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’s distance from the Earth varies because its orbit is not perfectly circular – it is 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 varies by around 10%, between 363,000 km and 405,000 km.
This means that its size in the night sky also varies over the course of each month, by around 13%. It brightness also varies slightly – the Moon appears a little brighter when it is closer to the Earth. In practice, however, this variability is swamped by the much stronger effect that the Moon’s changing phases have over its brightness.
The Moon’s distance varies between perigee (closest approach), apogee (furthest recess) and back again once every 27.555 days – a period of time called an anomalistic month. This is very close to the Moon’s orbital period (27.322 days), but slightly longer. For more information on why these periods don’t exacty match, see In-The-Sky.org’s glossary article for the term month.
As the perigee of 14 January 2016 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 369,000 km of the Earth, and appear with an angular diameter of 32.31 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.
All articles in this post were found at: in-the-sky.org