There's a lunar eclipse tonight.

Consult the map above to find out if and when the eclipse will be visible in your region of the world.

For those in Canada (with the exception of Eastern Canada) and the United States, the eclipse will already be in progress when the moon rises. Skygazers in Europe, Africa, and Western Asia can see the entire event during the middle of the night, while Eastern Asia will miss the end of the eclipse because of moonset.

Tonight's eclipse begins at 5:50 p.m. EDT, but the shading won't be visible until around 7:30 p.m. EDT, when at least two-thirds of the moon's disk is in Earth's outer-shadow. The most noticeable phase of the eclipse occurs at 7:50 p.m. EDT. The duration of the eclipse lasts for about 4 hours.

A lunar eclipse happens when the full moon passes through all or part of Earth's shadow. The shadow cast by the Earth is composed of two parts. The inner-part, or umbra, is the region that blocks all sunlight from hitting the moon. The outer-part, or penumbra, is a region that partly blocks the sun from reaching the moon (see the diagram below).

When the moon is in the umbra it's completely dark, but if it's in the penumbra, it's only shadowed.

F. Espenak/Nasa

During a total lunar eclipse, the entire moon passes through Earth's umbral shadow. In a penumbral lunar eclipse, like the one occurring tonight, the moon passes through Earth's penumbral shadow. Penumbral eclipses are typically very subtle, and usually go undetected by observers.

There is a full moon every month (when the sun, the Earth, and the moon line up in that order), but most of the time the moon passes above or below Earth's shadow. That's why lunar eclipses only happen two to four times a year when the moon moves through some part or all of Earth's shadow.

You can also watch the eclipse live online, which will be broadcast by the Slooh Space Camera.