This diagram shows what happens during a total solar eclipse

On August 21, around lunchtime in most cities in the United States, a total solar eclipse will pass through the country, moving from Oregon to South Carolina.

During a total solar eclipse, the moon crosses between the Earth and the sun, completely blocking out the sun's light. If you're in the moon's shadow today, the sky will go dark for just under three minutes.

Only a 70-mile-wide band of the country will see totality. But weather permitting, everyone will be able to see at least a partial eclipse.

The diagram below shows what will happen:

Ana Pelisson

Types of solar eclipses

The three main types of solar eclipses are total (when the moon appears to completely cover the sun), partial (when the moon appears to only cover the sun somewhat), and annular (when the edge of the sun remains visible as a bright ring around the moon).

A total solar eclipse is considered the most spectacular — globally, only about a third of all solar eclipses are total. The August 21 eclipse is the first total eclipse to travel coast-to-coast across the continental US since 1918. 

Ernie Wright/NASA

What causes a total solar eclipse

During a total solar eclipse, three key conditions happen at the same time: The moon is in the "new moon" phase; the moon crosses the plane of the Earth's orbit; and the moon is at its closest point to Earth in its orbit.

When those conditions align, the Earth, sun, and moon line up. Then, if you're in the path of totality, the moon appears to mask the sun.

The moon orbits Earth every 29.5 days. But the moon is not in line with Earth's orbit relative to the sun, which is why we don't have a solar eclipse every month. The moon's orbit is tilted about five degrees, which is large enough to keep its shadow off the Earth and the Earth's shadow off the moon most of the time.

There are two points — called nodes — where the moon's orbit crosses the Earth's plane. On the diagram above, the moon is lined up on a node.

The moon aligns with the nodes and the sun about twice per year, which is how we get eclipses. You get a solar eclipse when the moon is between the Earth and sun. You get a lunar eclipse when the moon is on the other side of the Earth, farthest from the sun.

What happens on Earth during a total solar eclipse

A solar eclipse is when the moon's shadow falls somewhere on the Earth's surface.

The faint outer shadow is called the penumbra, and the umbra is the dark inner shadow. The shadow's placement determines what the solar eclipse looks like on Earth.

Getty ImagesThe total solar eclipse has 10 distinct phases, each with different amounts of the sun visible from the ground level. The phenomenon kicks off with what's called first contact, when the moon starts to pass across the sun. After about an hour, the moon will almost completely mask the sun, and you'll start to see a bright light radiate out of the sliver of remaining sun, known as the "diamond ring."

During this eclipse, totality will last a maximum of 2 minutes and 43 seconds. After that, the moon will continue to travel across the sky to form another crescent. The eclipse ends when the moon ceases to cover the sun.

How to watch the eclipse

If you plan to watch the eclipse, make sure you are wearing certified eclipse glasses. (If you stare directly into the sun for an extended period of time, you risk damaging your eyes.) The only safe time to look at the eclipse without glasses is during the minute or two of totality.

You can find more information about today's eclipse on our full eclipse guide.

NOW WATCH: 6 major US cities could be underwater within 80 years — here are the disturbing ‘after’ images