SPACE: MEASUREMENT OF:

The measurement of space is always a difficult problem, even for near-by objects (see my “Psychology for Beginners”). When applied to celestial bodies, it becomes immensely complicated, and the only wonder is that such apparently accurate measurements have in fact been made! Such measurements cannot, of course, ever be made directly, but must depend upon trigonometry and abstruse mathematical calculations. Most of them are based upon the following principles: If we observe a distant object from two different points-of-view, at a known distance apart, the angle formed by imaginary lines running from the object to one position, and to the other, can readily be calculated. Knowing this angle, much can be ascertained as to the size, distance, etc., of the distant body. If a distant star be viewed from opposite sides of the earth, we have here a known base-line of slightly more than 8,000 miles. But this is altogether too small for astronomical distances! A much longer base-line must be sought. Accordingly, observations are made of a distant star when the earth is (so to say) “north” of the sun, and further observations of the same star when the earth is (so to say) “south” of it—six months later, when the earth has traveled half-way through its orbit round the sun. The diameter of the earth’s orbit being known (186,000,000 miles, almost) we have here a base-line of this size for use in our measurement of the angle and subsequent calculations. Immense as this base-line is, however, it is too small for our purposes, for so immense are astronomical distances, that no change whatever can be observed in the relative positions of certain fixed stars—even when studied from such different positions in space! In other words, the star is so far distant that, when viewed from two positions in space, distant from one another nearly one hundred and eighty-six million miles, it appears to occupy the same position! But a mere summary of this question, and its details would involve an entire volume in itself!