Celestial Distances

Scientific American

Nothing in all the realm of astronomy impresses the imagination more than its enormous distances. Indeed, they are so vast that it is hardly credible that they can be measured at all.

The principle used by astronomers to determine them is exactly that of the range-finders employed by modern gunners. Bearings are taken on the distant target from two points as far apart as is practicable. Knowing the distance between these points, the distance of the target can be calculated from the difference of its bearing at the two points.

When we come to apply this method to the stars, we find that the whole diameter of the earth is far too short a base-line. Fortunately, we have a longer one available, the diameter of the earth's orbit. By taking observations of a star at properly chosen dates, six months apart, we have a base 186,000,000 miles long, and can proceed with some chance of success.

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The Lectures
February 12, 1903

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