As we pay homage on Monday 14 November to the closest supermoon since 1948, it is time to inject a note of realism into the hype that has developed around supermoons of late.
Before I try to do so, though, I refer to our diagram which depicts (at ten-day intervals) the imminent climb and convergence of Mars and Venus as they loop across our SW evening sky. They draw to within 5.4° of each other on 2 February when Venus reaches its brilliant evening-star best at mag –4.6. Mars is much fainter at mag 0.5, dimming to 1.1 by February and 1.5 by April.
Turning back to supermoons, the term was coined in 1979 by astrologer Richard Nolle to describe the near-coincidence of a full moon with the lunar perigee, the point in the Moon’s monthly elliptical orbit that lies closest to the Earth. At such times, the Moon appears at its largest and I accept it is easier to talk of a supermoon than to use the technically accurate phrase of “a perigee-syzygy of the Earth-Moon-Sun system”.
The Moon’s average distance varies from about 362,600km at its perigee to 405,400km at apogee, but the varying gravitational tugs of the Sun, Earth and other planets on the Moon mean that its orbit’s precise dimensions fluctuate. The perigee on the 14th is at 356,509km by my calculation, the closest since one of 356,461km in 1948. It also comes (at 11:17 GMT) only 2h 35m before the Moon reaches full phase, enough to qualify this as a supermoon – as, incidentally, was also the case in 1948.
You will hear and read sensational claims that this makes the Moon appear around 14% wider and 30% brighter than usual. These are false. The Moon appears only this large and bright when compared with a full moon near apogee, which is almost as rare as one near perigee and certainly not “usual”. It would be more honest to say that it appears 7% wider and 15% brighter than average, but to do so would tone down the sensationalism.
A 7% increase is far from obvious, especially as we have nothing to compare it to in the sky at the time. Indeed, it is dwarfed by the impact of the Moon illusion which makes our satellite always appear larger when it is low in the sky.
It is always the case, too, that there is a sharp peak in the lunar brightness when full, thought to be due to the way light reflects from its pitted airless surface. Indeed, by the evening of the 16th it will be half as bright as it is on the 14th even though the illuminated part of the disc only reduces from 100% to 92%. So, don’t be fooled into thinking that any supermoon effect persists for more than a day or so.
