A manual of spherical and practical astronomy Volume 2 ; embracing the general problems of spherical astronomy, the special applications to nautical ... instruments, with an appendix on the - Tapa blanda

Sinopsis

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. 1906 Excerpt: ...in Art. 22. But, with the aid of the telescope micrometer, we can avoid the wror of runs, as follows. In observing the nadir point, set the circle so that an exact division is under or nearly under the zero of one of the reading microscopes, that is, so that all the microscopes will read nearly 0": their mean will not require any sensible correction for runs. But the fixed thread will then not be in coincidence with its image. Measure the distance of the fixed thread from its image by the micrometer. One-half this distance, being applied to the circle reading, will give the reading for absolute coincidence. In like manner, in observing the star, set the circle again upon an exact division, and bisect the star with the micrometer thread; the distance of the micrometer thread from the fixed thread, being applied to the circle reading, will give the required reading C". But, when the micrometer is employed, it is altogether preferable to dispense with the fixed thread and to depend solely upon the movable one. Thus, to determine the nadir point, having brought the circle division which is nearest to the nadir point reading under microscope A, let the mean reading obtained from all the microscopes be called C0. Bring the micrometer thread into coincidence with its image, and let the micrometer reading be M0, which we shall suppose to be converted into arc by multiplying by the value of a revolution found according to Art. 46 or 47. It is now evident that when the telescope is directed upon a star, if the micrometer reading remains Mt while the thread bisects the star and the circle reading is C", the nadir distance is C--C„, precisely as if the micrometer thread were fixed. But the reading C will, in general, involve an error of runs, to av...

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