Treatise on Light

OF LIGHT. CHAPTER I.

annual path of the Earth. Let A B C be a straight line, which I suppose meets the path of the Moon, represented by the circle C D, at point C.

A geometric diagram showing a horizontal line extending from point A on the left, through point B, to point C on the right circumference of a circle. A point D sits at the top of the circle, connected to point B by a vertical line segment. A second straight line originates from point A, passing through the circle at point E (where it intersects the line segment DB) and point G (on the upper right circumference).

Now, if light requires time—for example, one hour—to cross the space between the Earth and the Moon; it will follow that when the Earth has reached B, the shadow it causes, or the interruption of light, will not yet have reached point C, but will only arrive there one hour later. It will therefore be one hour later, counting from the time the Earth was at B, that the Moon, arriving at C, will be darkened there. But this darkening or interruption of light will not reach the Earth for another hour. Let us suppose that in these two hours [the Earth] has reached E. The Earth, therefore, being at E, will see the Moon eclipsed at C, from which the light departed an hour before, and will see at the same time the Sun at A. For [the Sun] being immobile, as I suppose along with CopernicusNicolaus Copernicus (1473–1543) proposed the heliocentric model where the Sun is the center of the universe and the Earth revolves around it., and light extending in straight lines, it must always appear where it is. But it has always been observed, they say, that the eclipsed Moon appears at the place on the EclipticThe Ecliptic is the apparent path of the Sun across the sky; for an eclipse to occur, the Moon must be exactly opposite the Sun on this path. opposite the Sun; and yet here it would appear behind that place, by the angle G E C, the complementIn this context, the geometric difference required to make the lines equal to 180 degrees (two right angles). of A E C to two right angles. Therefore, this is contrary to