Starting in mid-November, a mystery began to take hold in northern New Jersey, when residents of Morris County began posting videos that appeared to show brightly lit flying objects hovering or soaring across the night sky, in some cases above their homes.

Many believed they were drones, and various theories followed, with people claiming they were part of secret government programs or even alien invasions. Within weeks, residents across several states began reporting hundreds of sightings, and posting videos and photographs of their encounters.

To better understand what some New Jersey residents have likely been seeing — as well as how perceptions of objects in the night sky can be distorted — The New York Times mapped local airports, tracked air traffic patterns and, along with aviation experts, analyzed hundreds of videos of aircraft purportedly filmed in or near Morris County in November and December.

The vast majority of the videos appeared to show planes or helicopters, moving across a part of the state that brims with airports. None of the videos analyzed by The Times conclusively showed drones, though in some cases that remains a plausible explanation.

One video in particular, filmed in Somerset County, N.J. and posted on Facebook on Dec. 4, is an example of how difficult it can be to deduce what an object might be. It shows a helicopter and a second flying object, purported to be a drone; experts were unable to determine exactly what the object was, although they said it resembled a plane.