Page:Popular Science Monthly Volume 6.djvu/228

216 to later; we can leave no mark behind to denote the point in the void of space the earth has quitted. Our motion round the sun is therefore no help in finding its distance, and we may, in fact, for the sake of simplicity in illustration, treat the earth as standing still in its orbit, since the essential difficulty is thus nowise heightened. This difficulty, arising from the want of a proper base-line, is similar in degree and kind to that a surveyor would labor under, if he were called on to measure the distance of an object of unknown size at least half a mile away, without moving from his place. Success under such circumstances may well seem, not so much difficult as impossible; yet this is a fair simile of the apparent impracticability of measuring the distance of the sun without stepping beyond the limits of our little earth, a body so small by comparison with the sun's remoteness that, to an observer at that distance, a three-cent piece, held one hundred and fifty yards from the eye, would completely cover our globe and hide it from his view.

Within such narrow bounds we must work, or not work at all, and the reader, if he have not, from what he has just read, gained a definite conception of the principle on which all such distance measurement rests, may find aid in a very simple experiment. If any small object, such as a pencil, be held in front of the eyes as near as it can be conveniently seen, we may easily note the point on the opposite side of the room which it appears to cover, as viewed first by the right eye and then by the left. Though itself unmoved, it will appear to shift its place on the wall, when the latter is distant, in a notable degree, owing both to the difference of direction under which either eye views it, and the remoteness of the background, and the amount of this shifting will diminish progressively as it is carried directly away from the eyes, owing to its being now seen more nearly in the same direction by both, and to its approach to the wall. The change of direction due to the distance from the eyes only, but, this being constant, the amount of its displacement on the wall is due only to the distance of the latter, as is easily proved by walking toward it.

The distance of the wall might. conceivably be reckoned without going to it, by preparing tables which should show how this distance was proportioned to the apparent motion of the pencil on it, since one of these things evidently depends on the other, or which should tell the distance of the pencil, by the difference of direction under which we saw it. Such are the trigonometrical tables in common use, which give the distance when this change of direction and place is known. But this change as viewed by one eye or the other is the parallax of the pencil, the known distance between the eyes being a little "baseline," which plays the same part as the surveyor's longer one; and now, if we suppose ourselves in possession of tables which give the distance of any object, directly its parallax is known, we may substitute the earth for the head, two observers as far apart on it as they