An Andromeda plate captures a future clue

Observers at Sobral and Príncipe took eclipse images to test a prediction of general relativity.

The Brazilian observing site recorded stars whose apparent positions could be compared with earlier images.

Observations from the West African island provided an independent route to the same scientific test.

With the Sun obscured, stars close to its edge became visible; interpretation of the plates came afterward.

On the night of 5–6 October Hubble photographed a spiral arm of Andromeda.

The 100-inch telescope recorded faint points in Andromeda that could be compared across nights.

The photographic plate later helped establish that Andromeda lies outside the Milky Way; this conclusion was later, not on the exposure date.

Clyde Tombaugh found a moving point in the photographic plates he had been comparing.

By comparing images of the same star field, Tombaugh detected the movement of the object later named Pluto.

The discovery itself took place on 18 February; public announcement and naming came later.