An Explanation of the Gnomonic Projection of the Sphere; And of Such Points of Astronomy as Are Most Necessary in the Use of Astronomical Maps. Being a Description of the Construction and Use of the Larger and Smaller Maps of the Stars as Also of the Si (Paperback)

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. 1836 Excerpt: ...produces an alteration of the apparent direction of the rays of light which meet the eye; being a phenomenon of the same kind as the alteration of the apparent direction of a ship's course, when seen from another ship which is in motion. If we suppose the star to move in consequence of precession and nutation, as in the second supposition of the preceding descriptions, the effect may be represented as follows. Instead of supposing the star to describe the waving curve alluded to along its parallel of latitude, let the centre of the ellipse of aberration describe that curve. Let the ellipse always present its flat part to the pole of the ecliptic, and let the star describe the ellipse in the course of a year. Refraction is the effect of the atmosphere upon the rays of light, by which they are bent a little towards the earth, in consequence of which the star appears somewhat higher than it really is. At the horizon, where it is greatest, it is somewhat more than half a degree; at the zenith it is nothing; and of any two stars, that is the most altered by refraction which has the least altitude. Owing to the difficulty of laying down the horizon on our maps, it is not easy to represent the effect of refraction upon them. The preceding complicated motions cannot be well understood by any but a mathematician; it is evident that the three, precession, nutation, and aberration, will sometimes conspire, or each increase the effect of the other, and sometimes destroy each other's effects either wholly or partially. The following figures show the species of waving curve in which four different stars describe their parallels of latitude. The first is that of S Orionis, the second that of a Leonis or Regulus, the third that of the Pole star, and the fourth that of 7 Dr...