"You will see this streaming witch hair coming out in every direction," said Jason Craig of the space agency's Jet Propulsion Laboratory in Pasadena, who helped develop the Eyes Eclipse 2017 web application.
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The free app is available from https://eyes.nasa.gov/. It simulates what the sun will look like on the approximately 70-mile-wide path of totality, where the moon will completely obscure the sun - turning daytime into darkness.
"It will get colder. It will get darker. Animals will kind of freak out a little bit," Craig said.
The celestial event won't last long, with its duration changing as it stretches from the Oregon coast to South Carolina - making the total eclipse exclusive to the continental United States.
The app also shows what sky-watchers can expect in Southern California: a partial solar eclipse that blots out about 60 percent of the sun.
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"So you get the whole enchilada or just a piece of it," Craig said. "Now, a piece of it is cool but you really want the whole thing."
His recommendation: "If you know anyone in Oregon - or Wyoming or Idaho and so on - you should try to go there. And you should probably have already planned that."
NASA officials remind eclipse enthusiasts to avoid permanent eye damage by never looking at the sun without appropriate eye protection.