Cape Town - If you were one of those skywatchers who spent a few moments on Saturday evening gazing in awe at the huge full moon, you’re in for a treat – the next two months will also produce these so-called “supermoons”.

And the one in August could even be called an “extra-supermoon”, says Dr Tony Phillips, production editor of the online news service of US space agency Nasa.

Sometimes the moon appears bigger and brighter than at other times. In June last year, for example, the supermoon was 14 percent bigger and 30 percent brighter than other full moons of the year. The scientific term for this is “perigee moon”.

“Full moons vary in size because of the oval shape of the moon’s orbit. The moon follows an elliptical path around Earth with one side – ‘perigee’ – about 50 000km closer than the other – ‘apogee’. Full moons that occur on the perigee side of the moon’s orbit seem extra big and bright.”

The moon is on average about 384 400km from Earth. At perigee, it averages 362 600km (ranging from about 356 400km to 370 400km) and at apogee 405 400km (about 404 000km to 406 700km).

The perigee moon happens three times this year: last Saturday and again on August 10 and September 9.

In July and September the moon becomes full on the same day as perigee, but next month it becomes full during the same hour as perigee, arguably making it an “extra-supermoon”, says Phillips.

Such a sequence isn’t rare and occurred last year as well, although with little media fanfare.

“Generally speaking, full moons occur near perigee every 13 months and 18 days, so it’s not all that unusual,” says Geoff Chester of the US Naval Observatory.

While it’s nice to imagine something significantly bigger and brighter, Phillips says that it’s not always easy to tell the difference.

“A 30 percent difference in brightness can easily be masked by clouds and haze. Also, there are no rulers floating in the sky to measure lunar diameters. Hanging high overhead with no reference points to provide a sense of scale, one full moon looks about the same size as any other.”

Cape Argus