Lunar and Horary Tables, for New and Concise Methods of Performing the Calculations Necessary for Ascertaining the Longitude by Lunar Observations, or Chronometers; With an Appendix, Containing Directions for Acquiring a Knowledge of the Principal

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. 1831 Excerpt: ...will appear 4000 miles, for 240,000 + 4000 = 60; and as the Moon's semidiameter, when at her mean distance from the Earth, is about 15' 42," the greatest augmentation of the Moon's semidiameter will in this case be 15" 42"', that is, one 60th of 15' 42." When the Moon is nearer to the Earth than her mean distance, the greatest augmentation of semidiameter exceeds 15" 4!i'," and the contrary is the case, when the Moon's distance is greater than her mean distance from the Earth. When the Moon is in the horizon of any place, her distance from that place is so nearly the same as the distance from the Earth's centre, that the augmentation is insensible. Supposing the Moon's distance from the earth to remain the same, as her altitude increases she approaches an observer, and therefore, at any altitude between 0 and 90, the augmentation will be between 0," and the Moon's greatest augmentation of semidiameter. TABLE V. Contraction of Semidiameter of the Sun or Moon. As the effect of refraction is greater on the lower limb of the Sun, or Moon, than on the upper limb, the apparent vertical diameter is always less than the apparent horizontal diameter, in a quantity equal to tie difference of the refractions on the upper and lower limbs. Thus, if the observed altitude of the Sun's lower limb be 3 0', when nis diameter, by the Nautical Almanac, is 32', the observed altitude of the upper limb will not be 3" 32', but only 3 30' 30," because the refraction on an object, when it is apparently 3 above the horizon, is 14'36"; but when the apparent altitude of an object is 3 0, the refraction is only 13' 6," in this case, therefore, the horizontal diameter of the Sun would be greater than the vertical diameter by 1'...