This week, part of the world will be treated to a rare kind of lunar eclipse, as the moon will turn red and become a so-called Blood Moon.

This month's full moon -- the Worm Moon -- will completely enter Earth's shadow on the night of March 13 or early morning of March 14. Unlike a solar eclipse that is only visible within a narrow stretch of a few hundred miles, the "Blood Worm Moon" will be visible in the entire Western Hemisphere, including all of North and South America and the far western parts of Africa and Europe.

But what is a lunar eclipse and what makes this month's total lunar eclipse so special?

The answer to both questions comes down to "a little bit of luck and a little bit of geometry," says Jacqueline McCleary, an assistant professor of physics at Northeastern University.

Both lunar and solar eclipses are "essentially a geometric effect," she says. The orbit of Earth around the sun and the orbit of the moon around Earth are offset by about five degrees, which means that most of the time they are not perfectly aligned. However, every so often the sun, Earth and moon fall into perfect alignment and create either a lunar or solar eclipse based on how the three are overlapping.

When the Earth sits directly between the moon and the sun, leaving the moon in the Earth's shadow, we get a lunar eclipse, McCleary explains. After about two weeks, the moon orbits to the other side of Earth, covering the sun and creating a solar eclipse.

The striking appearance of a lunar eclipse -- an Earth-darkened moon -- also comes down to a remarkable coincidence, McCleary says.

"The reason they are as dramatic as they are on Earth is because, by a remarkable coincidence, the sun has a size that is about 400 times larger in the sky than the moon, but it is also about 400 times further away than the Earth," McCleary says. "So, the moon and the sun appear to have about the same size in the sky even though, of course, the sun is much, much bigger in reality."