Bill Cooke of NASA's Meteoroid Environmental Office said the flash appears to be "a single meteor event." He said it "looks to be a fireball that moved roughly toward the southeast, going on visual reports."
"Judging from the brightness, we're dealing with something as bright as the full moon," Cooke said. "The thing is probably a yard across. We basically have (had) a boulder enter the atmosphere over the northeast."
He noted that the meteor was widely seen, with more than 350 reports on the website of the American Meteor Society alone.
"If you have something this bright carry over that heavily populated area, a lot of people are going to see it," he said. "It occurred around 8 tonight, there were a lot of people out, and you've got all those big cities out there."
Matt Moore, a news editor with The Associated Press, said he was standing in line for a concert in downtown Philadelphia around dusk when he saw "a brilliant flash moving across the sky at a very brisk pace... and utterly silent."
"It was clearly high up in the atmosphere," he said. "But from the way it appeared, it looked like a plane preparing to land at the airport."
Moore said the flash was visible to him for about two to three seconds - and then it was gone. He described it as having a "spherical shape and yellowish and you could tell it was burning, with the trail that it left behind."
"Set as it was against a cloudless sky over Philadelphia, it was amazing," he said.
Derrick Pitts, chief astronomer at Philadelphia's Franklin Institute, agreed that the sightings had all the hallmarks of a "fireball." These include lasting 7-10 seconds, being bright and colorful, and seeming to cross much of the sky with a long stream behind it.
He said what people likely saw was one meteor - or "space rock" - that may have been the size of a softball or volleyball and that fell fairly far down into the Earth's atmosphere.
He likened it to a stone skipping across the water - getting "a nice long burn out of it."
Robert Lunsford of the American Meteor Society told USA Today "it basically looked like a super bright shooting star."
The newspaper reports that the sky flash was spotted as far south as Florida and as far north as New England.
Pitts said meteors of varying sizes fall from the sky all the time, but that this one caught more eyes because it happened on a Friday evening - and because Twitter has provided a way for people to share information on sightings.
He said experts "can't be 100 percent certain of what it was, unless it actually fell to the ground and we could actually track the trajectory." But he said the descriptions by so many people are "absolutely consistent" with those of a meteor.
Associated Press writer Norman Gomlak in Atlanta contributed to this report.