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Boys' Book of Astronomy


This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1914 Excerpt: ...there is no doubt it is entirely owing to this addition to the observatory that so many have been found. Before the photographic plate was invented the astronomer who would hunt for minor planets had first to carefully construct maps of all the stars visible in the particular zone which he decided to explore. This in itself was a most laborious task, as was also the methodical comparison of the maps with the actual stars as seen with a telescope, night after night. If on any night an object was seen which was not charted, it might or might not be a minor planet, but only by long and tedious comparison was it possible to make such a discovery. Nowadays, however, all this is done away with, for, as we have already explained, a minor planet has a proper motion among the stars. When therefore a photograph is taken of the stars, instead of coming out on the negative as a round dot--as the stars themselves do when the telescope is correctly adjusted--the minor planets leave a short trail or line (see Fig. 21). Thus their detection is a comparatively easy matter, resolving itself only Fig. 21.--Diagram showing the Difference between the Circular Images of the Stars and the "Trail" of an Asteroid as recorded by the Photographic Plate. into ascertaining whether or not the trail is that of an asteroid which has already been observed, or whether it is that of an entirely new minor planet. When it was evident that a large number of these tiny planets existed, it became the practice to distinguish them by a letter of the alphabet. Soon after this plan was agreed upon all the letters of the alphabet were used up, and so it was arranged to commence the alphabet again for any new asteroids which might be found. While the first series were known as A, B, C, etc., ... Hide synopsis

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