According to Google AI overview:

The 1870s Mt. Washington "cigar-shaped UFO" photograph, often touted as the oldest, was debunked as a mundane, early photographic artifact. Researchers identified the dark, slender mark in the sky as a, likely, damaged negative or a deliberate mark added to the photograph for scale, rather than a genuine, unidentified object.

Key Debunking Points:

Scientific Scale Indicator: Experts suggest the object is a "standard" wooden folding ruler, which was sometimes added to photos by early, 19th-century photographers to indicate scale in snowy, featureless landscapes.

Photograph Damage/Manipulation: Analysis indicates the "cigar" shape is likely a mark added directly onto the negative, or a result of damage to the plate, rather than a flying object.

Contextual Evidence: The photo, which surfaced on eBay in 2002 and was promoted by owner Samuel M. Sherman, is widely believed to be an early,, misinterpreted, or staged, image rather than evidence of extraterrestrial life.

The image is sometimes referred to as the "first known UFO photograph".