What Is a Lunar Eclipse? Next One in 2021 to Be Longest Partial Eclipse of the Century

A partial lunar eclipse is due to occur next week between November 18 and November 19, resulting in a visual treat for millions of people around the world with clear skies at night.

Lunar eclipses themselves aren't particularly rare—they happen at least two times per year—but the coming one is due to be the longest partial lunar eclipse of the century, lasting nearly 3.5 hours from start to finish, according to NASA's eclipse website.

What Is a Lunar Eclipse?

All planets and moons in the solar system are constantly moving around in particular orbits.

A lunar eclipse occurs when the Earth gets in the way of the sun's light and stops it from reaching the moon. This can happen fully, when the moon comes completely into Earth's shadow, or partially.

For observers, this causes the moon's light to fade during the night as it slips into the darkness cast by the Earth.

Lunar eclipses may also cause the moon to look red or orange.

This is because even though the Earth is blocking the sun's light, some of this light is still reaching the moon after passing round the edge of the Earth and through our atmosphere.

When sunlight passes through our atmosphere at a certain angle it tends to appear red. This is the reason sunrises and sunsets also look orange.

Lunar eclipses only last for a few hours at most. They can be seen by anyone living on the night-time half of Earth who has a clear sky.

When Is the Next One?

The next lunar eclipse is going to be a partial one, and it's due to occur overnight between November 18 and November 19.

The time at which it happens will be different depending on where people are on the planet.

For people living on the U.S. east coast, for example, the eclipse will start just after 2 a.m. local time on November 19, and reach its maximum at about 4 a.m., NASA states.

In the U.K., it will start just after 7 a.m. local time on November 19 according to TimeAndDate and reach its maximum just after 9 a.m.—which won't be visible.

People living elsewhere should check their local time zones.

People in some other locations may find that the moon will unfortunately not be visible at the time of the eclipse because of clouds, or simply because it is daytime.

For people who do miss it, there will be a full lunar eclipse between the 15 and 16 May in 2022 and another between the 7 and the 18 of November that year, according to TimeAndDate.

Partial lunar eclipse
A partial lunar eclipse as seen from Israel in August 2017. There are usually a couple of lunar eclipses every year. AFP / Getty/Jack Guez