PHOENIX - For the first time in 38 years, the United States will serve as the shadow backdrop to the solar eclipse.

And for the first time in 99 years, a 100 percent eclipse totality will run coast-to-coast.

Dr. Tyler Nordgren, an astronomer and teacher at the University of Redlands, explains eclipse totality:

"You get the sky going dark. The temperature dropping. The stars coming out. The sun turning black. The corona – the sun’s outer atmosphere that’s over a million degrees – suddenly becoming visible. All of that happens at 100 percent totality."

He witnessed his first solar eclipse in 1999. He said it was a life-changing experience. Usually you have to travel to see an eclipse. For 2017, the moon will directly align across the U.S.

"Twelve million people live within the path of totality," Dr. Nordgren said, "so there’s a vast amount of students and young people that are going to be able to go outside of their house, their apartment, maybe their school (if it’s back in session), look up and see this awe-inspiring sight."

Phoenix will only see a partial eclipse -- about 62 percent. Flagstaff will see about 70 percent. In order to witness the moon align 100 percent with the sun, Phoenix residents would have to drive 13 hours north to Idaho Falls, Idaho.

Fred Espenak, who speaks about eclipses worldwide, said it's worth the trip to see 100 percent totality if you can make it.

He argued, seeing an eclipse even at 99 percent or less: "It's the difference between getting five out of six numbers on the lottery and winning the jackpot."