path around the Earth each month, its
distance away from the Earth varies by more than 10%, between 363,000 km
and 405,000 km.
This means that its diameter in the night sky also varies over the course of
each month, by around 13%. This variation repeats itself once every
27.555 days – a period of time called an anomalistic month
which is very close to the Moon's orbital period. For more information on why
it is not quite the same length of time, see In-The-Sky.org's glossary article
for the term month.
Although the Moon's changing distance from the Earth does cause small changes
in its brightness – it appears brightest when it is closest to the Earth
– a much stronger effect is that the Moon's brightness changes over the
course of the month with its changing phases.
As the perigee of 7 February 2013 will occur close to the time of new moon, the moon will appear as no more than a thin crescent.
On this occasion the Moon will pass within a distance of 365,000 km of the Earth, and appear with an angular diameter of 32.69 arcmin. This may be compared to its average size of 31.07 arcmin.
The genuine variation in the Moon's angular size that is associated with its
changing distance from the Earth should not be confused with the Moon
illusion – an optical illustion that makes the Moon appear much
larger than it really is when it is close to the horizon. The reason why we
experience this optical illusion is still hotly debated.
The detailed circumstances of this event are:
The coordinates above are given in J2000.0.
The sky on Thu, 07 February 2013
The circumstances of this event were computed from the DE405 ephemeris published by the Jet Propulsion Laboratory (JPL).