THE CONSTANT OF ABERRATION. From what has been said above it may be seen that the quantity of apparent displacement depends on the velocities both of light and of the earth. The nature of that dependence is quite sim ple: the velocity of light is known in miles per second from laboratory experiments; the amount of possible aberration, while inversely proportional to the velocity of light, is large in proportion to the earth's speed. If, there fore, we could determine by direct observation of the stars just how much they are displaced, it would be possible to calculate the earth's orbital velocity from the size of the aberration. The aberration may be determined by the simple method of observing a star at intervals during. the year and noting how much its position changes. If we select a star most favorably situated for this purpose, we find that its position throughout the year will vary from the average by a little more than twenty circular seconds. This number (more exactly 20".47)
is called the constant of aberration. To meas ure this constant with the utmost possible pre cision has long been the object of very earnest efforts; and few other astronomical problems have received so much attention in recent years. Its particular importanCe, as we have seen, is clue to the computations rendered possible by a knowledge of the constant. Combined with the known velocity of light, it gives us the earth's orbital velocity in miles per second. From this we get the length of the annual terrestrial orbit in miles, and then by a simple calculation we find its semi-diameter, or the distance from the earth to the sun. This last is the funda mental unit for astronomical measures of dis tance, and its exact evaluation is considered the most important of all astronomical problems. See PARALLAX, section Solar Parallax; SUN.