LIGHT, Velocity of, the rate at which light is propagated. This may be found either from astronomical observations or by direct terrestrial measurements. Of the former there are two methods, the first by observations of the satellites of Jupiter and the second by measurement of what is known as the "aberra of the stars. The satellites of Jupiter revolve regularly around theplanet ; when the earth is farthest away from Jupiter there is a delay of 16 minutes, 38 seconds in the times at which they are seen to enter the planet's shadow, as compared with the corresponding times when the earth is nearest Jupiter. Thus it is found that it takes light 16 minutes, 38 sec onds to pass across the earth's orbit, a distance of 186,000,000 miles: the velocity thus found is 186,330 miles a second. As the passage of a moon into the planet's shadow is, however, a gradual phenomenon, the true instant of its disappearance cannot be accurately determined. This method of determining the velocity of light is hence not susceptible of accuracy; it has, however, a high historic interest, since it was in this way that Roemer, in 1675, first dis covered that light requires a definite time to pass through space.
The second astronomical method depends upon the aberration of the fixed stars. It can be shown that if the transmission of light is not instantaneous, then when a star is viewed from our moving earth it will appear constantly dis placed by a small amount from its true position. This displacement will be a forward one in the plane in which the direction of our own motion at the instant lies and which also contains the ray of light from the star; the amount of the displacement varies as the quotient of the observer's velocity divided by the velocity of light. The total effect of all displacements as the earth passes around its orbit is that each star appears to describe a small orbit in the course of a year about its true place. The mo tion of the earth in its orbit carries the spec tator at one time so as to increase the relative velocity of the light from the star and at an other time so as to diminish this velocity. The
maximum displacement of a star due to this cause is the so-called ((constant of aberration)); its value is 20.47". The velocity of light corre sponding to this is about 186,000 miles a second.
The most accurate method of directly meas uring the velocity of light on the surface of the earth is by causing a ray to be reflected from a very rapidly rotating mirror to a dis tant fixed mirror and finally back to the first mirror again. If the time occupied by the ray in passing from the first to the second mirror and back again is sufficiently long so that the first (revolving) mirror has taken a new direc tion, the ray after its last reflection will take a different course than it would have done had the first mirror been at rest. From the known velocity of rotation of the mirror, and the measured amount of the deflection, the time taken by the ray in passing twice over the course which separates the two -mirrors can readily he computed. In this manner Newcomb, in 1882, determined the velocity of light in air to be 186,273 miles a second. From theoretical considerations it is known that the velocity in a vacuum should be slightly greater, and upon applying a correction for this cause the final value of 186,324 (in a vacuum) resulted. An equally accurate determination afterward made by Michelson led to the value, 186,320. The uncertainty of these values hardly can exceed 15 or 18 miles a second.
It is a matter of importance and interest to ascertain whether the above value of the veloc ity for white light also represents the velocity of transmission of light waves of different colors. But this question is readily answered by a consideration of the behavior of eclipsing variable stars. Even with such stars so dis tant that their light requires from 30 to 100 years to come to us, we find that rays of all colors reach us simultaneously upon the eclips ing body being removed. It is thus certain that to a very high order of approximation the velocities of transmission of light waves of all different wave lengths are equal.