How Are the Stars Counted?
Stars are counted by means of the telescope and photography. The Astronomer-Royal for Ireland, Sir Robert S. Ball, in one of his lectures mentioned a photograph which had been obtained by Mr. Isaac Roberts representing a small part of the constellation of the Swan. The picture is about as large as the page of a copy-book, and it is so crowded with stars that it would puzzle most people to count them; but they have been counted by a patient person, and the number is about 16,000. Many of these stars are too faint ever to be seen in the greatest of telescopes yet erected. Attempts are now being made to obtain a number of similar photographs which shall cover the whole extent of the heavens. The task is indeed an immense one. Assuming the plates used to be the same size as that above mentioned, it would require at least 10,000 of them to represent the entire sky. The counting of stars by the telescope was first reduced to a system by the Herschels, who introduced “star-gauges,” which were simply a calculation by averages. A telescope of 18 in. aperture, 20 ft. focus, and a magnifying power of 180, giving a field of view 15 in. in diameter, was used for the purpose. The process consisted in directing this instrument to a part of the sky and counting the stars in the field. This, repeated hundreds of times, gave a fair idea of the average number of stars in a circle of 15 in. diameter in all parts of the sky. From this as a basis it is possible to reckon the number of stars in any known area.