Treatise on Light

OF LIGHT. CHAP. I.

[treat]ment of all subjects of Physics, and who certainly succeeded much better therein than anyone before him, has said nothing that is not full of difficulties, or even inconceivable, regarding Light and its properties. Huygens is likely referring to René Descartes, whose mechanical theory of light he both admired and sought to improve upon.

But what I was using only as a hypothesis has recently received the strong appearance of a constant truth through the ingenious demonstration by Mr. Rømer Ole Rømer (1644–1710), the first person to quantitatively measure the speed of light. which I am going to report here, while waiting for him to provide everything that should serve to confirm it. It is founded, like the previous one, on celestial observations, and proves not only that light employs time in its passage, but also shows how much time it employs, and that its speed is at least six times greater than the one I just mentioned.

A geometric diagram illustrating planetary orbits and the passage of light. A large circle at the bottom, labeled B, C, L, D, and E, represents the Earth's annual orbit around the Sun, which is labeled A at the center. Above it is Jupiter, labeled F, and the smaller circular orbit of one of its satellites, labeled G, N, and H. Two lines extend from F to create a shadow cone. Points G and H mark where the satellite enters and exits Jupiter's shadow. Several lines represent the path of light from these events to different observation points on Earth's orbit. Other labels include K and M on the vertical axis between the two orbits.

at least six times greater than that which I just mentioned.

He uses for this purpose the Eclipses suffered by the small Planets original: "petites Planetes." In the 17th century, the moons of Jupiter were often called "Medicean planets" before the term "satellite" became standard. which revolve around Jupiter, and which often enter its shadow; and here is his reasoning. Let A be the Sun, BCDE the annual orbit of the Earth, F Jupiter, GN the orbit of the nearest of its Satellites, for this one is more suitable for this research than any of the three others because of the speed of its revolution. Let G be this Satellite entering Jupiter's shadow, H the same exiting the shadow.

Suppose then that the Earth being at B, some time before the last quadrature Quadrature: A configuration where the Earth, Sun, and another planet form a right angle, marking a specific point in the planet's observational cycle., one has seen the said Satellite emerge from the shadow; it would be necessary, if the Earth remained in this same place, that after 42 and a half hours