What is a supermoon? Astronomer explains spectacular lunar phenomenon

'Worm Moon' will appear bigger and brighter than normal full moon

An astronomical phenomenon is set to take place this week, as the “Worm Moon” supermoon swings across the night sky.

As the name suggests, the supermoon will appear bigger and brighter than a normal full moon – but why does this happen?

Dr Emily Brunsden, director of the University of York’s Astrocampus, explained the lunar event happens when the full moon is slightly closer to the Earth in its orbit.

“The night of 20 March will have a familiar large moon in the sky – the third and final supermoon of 2019,” she said.

“It is not a big enough change to notice the size difference by eye, but the moon will seem brighter than usual.”

The pattern of the moon’s orbit around the earth means this is actually the third supermoon in a row, with sky gazers treated to a lunar eclipse combined with a supermoon in January, and a folllow-up supermoon in February.

Having three in quick succession is rare, however, and there won’t be another supermoon taking place until February 2020.

What makes this last one extra special is that it takes place on the same day as the March equinox, which marks the first day of spring.

This has given this month’s supermoon the name Worm Moon, after the observation by Native Americans that worms emerge at the start of spring as the ground thaws.

A supermoon, which occurs when a full moon passes closest to the Earth, can appear even bigger when it is close to the horizon thanks to an optical illusion (AFP/Getty Images)

The spring equinox is the exact date that the Earth’s equator is closest to the sun.

Dr Brunsden explained: “It is an astronomical event when the Earth’s tilt and orbital position makes day and night nearly equal. For us that means daylight will be longer than darkness until the September equinox.”

Those hoping to see the final supermoon of 2019 in the UK will hope that skies stay relatively clear.

Current weather forecasts from the Met Office suggest the best place to witness the Worm Moon in the UK is in the Midlands at 01.43AM – the exact time the moon passes closest to the Earth.

The moon will also appear bigger than normal on Thursday night, while an effect known as the “moon illusion” means it will look even larger when it rises and sets above the horizon.

The reason for this optical illusion is still up for debate, though one explanation is that its relative size to objects on the horizon like skyscrapers and trees make it appear bigger than when it is surrounded on all sides by the expanse of the sky.