Familiar talks on astronomy, with chapters on geography and navigaton . ion of a line drawntangent to the curve at the moment it entersthe eye. He therefore sees it too high. Inthe diagram (Fig. 13) let O be the centre ofthe earth, and A an observer on its B C represent the upper limit of the atmos-phere. Let S be any celestial body, and S Da ray of light from it which enters the earthsatmosphere at D; it will then be refracteddownwards, in a curve, until it reaches the eyeof the observer at A; and he will see it in thedirection of the line A S\ tangent to the curveat A, where it e
Familiar talks on astronomy, with chapters on geography and navigaton . ion of a line drawntangent to the curve at the moment it entersthe eye. He therefore sees it too high. Inthe diagram (Fig. 13) let O be the centre ofthe earth, and A an observer on its B C represent the upper limit of the atmos-phere. Let S be any celestial body, and S Da ray of light from it which enters the earthsatmosphere at D; it will then be refracteddownwards, in a curve, until it reaches the eyeof the observer at A; and he will see it in thedirection of the line A S\ tangent to the curveat A, where it enters the eye of the is, he sees it too high; hence the correc-tion for refraction must be subtracted from anobserved altitude. Refraction, you observe, de-pends upon the altitude of a heavenly body; itis greatest when the object is in the horizon,and nothing when it is in the zenith. Tables of Refractio7i. 223 refraction are calculated for all altitudes, fromo° to 900, and inserted in works on is a small quantity. The refraction for o°,. Fig. 13. that is, when the body is on the horizon, is33; and for 900 it is zero. As the apparent,1 or angular diameter of thesun is about 33, the effect of refraction is to 1 The apparent enlargement of the sun and moon in the hori-zon arises from an optical illusion. These bodies, in fact, arenot seen under so great an angle when in the horizon as whenon the meridian, for they are nearer to us in the latter casethan in the former. The distance of the sun is, indeed, so greatthat it makes very little difference in his apparent diameterwhether he is viewed in the horizon or on the meridian ; butwith the moon the case is otherwise; its angular diameterwhen measured with instruments is perceptibly larger at thetime of its culmination. (Olmsted.) Yet, like the sun, seems larger at his setting. (Robert Blair, 1700.) 224 Familiar Talks on Astronomy, etc. elevate it by just its diameter when it is risingor setting; so th
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