SAAO - South African Astronomical Observatory
03.05.2012 00:00 Age: 6 yrs
Category: SAAO Press Releases
By: Dr Nicola Loaring

"Super Moon" this month!

The Moon will make its closest approach to the Earth (at perigee) for the year in May. It will happen early morning on Sunday 6th May at about 5:33 AM and at this time the Moon will be 357,000 km from the Earth. This will result in the largest apparent full Moon for the year, which also occurs on Sunday 6th May at 5:35 AM purely by coincidence! The full Moon will have an angular size of 33.7 arc min at this time and is referred to as a perigee full Moon. This coincidence, whereby the full Moon occurs at the same time as the Moon's closest approach in its orbit happens roughly every 18 years.

The Moon will be at its furthest distance from Earth (at apogee) for the year on December 25th when the Moon will be 406,100 km away from the Earth. The full Moon in December, occurring on the 28th, will therefore be the smallest apparent full Moon with an angular size of 29.1 arc min. Nearby perigee Moons are about 14% bigger and 30% brighter than lesser Moons that occur on the apogee side of the Moon's orbit.

(60 arc min = 1 degree)

Note that the size of the Moon is not really changing, it just appears larger or smaller from our perspective according to its distance from us. The Moon orbits the Earth in an elliptical orbit (oval) rather than a circle and so the distance between the Earth and the Moon varies along its orbit. Viewing the Moon just as it rises or before it sets when it is close to the horizon, close to distant buildings or trees will exaggerate the effect because of an optical illusions making the Moon seem even larger.

The closeness of the Moon on the 6th May will have a slight effect on tides around the world. It will result in extra-high "perigean tides". However, in most places the tides will only be a few centimetres higher than usual. Local geography can amplify the effect to about 15 centimeters, so no great floods or natural disasters! For example in Cape Town the average height of spring high tides over the last 19 years is 1.74m. The two high tides on the 6th May are predicted to be about 1.88m and 1.80m respectively, a difference of only 14cm and 6cm respectively over average values.