Nautical Tables, Revised by J.W. Inman. with Navigation and Nautical Astronomy, Revised by J.W. Inman

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. 1872 Excerpt: ...E., the sun's amplitude at rising was by compass E. 12 40' S: required the variation. (1) (2.) Sh. Oct. 13.. 5" 30TM Sh. Aug. 23.. 17" 30TM Long, in T... 3 21 W. Long, in T... 10 2 E. Var. 20 22 E. Var. 31 5 15 W. In the above rule, the true altitude of the sun's centre is supposed to be 0, or its true zenith distance 90. When this is the case, the apparent altitude of the sun's centre must be nearly 0 34'; since it is raised so much by the effect of refraction, while parallax, which on the other hand depresses it, is inconsiderable. The proper time therefore for taking the amplitude of the sun by the compass at sea is when its centre appears to be raised above the horizon about 34', that is, a space equal to the diameter of the sun; or, what may be a better direction, when its lower edge appears to be half way between the horizon and centre. If a fixed star or a planet be observed, its bearing must be taken when it appears about 34', or half a degree above the horizon, or an app. diameter of the sun. With respect to the moon, the effect of horizontal parallax is to depress it about two of its diameters, whilst refraction raises it only one. This body therefore cannot be seen, when it is truly 90 from the zenith, or its true altitude 0. In this case the bearing should be taken, as soon as sufficient of the moon's disk is perceptible to allow of its being bisected by the thread of the azimuth-compass. This will be quite correct enough for all the purposes for which the variation is wanted at sea. The amplitude of the sun at rising and setting, and also the time at which it rises or sets, may be taken by inspection from a table constructed for that purpose (see Nautical Tables, p. 373). It is entered with the declination at the top, and the latitude...