Treatise on Light

...and a half hours, one would see another such emergence; because this is the time in which it completes the circuit of its orbit, and returns to opposition with the Sun. And if the Earth remained always at B during 30 revolutions, for example, of this satellite, it would see it emerge from the shadow again after 30 times 42 and a half hours. But the Earth having moved during this time to C, while moving further away from Jupiter, it follows that if light takes time for its passage, the illumination of the little planet Huygens refers to Jupiter's moon, likely Io, as a "little planet." will be perceived later at C than it would have been at B; and that one must add, to this time of 30 times 42 and a half hours, also that which the light employs to cross the space MC, the difference of the distances CH and BH. Likewise, toward the other quadrature A point in a planet's orbit where it appears 90 degrees away from the Sun as seen from Earth. when the Earth has come from D to E, while approaching Jupiter, the immersions of the satellite G into the shadow must be observed earlier at E than they would have appeared if the Earth had remained at D.

Now, by a quantity of observations of these eclipses, made during ten consecutive years, these differences were found to be very significant, such as ten minutes or more; and it was concluded that to cross the entire diameter of the annual orbit KL The "annual orbit" refers to the diameter of Earth's orbit around the Sun., which is double the distance from here to the Sun, light? needs about 22 minutes of time. Rømer’s original estimate of 22 minutes is slightly higher than the modern measurement of approximately 16.7 minutes for light to cross Earth's orbital diameter.

The motion of Jupiter in its orbit, while the Earth passes from B to C or from D to E, is included in this calculation; and it is shown that one cannot attribute the delay of these illuminations, nor the anticipation of the eclipses, to the irregularity found in the motion of this little planet, nor to its eccentricity.

If one considers the vast extent of the diameter KL, which in my view is some 24 thousand diameters of the Earth, one will recognize the extreme speed of light. For supposing that KL is only 22 thousand of these diameters, it appears that—