Gabriela Aguilar grew up in Michigan, sometimes chasing the northern lights in the state’s Upper Peninsula. But it wasn’t until Thursday night, when she climbed to the roof of her apartment building in Harlem, that she saw the celestial show for the first time.

“I’m just shocked that it took my entire life to be able to see it,” said Dr. Aguilar, 37, who stood with her dog, Gomez, and watched the sky turn pink, purple and green until the autumn chill drove her back inside. “And — of all places — seeing it in New York City!”

New Yorkers were treated to a rare light show Thursday night as the aurora borealis, also known as the northern lights, spread an ethereal smear across the sky. On social media, people as far south as Washington, D.C., and Kentucky reported seeing the lights, which in pictures seemed to vary in color and intensity from neon pink to a subtle hazy purple.

Udi Ofer, a professor of public affairs at Princeton University, was at home shortly after dusk when a neighbor texted to alert him to the sky. He rushed to his backyard in Princeton, N.J., with his 9-year-old daughter and 4-year-old son.

For about 20 minutes, they watched stripes of pink and purple as the stars began to come out.

“I think the thing that’s most remarkable about it are the streaks of light, which I just didn’t expect,” Mr. Ofer, 49, said. He called them “pretty magical.”