Roughly once every year and a half, a total solar eclipse darkens some spot on Earth, but when eclipses make landfall on highly populated areas, they generate a lot of notice.

That’s the case this April 8, when a total solar eclipse will pass over the United States, traveling east from southern Texas to northern Maine. The total eclipse will also be seen in Oklahoma, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire.

A total solar eclipse happens when the Moon passes between the Sun and Earth, completely blocking the face of the Sun. The sky will darken as if it were dawn or dusk.

We asked Anna Sajina, associate professor of astronomy and astrophysics, to tell us more about the eclipses, and why they matter.

Tufts Now: How often do total solar eclipses occur, and is there a regularly occurring pattern?

Anna Sajina: Yes, these are regularly occurring phenomena, as the Moon’s orbit around the Earth and the Earth’s orbit around the Sun are very consistent. The question on how often depends on whether you mean “from anywhere on Earth” or from a specific place on Earth. The first is pretty often, actually; roughly once every 18 months, somewhere on Earth experiences a total solar eclipse.

However, the path of totality—where a total solar eclipse can be seen—is quite narrow, so any given eclipse is visible from a very small portion of the Earth. Any given place on Earth will experience a total solar eclipse only once in about every 360 years, though it varies by location.

Why is it that some neighboring areas might see a partial eclipse?

It is because they are outside the path of totality. The eclipse is the result of the Moon completely blocking the Sun, but only if you’re looking at it from the right point of view—basically where your location on Earth, the Moon and the Sun form a straight line. 

What’s the safest way to view the eclipse?

Never view an eclipse directly or through binoculars or telescopes that are not equipped with special solar filters. The easiest and cheapest way is through solar glasses, which are inexpensive and widely available.

What can scientists learn from eclipses?

The best scientific use of total solar eclipses is that they allow us to study the solar corona, an aura of plasma that surrounds the Sun that is hard to observe otherwise.