For the first time in 99 years, a total solar eclipse will be visible on Aug. 21 to spectators across the contiguous United States.

As the moon moves between the Earth and the sun, the 68-mile-wide shadow it casts (called the umbra) will sweep across the Earths surface, moving around 1,200 mph along a path stretching from Oregon to South Carolina.

People in the center of the shadows path will be plunged into darkness for no more than about two and a half minutes. The temperature may drop 20 or so degrees, depending on the location. The wind may suddenly change direction . Stars, planets and the moon will become visible. Some animals might prepare for bed. (Probably not cows, though. A 1999 study found eclipses have little, if any effect on the ruminants.)

Whats it like?

Heres how Mabel Loomis Todd, author of Total Eclipses of the Sun , described what she witnessed in 1894:

Then, with frightful velocity, the actual shadow of the moon is often seen approaching, a tangible darkness advancing almost like a wall, swift as imagination, silent as doom. The immensity of nature never comes quite so near as then, and strong must be the nerves not to quiver as this blue-black shadow rushes upon the spectator with incredible speed. A vast, palpable presence seems to overwhelm the world. The blue sky changes to gray or dull purple, speedily becoming more dusky, and a death-like trance seizes upon everything earthly...

Then out upon the darkness, gruesome but sublime, flashes the glory of the incomparable corona, a silvery, soft, unearthly light, with radiant streamers, stretching at times millions of uncomprehended miles into space, while the rosy, flame-like prominences skirt the black rim of the moon in ethereal splendor. It becomes curiously cold, dew frequently forms, and the chill is perhaps mental as well as physical.

In short, its something you probably dont want to miss.