"Supermoon" over Seattle, Washington, USA, January 1, 2018 [Photo: Gary Peltz[

Sometimes the media are full of stories about a supermoon. They may even include dire predictions of earthquakes, tsunamis and other disasters. What is a supermoon and should you be worried — or excited? Let's have a look.

“Supermoon”
Supermoon isn't an astronomical term, but has come intocommon use. To understand what a “supermoon” is, we need to remember two things about the Moon.

Firstly, the Moon's orbit isn't perfectly circular. It's an ellipse, a slightly squashed circle. That means the Moon isn't always the same distance from us. Rounding off the distances, its average distance is 384,400 km (240,250 mi), but it can get as far away as 405,400 km (253,400 mi) and as close as 362,600 km (226,600 mi). When the Moon is farthest away, we call it apogee and when it's closest to us, it's perigee.

Secondly, since the Moon shines by reflecting sunlight, half of it is illuminated as it orbits us. So the Moon seems to change shape because we only see part of the lit portion. The only time we see the entire illuminated half of the Moon is during a full Moon when it faces both us and the Sun.

The “supermoon” is simply a full Moon that occurs at perigee.

Does a “supermoon” look bigger and brighter than usual?
Compared to a full Moon at apogee, a full Moon at perigee can appear around 14% larger and maybe 30% brighter. The brightness varies, as it is also affected by factors such as atmospheric conditions.

But if you see a “supermoon”, it's not going to look different from the full Moon of the month before or the following month. The percentage change from month to month is much less than the maximum 14%. In addition, with nothing to compare it to, to see a percentage change of this amount needs photographs or measurements. Without the publicity, anyone just looking up at the sky wouldn't see anything remarkable in a perigee full moon.

Interestingly, many people do think they've seen an extra-large Moon on the night of a “supermoon”. This is a perceptual illusion known as the horizon effect which makes objects low in the sky look bigger to us. It makes a rising full Moon look large at any time.

In addition, many of the lovely pictures we see after a “supermoon” show a very large Moon. If you take a picture of the Moon at a distance from the foreground objects, as in this photo, the Moon looks enormous. [Image credit: VegaStar Carpentier]

Earthquakes? Floods? Tsunamis?

Gravity and tidal effects
The gravitational pull of the Moon, with some help from the Sun, distorts the Earth's crust and produces tides.

Isaac Newton showed that there are two factors that determine how strong a gravitational force is: mass and distance. The Sun's mass is very large, but it's far away. Newton's inverse square law tells us that the force decreases rapidly as the distance increases. Although the Moon's gravity is comparatively weak, the Moon is so close that humans have been there.

When the Moon pulls on the Earth, obviously, every part of the Earth can't be at exactly the same distance from it. So the gravitational force on the Earth varies. This squeezes the Earth slightly out of shape. It has a small effect on the rigid part of the Earth, but a more noticeable one on the oceans.

In addition, if the Moon is full or new, it's lined up with the Earth and the Sun. In both cases, the gravitational forces of the Sun and Moon reinforce each other. This produces spring tides, which are extreme tides, with high tides higher and low tides lower.

It's reasonable to wonder if this effect could be dangerous when the Moon is closer to us. Let's consider the evidence.

Looking at evidence
There is some evidence that occasionally a low level quake might have been triggered when there was a full moon or new moon at perigee. Certainly, there may be some flooding in some low-lying areas that wouldn't normally occur even at the spring tides. But lunar perigees have been happening for millions of years and there is no pattern of related extreme events.

The “supermoon” of March 19, 2011 was accompanied by well-publicized predictions of disaster. According to one internet site, during the period from 16th to the 22nd, among other things, we could expect “a surge in extreme tides”, “a rash of moderate-to-severe seismic activity (including Richter 5+ earthquakes”, “volcanic eruptions” and “powerful storms”. These would be big stories, but they didn't feature in the news during those eight days.

As for the seismic activity, I checked it using the website of the US Geological Survey. It gives the average annual figures for earthquakes of different magnitudes, and from this I calculated a weekly average of 28 for those of at least 5 on the Richter scale. The USGS website also includes a map showing quakes and their magnitudes for the previous seven days. Late on the 22nd I counted those of Richter 5 or more and found 24, the strongest being 6. Far from an increase in seismic activity, this was slightly below average.

Full Moon at perigee
So a supermoon is a full Moon at perigee, or perhaps a new Moon at perigee. Without precision observation, the full Moons aren't noticeably larger and brighter than other full Moon. It's not connected with disasters, so enjoy the beautiful sight of this full Moon — or any other full Moon.