Know, why this perigee full moon is quite so ‘super’

The moon came closer to earth than any full moon since 1948 on Monday night. NASA says we won’t see another like it until 2034.

The reason this perigee full moon is quite so “super” is because perigee and full moon happen at almost exactly the same time, so the moon is at its closest possible when it’s also at its fullest.

Astronomers suggest if sky-watchers take a photograph of the moon on Monday night and then again in a few weeks’ time they’ll appreciate the difference in scale.

If you miss it, you’ll have to wait to around November 25, 2034 for another chance.

Associate Professor John O’Byrne from the University of Sydney’s physics school says, “Unless you are really paying attention it’s going to be hard to notice,”

“(But) the moon will look genuinely fully illuminated with no hint of a shadow at the edges.”

“The difference between the moon at its minimum and maximum is 14 per cent in diameter and that translates to about 30 per cent in area.”

That larger surface area means a supermoon reflects almost a third more light than when the moon is farthest from earth. Hence it appearing so bright.

The occurrence of a supermoon, or to give it its proper name, a “super perigee full moon”, is not particularly uncommon.

It is the result of two regular astronomical events happening at about the same time.

As the moon orbits the Earth it moves around the sky relative to the sun. This means we see different proportions illuminated from one night to the next—an effect known as the phases of the moon.

Once per orbit the moon is opposite the sun in the sky, meaning that the side facing the Earth is fully illuminated. This happens about once a month, so hopefully isn’t that unfamiliar to most people.

The other regular event requires little explaining: it is simply the moon’s “perigee”, or closest approach to the Earth.

Although the moon orbits the Earth once every 27 days or so (actually about 27 days and eight hours), it doesn’t go round in a perfect circle.

Instead it traces out an ellipse or oval shape, getting slightly closer and further from the Earth over the course of its orbit.

At its closest the moon is just under 360,000 kilometers away, while at its furthest it is at a distance of around 405,000 kilometers.

The closer an object like the moon is to Earth the larger it appears in the sky.

At around 1:30 p.m. (GMT) on November 14, the Earth, sun and moon will be almost perfectly in a line (an effect known as a “syzygy”), with the moon directly opposite the sun.

A couple of hours earlier, at about 11:30 p.m., it will also have reached its closest point to Earth—perigee—at a distance of 356,500 kilometers.

It will then begin moving further away very slowly—but by the time the sun sets and the moon rises in the U.K. it will only be about 50 kilometers further away, which isn’t much in the grand scheme of things.

Both of these effects happen about once a month, so you might think that there’s no reason why every full moon shouldn’t be the same.

However, there’s another effect that means they get out of sync—the fact that the Earth is orbiting the sun.

This means that the moon’s closest approach can occur at any point in its orbit around the Earth.

Moon's orbit showing perigee

Moon’s orbit showing perigee

To see why, fast forward to when the moon will once again be back at the same point in its orbit—in about 27 days.

But over those 27 days the Earth has moved round the sun a bit, so the moon has to “catch up” over the course of a couple of days to get back to being opposite the sun, by which point it’s not at perigee anymore.

This “catching up” is why the moon orbits the Earth in 27.3 days, but there are between 29 and 30 days between full moons.

Over time, the perigee and full moon get more and more out of sync, but after a year or two they get more or less back in sync again.

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