Internet legend suggests it was astrologer Richard Nolle who first came up with the term supermoon, which he defined as “… a new or full moon which occurs with the moon at or near (within 90 per cent of) its closest approach to Earth in a given orbit”.
Five years ago – when the closest and largest full moon fell on March 19, 2011 – many began using the term supermoon, which we’d never heard before. In the following years, we heard this term again to describe the year’s closest full moon on May 6, 2012, and again on June 23, 2013, and again on August 10, 2014, and yet again on September 28, 2015.
Whilst supermoon is an astrological term, the scientific name for the occurence is perigee-syzygy, but since supermoon is catchier the media use it to describe our celestial neighbour when it gets up close.
Astronomers call it a perigee full moon describing the moon’s closest point to Earth for any given month.
Today’s event is the biggest and best in a series of three super moons, the first of which was on 16 October and the third is due on 14 December.
The moon won’t come this close to Earth again until November 25, 2034.
In addition to today’s moon making the moon appear bigger and brighter in the sky, there will also be a “low hanging moon” effect; an optical illusion caused by the moon being close to the horizon making it easy to measure against familiar landmarks or objects such as trees or houses.
The full moon story:
To observers, the moon will appear approximately 7% larger than normal, around 15% brighter – although to the human eye this is barely discernable.
As the Moon traces its orbit around the Earth, we see different proportions illuminated by the Sun. Once in each orbit, our satellite is totally illuminated – a full moon.
And as the Moon orbits the Earth every 27 days or so, it travels in an elliptical or oval shape.
This means that its distance from our planet is not constant but varies across a full orbit.
But within this uneven orbit there are further variations caused by the Earth’s movements around the Sun.
These mean that the perigee – the closest approach – and full moon are not always in sync.
But occasions when the perigee and full moon coincide have become known popularly as supermoons.
To observers, the differences between a supermoon and a normal full moon are quite subtle.
Generally, supermoons can be up to 14% larger and 30% brighter, but only when compared with the furthest point the Moon gets to within its orbit.
main image by: Monday super moon shot by Rob Pettengill as part of his Austin Super Moonset