Society for Popular Astronomy

Tonight’s Moon – how super is super?

The media are full of excitement about tonight’s full Moon. But will it be as spectacular as they are making out?

A deep red total lunar eclipse. Photo: Robin Scagell

They are going on about a ‘Blue blood supermoon’ and pointing out that this is the first time such an event has happened for ages. There’s even a documentary on the BBC tonight to celebrate the event.

While we are all in favour of people looking at the Moon, we feel we should point out that to astronomers, this is nothing out of the ordinary. Indeed, even on this morning’s Breakfast programme, our Vice President Prof Tim O’Brien was referring to it as ‘fake news’, though he did go on to encourage people to take a look. Just don’t expect too much, particularly from the UK!

Where do these terms come from? The ‘blue’ bit comes from a suggestion that the second full Moon in a month is known as a blue moon, but this is a misunderstanding anyway. It’s not really blue at all! It just happens fairly rarely, and was invented long after people were saying ‘Once in a blue moon’ to describe something rare. True blue moons, where the Moon really does appear blue, are caused by dust in the upper atmosphere, such as following a massive volcanic eruption – though even then, the Moon is more likely to go red than blue.

And the ‘blood’ Moon? Another recent media invention, referring to the colour that the Moon sometimes appears during a total lunar eclipse, when it passes through Earth’s shadow. Some eclipses do appear red, but others are less vivid. Today’s lunar eclipse is not visible from the UK – it ends well before the Moon rises.

What about the ‘super’ bit? As most amateur astronomers know already, the Moon’s distance from Earth varies during the month because like all the planets and their moons it has an elliptical rather than circular orbits. So when it’s at its closest (called perigee), it is larger than when at its most distant (apogee), as you can see in this photo comparing the two sizes:

The sizes of the Full Moon at perigee and apogee compared. Credit: Skymania.com

The term ‘supermoon’ was invented not by an astronomer but by an astrologer, Richard Nolle, in 1979, to describe a perigee New or Full Moon, which he claimed could result in geophysical stresses – though there’s no evidence that this is the case. Until very recently, the closeness of the Moon was of minor interest to astronomers.

This month, the Moon was at perigee yesterday, so it isn’t even very super! However, it might well appear large when it’s rising. The size that the Moon appears in the sky is very subjective. Whenever the Moon, whatever its phase, appears close to the horizon it always appears bigger than when it is high in the sky. This is known as the Moon Illusion. It’s hard to shake off this illusion, even when you know that it isn’t really larger, but it applies not just to the Moon but even the Sun and constellations. When the Moon is high up it’s hard to tell whether it’s super or just plain ordinary!

The illusion is caused by the brain’s perception of size rather than by any magnification by the atmosphere. Measure it and prove the point. But the increase in size of the supermoon compared with the average, which is 7% smaller, is hard to judge without accurate measurement.

So while tonight’s Moon may not be either blue or red, and not especially super, it will look as fantastic as it always does. Take a look with binoculars, or a telescope if you have one. Enjoy it for what it is – our amazing natural satellite.