Casper will be in the dark for more than two minutes on Aug. 21, as the moon passes directly in front of the sun -- a total solar eclipse. Here's why that's happening and what observers can expect to see. 

What happens during a solar eclipse?

The new moon crosses directly in front of the sun, so the moon’s shadow is cast onto the Earth.

A solar eclipse happens only during a new moon phase, when the side of the moon that faces the Earth is in darkness. 

Every month has a new moon, but solar eclipses are rare because the moon’s orbit is tilted 5 degrees compared with Earth’s. This means the moon's shadow often either goes above the Earth or below. An eclipse happens when the two celestial bodies align.

What will people watching the eclipse see?

In Casper and other places along the path of totality, which stretches from Oregon to South Carolina, observers will see the dark disc of the moon slowly encroach into the sun, bit by bit, until the sun is covered. But that cover doesn’t mean it will be as dark as night – the shadow won’t be strong enough for that. It will probably be more like twilight.

Those in Casper will see "first contact" – where the moon first touches the sun – at 10:22 a.m. Full totality will occur at 11:42 a.m., and for Casper viewers, it will last 2 minutes and 26 seconds. The duration of the eclipse varies depending on where observers are located. In nearby Alcova, for example, totality is expected to last only 1 minute and 45 seconds.

People outside the path of totality will see a far less dramatic effect. Most people in the United States will see at least a partial eclipse – where the moon encroaches on the sun but never completely covers it. Others farther away will see no eclipse at all.

What else should observers look for?

The moon isn’t a perfect circle; it has mountains and canyons, just like the Earth. That means some light from the sun will be visible around the perimeter of the moon, and that leads to unusual effects, usually seen just before totality.

  • Baily’s beads: These bits of bright sunlight stream out from behind the moon.
  • Diamond ring: When one stream of sunlight is more visible than the others, the eclipse takes on the appearance of a jeweled ring – a slender band of illumination sporting a gemstone of sunlight.

There might also be effects on the ground. Some animals might behave as if night was descending – such as chickens roosting. Temperatures are likely to drop quickly as the sun is blotted out. Observers may also see a bright light at the horizon, similar to a sunset, and planets and constellations may stand out.

Sources: Casper Planetarium educator Rod Kennedy, NASA.