Full Moon. Image courtesy of Tom Ruen.
A supermoon is, according to some publications, a full moon or new moon which occurs when the Moon is closer than usual to the Earth.
The term originated among astrologers in the late 1970s, and does not have a precise definition. Although the term has recently begun to be used by stargazers as well as astrologers, in practice a supermoon looks little different from any other moon.
The term first appeared in the Dell Horoscope in 1979, where it was introduced by the astrologer Richard Nolle. In subsequent publications, Nolle has presented various differing definitions of how close the Moon needs to be to the Earth to qualify as a supermoon, and as a result, lists of such events are rarely consistent. These various definitions are summarised in detail by Wikipedia here.
The Moon's elliptical orbit
The Moon orbits the Earth in an elliptical orbit, which means its distance from the Earth changes over time, oscillating with a 27.55-day period, called an anomalistic month. The variation is extremely slight, however: the Moon never comes closer than 362,600 km (perigee), or recedes further than 405,400 km (apogee) – a maximum difference of only 12%. This leads to an extremely slight change in the Moon's apparent size of about 14%.
The chart below illustrates the two extremes of this variation in the size.
Even in months without supermoons, the Moon's apparent size subtly changes from week to week, oscillating between these two extremes. The effect goes entirely unnoticed, since the change is so small that it is right at the limit of what even the best human eyesight can perceive.
The only way in which a supermoon is any different from other months is that the date of full moon happens to coincide with when the Moon appears at its largest. In the other months of the year, the Moon's size will fall somewhere between the two extremes shown above on the day of full moon.
For this reason, supermoons are of little interest to stargazers. Anyone who is tempted to rush outdoors to observe one is likely to be disappointed. Indeed, if the Moon does seem to appear abnormally large, this is far more likely to be an optical illusion than anything to do with the Moon's distance from the Earth. This extremely common illusion is called the Moon illusion.
The Moon illusion
Although the angular size of the Moon only changes by a very modest amount in reality, it is very common for the Moon to be perceived as appearing much larger than it really is when it is close to the horizon. This is called the Moon illusion – and is nothing more than an optical illusion. Any photograph will reveal that the Moon is exactly the same size regardless of whether it appears on the horizon or directly overhead.
The reason why we perceive this optical illusion is hotly debated. However, it may explain why some people who go out in the early evening to observe a supermoon may be convinced that the Moon appears unusually large, despite the actual changes in its true size being so small.
Defining a supermoon
As a result of Richard Nolle's multiple inconsistent definitions of what qualifies as a supermoon, various websites produce different lists of such events, based on their own definitions. For example, Sky & Telescope uses the term when the Moon is closer than 358,884 km. TimeandDate.com, meanwhile, uses the term when the Moon is closer than 360,000 km.
Fred Espenak has attempted to interpret Richard Nolle's definitions literally – which leads to a more complicated definition. Espenak compares the Moon's distance on the day of full moon with the distances of its closest approach (perigee) with its furthest recess (apogee) in that same month. The full moon is said to be a supermoon if the Moon's distance on that day is at least ten times closer to its perigee distance than its apogee distance.
A term which has come into even more recent usage is micromoon. This refers to a full moon which occurs when the Moon is close to apogee, and is therefore at its further recess from the Earth. Needless to say, micromoons are of even less observational interest than supermoons.
List of supermoons 1950–2299
The following full moons occur when the Moon is closer than 358,884 km from the Earth: