The truth about this month’s supermoon and what you will see

Once again, popular news sources have been proclaiming this month’s Full Moon a “supermoon”. It is in danger of becoming a “hypermoon” with all the fuss that is being made about it.
How different in size the Full Moon will appear at its closest (perigee) and at its most distant (apogee). Image credit: Paul Sutherland/Skymania

We originally wrote about this in November 2016 when many news organisations are proclaiming that the Moon would be closer to the Earth than at any time since 1948. But the principle is the same, so we are happy to bring our article up to date.

What the reports should actually have said was that it would be the closest FULL Moon in that time. That is because the Moon comes just as close to us during every one of its orbits around the Earth. So it last came this close only a month earlier!

The reason is because the Moon’s orbit is not circular but elliptical and its distance from Earth during each revolution varies from a maximum of 407,119 km (252,972 miles) to a minimum of 356,653 km (221,614 miles).

The Full Moon on November 14th, 2016, occurred at around 11.30 am UT, at a distance from Earth’s centre of 356,509 km, so very close to the minimum distance possible. The final “supermoon” in 2021 is the Full Moon on June 24, also known in America as the Strawberry Moon.

The closest point in the Moon’s orbit around the Earth is called the perigee and the furthest point is termed its apogee. (Strictly speaking, the Earth and Moon both revolve around a point that lies about 4,671 km (2,902 miles) from the centre of the Earth, a point called the barycenter.)

So the Moon can be at perigee at various phases, and the New Moon on 10 January, 2005, was actually closer, at a distance of 356,571 km (221,563 miles).

We have produced an image to compare the relative diameters of the Full Moon at perigee and apogee. Seen together, it is clear that the perigee Moon, or supermoon, is larger. But to be honest, the casual observer is unlikely to notice the difference.

Complicating matters is the “Moon illusion”, a well known phenomenon which makes the Moon appear much larger when close to the horizon than when high in the sky. Though not fully understood, this is a psychological effect because the angular diameter remains the same, whatever its elevation.

At perigee, this angular diameter is a little over half a degree (33 min. 33.20 sec.), and at apogee, it is just under half a degree (29 min. 23.65 sec.).

Regardless of the hype, the Moon is always a fascinating object to observe and the only world we can explore in such detail, due to its proximity. We have a special guide to help you explore it with a small telescope.

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