The supermoon: how much larger the Moon really is at perigee
Where the word supermoon comes from, the real figures behind it, the Moon illusion explained by Ibn al-Haytham and its modest effect on the tides.
In recent years the word supermoon has become familiar, accompanied by photographs of huge discs rising behind cities and mountains. The phenomenon is real, but much subtler than the images suggest. To understand it one must distinguish between the actual change in the Moon's distance and the way our brain perceives the Moon near the horizon.
A recent word
The term supermoon was not born in astronomy. It was coined in 1979 by an American astrologer to denote a new or full moon falling when the Moon is close to perigee, and it spread above all from 2011 onwards thanks to the media. Astronomers prefer to speak of a perigee full moon, or perigee syzygy, and there is no universally accepted distance threshold for defining the phenomenon.
The figures behind it
The distance between the Earth and the Moon varies during the month by about fifty thousand kilometres. A full moon at perigee appears about 14 per cent larger in diameter and about 30 per cent brighter than a full moon at apogee. Compared with a full moon of average size, however, the difference shrinks to a few percentage points and is hard to perceive with the naked eye without a photographic comparison.
The most striking supermoons occur when full moon coincides almost exactly with perigee. On 14 November 2016 the full Moon was about 356,500 kilometres from the Earth, the closest since 1948; a comparable approach is expected on 25 November 2034. At the other extreme, the full moon at apogee is sometimes called, with an equally recent term, a micromoon.
An illusion stronger than reality
Far more evident than the supermoon is the so-called Moon illusion: the Moon near the horizon looks enormous, while high in the sky it seems smaller, even though its angular size is almost identical. The phenomenon was already discussed by Ptolemy and was analysed in the eleventh century by the Arab scientist Ibn al-Haytham, who attributed it to the way the mind judges distances. Its full explanation is still debated among scholars of perception.
Tides and beliefs
Full and new moons near perigee bring slightly larger tides than normal, known as perigean tides. The increase amounts to a few centimetres or a few tens of centimetres depending on the place, and is noticeable mainly along coasts with a large tidal range. There is no evidence, on the other hand, that supermoons cause earthquakes, eruptions or changes in human behaviour, as one sometimes reads.
In tradition a particularly bright full moon was often associated with vigils, feasts and omens, but ancient cultures had no specific name for the Moon at perigee. The supermoon is a modern label applied to a phenomenon our ancestors saw without distinguishing it from other full moons, and which symbolism linked to the fullness of light rather than to distance.
The next time a huge full moon rises above the horizon, it is worth remembering that much of its size is born in our eyes and our mind. The supermoon nonetheless remains an excellent occasion to watch the sky, to reflect on the Moon's elliptical orbit and to grasp the difference between measurement and perception.