An Introduction to Practical Astronomy: With a Collection of Astronomical Tables

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Harper & Bros., 1855 - Spherical astronomy - 498 pages
 

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Page 99 - ... to the left of zero, and looking towards the sun, the two images will appear either nearly in contact or overlapping each other; then perfect the contact, by moving the tangent-screw, and call the minutes and seconds denoted by the vernier, the reading on the arc. Next place the index about the same quantity to the right of zero, or on the arc of excess, and make the contact of the two images perfect as before, and call the minutes and seconds on the arc of excess* the reading off...
Page 99 - ... to make the objects appear on the other wire ; if the contact still remains perfect, the axis of the telescope is in proper adjustment ; if not, it must be altered by moving the two screws which fasten, to the up-and-down piece, the collar into which the telescope screws. This adjustment is not very liable to be deranged.
Page 95 - MM, is connected with the horizontal axis in a manner similar to that of the transit instrument. Upon the axis, as a centre, is fixed the double circle NN, each circle being placed close against the telescope. The circles are fastened together by small brass pillars, and the graduation is made on a narrow ring of silver, inlaid on one of the sides, which is usually termed the face of the instrument. The reading microscopes, AB, for the vertical circle, are carried by two arms, PP, attached near the...
Page 474 - When water is heated, the elastic force of the vapor produced from it gradually increases until it becomes equal to the incumbent weight of the atmosphere. Then, the pressure of the atmosphere being overcome, the steam escapes rapidly in large bubbles, and the water boils.
Page 101 - At the same time, move the sextant slowly, making the axis of the telescope the centre of motion ; by which means the objects will pass each other, and the contact be more...
Page 48 - Direct the telescope to some small distant well-defined object, (the more distant the better,) and bisect it with the middle of the central vertical wire; then lift the telescope very carefully out of its angular bearings, or Y's, and replace it with the axis reversed ; point the telescope again to the same object, and if it be still bisected, the collimation adjustment is correct; if not, move the wires one half the error, by turning the small screws which hold the diaphragm near the eye-end of...
Page 121 - Astronomers, with a view of obtaining a convenient and uniform measure of time, have recourse to a mean solar day, the length of which is equal to the mean or average of all the apparent solar days in a year. An imaginary Sun, called the mean Sun...
Page 141 - A cos 6 = cos a cos c + sin a sin c cos B cos c = cos a cos 6 + sin a sin 6 cos C Law of Cosines for Angles cos A = — cos B...
Page 98 - ... equal to the error itself; therefore, in modern instruments, there are seldom any means applied for its correction, it being considered preferable to determine its amount previous to observing or immediately after, and apply it with its proper sign to each observation. The amount of the index error may be found in the following manner; clamp the index at about 30...
Page 98 - ... be moved in a contrary direction to that which the object appears to take, in order to keep it in the field of view. Four dark glasses, of different depths of shade and color, are placed at K, between the index and horizon glasses ; also three more at N, any one or more of which can be turned down to moderate the intensity of the light, before reaching the eye, when a very luminous object (as the sun) is observed. The same purpose is effected by fixing a dark glass to the eye-end of the telescope:...

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