Page:Popular Science Monthly Volume 6.djvu/234

222 simple computation, just what part of a degree it went over in passing, or in its shadow's passing, from one eye to the other; the angle in other words, that the distance between his eyes would appear under as seen from the light. But this is the parallax of the light, and it gives him its distance at once (that between the eyes—the base-line—being known).

This suggests the principle of a method of obtaining the sun's parallax, on which the English astronomers will largely rely.

For, neglecting matters of detail, and supposing Venus to pass centrally across the sun, since she completes her revolution of 360° in 225 days, nearly, we find, on dividing 360° by the number of minutes in that period, that in one minute she moves through an arc of 4.", and dividing 360° by the number of minutes in our year, that the earth moves through 2”.46 in the same time. Hence, as Venus is gaining 1”.54 every minute, the case is the same as though the earth stood still, and the shadow of Venus (could she throw one so far) passed over the earth at that rate as seen from the sun.

Suppose an observer on the left or eastern side of the on be had his view of part of the left side of the sun intercepted by the interior planet at nine o'clock, and one placed opposite the centre of the globe (at half the earth's diameter west of the first), five and three-quarter minutes later, then, since 5¾ times 1”.54 is 8”.85, this angle 8”.85 represents the difference of directions in which the sun would be seen by the two observers, or, what is the same thing, the angle the earth's semi-diameter would fill to an eye at the sun. This is the solar parallax, and on reference to our tables we should find that such a difference of direction could only be caused by an object nearly 92,000,000 miles off. In practice, observers are not stationed at the extreme edge of the earth (as seen from the sun), because from such a station the sun itself would be seen in the horizon, where vision of it is obscured and rendered unsteady by the vapors of our atmosphere. Neither is it needful to place observers just half a diameter of the earth apart, since it is easy to allow for the effect of greater or less



distance, and, in reality, the time would be longer than that supposed, because Venus's path lies aslant to the sun's edge, and it takes her longer to cross it. But it will, of course, be understood that such matters as these, and such complications as arise from the elliptical form of the orbits, the real inequality of the motion, the fact of the earth's