The last solar eclipse of the year is set to occur in several countries of the world including Pakistan on Monday.

The first solar eclipse happened in the beginning of the year on February 26, 2017. For the first time in 99 years, a total solar eclipse will march across the entire United States.

“The Great American Eclipse” will cast a shadow over the whole country, moving diagonally from Oregon in the northwest to South Carolina in the southeast.

Here are some facts on this eclipse, and what makes it so unique.

This is the first eclipse to pass over the United States in the 21st century.

It is the first total eclipse on American soil since 1991, when one was visible from the Big Island of Hawaii.

But it has been 38 years since the mainland United States glimpsed a total eclipse. The last one was in 1979, and that swept only a handful of northwestern states.

Having a total solar eclipse move across entire the United States is quite rare. The last time it happened was in 1918.

A different kind of eclipse — called an annular eclipse, or “Ring of Fire” eclipse — did cross the United States from coast to coast in 1994.

Because the Moon was near its farthest point from Earth at that time in its orbit, it blocked about 94 percent of the Sun’s light.

Solar eclipses occur when the Moon moves between the Earth and the Sun, blocking light from the Sun.

Total solar eclipses happen because the Sun’s diameter is 400 times wider than the Moon’s, but it is also 400 times farther away. From Earth’s perspective, this geometry makes the Sun and Moon appear to be the same size.

When the two line up just right, the Moon obscures the entire Sun, and the skies go dark. These total eclipses happen every 12 to 18 months somewhere in the world, often over the open ocean since most of the Earth is covered by water.