Enjoy this film of a special box, custom made for the Wenner Collection! This is a lovely tricksy book-shaped enclosure that Wenner titled Speed of Light - Roemer. It features an article in the tiny Journal des sçavans, “Démonstration touchant le mouvement de la lumière” by Danish astronomer Ole Roemer on the subject, published in 1676. The other item in the box is a monograph about Ole Roemer by Bernard Cohen called Roemer’s Basis for the First Estimate of the Speed of Light, published in 1942.
The white card is the first thing that I picked out of the box. Many items in the collection contain these white bookmarks; they are written by David Wenner and describe the significance of the items in the box, publication information, their connection to other items in the Collection, and sometimes provenance information. Here is an excerpt from the bookmark in this video:
"Scientists and philosophers believed that the speed of light is infinite until the late 17th century. Early that century, Johannes Kepler stated that the speed of light is infinite since empty space presents no obstable to it. René Descartes also believed that the speed of light is infinite, but for the opposite reason that he did not believe there is a void. Galileo Galilei proposed to measure the speed of light by observing the delay between uncovering a lantern and its observation some distance away, but if he ever conducted the experiment it was inconclusive.
The belief in infinite light speed became a problem in maritime celestial navigation when the timing of the occultations of the moons of Jupiter, especially Io, led to errors in navigation. In 1668, Giovanni Cassini observed that the timing of these occultations depended on the position of Earth relative to Jupiter, and considered but ultimately rejected the notion that a finite light speed could be a cause.
Danish astronomer Ole Roemer, Cassini's assistant, used Cassini's data and additional observations of his own to determine the relationship between Earth's distance from Jupiter and the timing of Io's occultations. He used his measurements to estimate the time light took to traverse the diameter of Earth's orbit to be approximately 22 minutes. Roemer presented his results to the French Academy of Sciences, and it was summarized in December 1676 by a reporter in a short paper in this volume of Journal des sçavans: “Démonstration touchant le mouvement de la lumière” (Amsterdam printing of Journal des sçavans pp. 276-279, 7 December 1676)." - David Wenner