Comet Lemmon will appear in the Philly-area skies the next several evenings

Come Monday around sunset, Teddy Kareta plans to ascend to the roof of a building near Lancaster Avenue on the Main Line to catch sight of something in the evening sky that hasn’t been around for about 1,300 years.

To the surprise of astronomers, a “cosmic snowball” named Comet Lemmon has become so bright “that people don’t have to be in the most perfect dark location” to see it, said Kareta, assistant professor of astrophysics and planetary science at Villanova University.

Binoculars and telescopes certainly can’t hurt, but he said people should be able to view it without any special optical equipment, and “they probably have all of next week to give it a go.”

He added that while it may appear as a celestial “smudge” to the unaided eye, it should be quite photogenic, even in the eyes of the average smartphone.

NASA estimates that the comet will be making its closest approach on Tuesday, when it will be about 45 to 50 million miles away, about half the distance between the Earth and the sun.

Kareta said the peak opportunity for seeing it would be 30 to 90 minutes after sunset Monday and Tuesday.

As the sky darkens it will appear in the western sky about 40 degrees above the horizon (think about halfway between flat and straight up). It will stay visible for perhaps two hours before it follows the sun and calls it a night.

Kareta said he aims to head to the roof of the Mendel Science Center on Monday, since Monday looks like the better bet given the forecast. Some clouds could interfere with the show on Tuesday.

But skies are forecast to clear Wednesday, and the comet should be visible into next weekend, with no moon interference, he said.

NASA describes comets as “cosmic snowballs of frozen gases, rock, and dust.”

As a comet approaches the sun, it gets cooked, and it spews gases and other material to form the signature tail.

By NASA’s count, “there are likely billions” of these things orbiting our sun, meandering for hundreds and thousands of years, and the fact that Lemmon is visible from Earth is mighty exceptional.

It has been further distinguished by its surprising surge in brightness, a quality always difficult to predict considering the distance comets travel to reveal themselves to earthlings, said Kareta.

The comet was discovered in January by the Mount Lemmon SkyCenter Observatory (thus the name) in Arizona, and it passed a good part of the summer in obscurity.

“There were a couple of months when we couldn’t see it because it was behind the sun,” Kareta said.

“When it reemerged in early August, it was significantly brighter than anticipated,” and it’s been getting brighter, said Kareta.

Come Monday it could get to such a magnitude that “people might notice it in a dark sky without meaning to,” he said.

He said he expects the comet to be visible in areas just outside Philly — for example, Radnor Township, where Villanova is located.

Not that you’re going to see much if you’re hanging around Liberty Place: “If you’re in the city, you’re probably out of luck,” he said.

But for people in some of the region’s more light-intensive corridors, to get a glimpse of Lemmon “they won’t have to go super far away.”