What is a total solar eclipse?

You've probably heard by now to watch out for a total solar eclipse in the United States on Aug. 21. But do you know exactly what it is and what will happen when it occurs?

Here's what you need to know:

What is a total solar eclipse?

To truly understand a total solar eclipse, you must be familiar with the different types of eclipses.

An eclipse is when one astronomical body, such as a moon or planet, moves into the shadow of another astronomical body. There are two types of eclipses on Earth: a lunar eclipse and a solar eclipse.

A lunar eclipse occurs when Earth moves between the sun and the moon, with its shadow blocking the sunlight that causes the moon to shine. This can only occur when the moon is full, according to NASA.

A solar eclipse occurs when the moon moves between the sun and Earth, blocking the sunlight and casting a shadow onto Earth. There are four main types of solar eclipses: partial, annular, total and hybrid, according to NASA.

A total solar eclipse is when the moon moves between the sun and Earth, lasting for up to about three hours from beginning to end.

Total solar eclipses occur once every 12 to 18 months while partial solar eclipses, when the moon blocks only part of the sun, occur more frequently, though visibility varies, according to NASA.

You must be in the path of totality to witness a total solar eclipse. The path of totality for the Aug. 21 eclipse is a 70-mile-wide ribbon that will arc across the continental United States from west to east. This stretches from Lincoln Beach, Oregon, at 9:05 a.m. PDT to Charleston, South Carolina, at 2:48 p.m. EDT.

From there, the moon's shadow leaves the country at 4:09 EDT.

What happens during a total solar eclipse?

During a total solar eclipse, the lunar shadow will darken the sky and temperatures will drop while bright stars and planets will appear at a time that is normally broad daylight.

Retired NASA astrophysicist and photographer Fred Espenak said the experience usually lasts for just a couple minutes, but it's truly out of this world.

"It is unlike any other experience you've ever had," Espenak, popularly known as Mr. Eclipse, told ABC News. "It's a visceral experience; you feel it. The hair on your arms, on the back of your neck, stand up. You get goosebumps.

"You have to be there," he added.

Espenak said the rare and striking astronomical event can last as long as seven minutes. For the Aug. 21 eclipse, NASA anticipates the longest period when the moon obscures the sun's entire surface from any given location along its path will last about two minutes and 40 seconds.

Some animals may react strangely to the celestial phenomenon. Rick Schwartz, an animal behavior expert with the San Diego Zoo, said there have been observations of animals going to sleep during total solar eclipses.

"The animals take the visual cues of the light dimming, and the temperature cues," Schwartz told ABC News.

"You hear the increase of bird calls and insects that you usually associate with nightfall," he added. "Farmers have said that the cows lay down on the field or the chickens go back into the coop."

ABC News' Gina Sunseri and Catherine Thorbecke contributed to this report.