A solar eclipse occurs when the sun, the moon and Earth align, with the moon passing in front of the sun and blocking its light.

During a total solar eclipse, the moon fully obscures the sun’s light and skies darken as if it were dawn or dusk. Weather permitting, the sun’s outer atmosphere, which is normally obscured by the bright face of the sun, becomes visible during the moments of totality.

In a partial solar eclipse, the moon blocks only part of the sun’s light, appearing to take a bite out of it.

Another type of solar eclipse, known as an annular solar eclipse, occurs when all three celestial bodies align when the moon is at or near its farthest point from Earth. Because of this distance, the moon does not completely cover the sun. Instead, it appears as a dark disk on top of a brighter disk, an effect that leaves an orange-reddish ring around the moon.

Photos from Villalibado, Spain, show NASA staff guarding helium cylinders and visitors setting up telescopes in preparation for the total solar eclipse today.

Gabriel Luengas / Europa Press via Getty Images<!–>

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Gabriel Luengas / Europa Press via Getty Images<!–>

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Gabriel Luengas / Europa Press via Getty Images<!–>

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Around 450,000 tourists were expected to arrive in Spain this week in search of the best views of the first total solar eclipse visible from mainland Europe since 1999.

The eclipse’s path of totality — when the sun is fully blocked by the moon — will pass over Greenland first, then parts of Iceland, followed by northern Spain.

Spain will reap the most economic benefits from tourists looking to witness the “magic” moment. The country’s government estimates the revenue from eclipse-related tourism will net 347 million euros, or $400 million.

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One of the year’s most highly anticipated skywatching events is finally here!

A total solar eclipse is set to darken late-afternoon skies for millions of people in Spain, a small part of Portugal, Greenland, Iceland and northern Russia.

The eclipse will follow a path that cuts across the Arctic and through southern Europe. It will be the first time since 1999 that a total solar eclipse is visible from mainland Europe.

Weather permitting, most of Europe, Canada and northwestern Africa, along with people in more than 20 northern U.S. states, will see a partial solar eclipse.