. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. enmade at the centre of the sun. Fig. 21, and the formulae obtained from it, will apply equally well to this case, by makingthe necessary changes in the description of the figure and inthe names of the angles. Let S be still a celestial body, but let C be the centre of thesun, and L that of the earth. The angle p will then repre-sent the heliocentric parallax, and the angle 8LG the angulardistance of the body from the sun, as measured from theearths centre, or, as it is called, the bodys elongation. The
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. enmade at the centre of the sun. Fig. 21, and the formulae obtained from it, will apply equally well to this case, by makingthe necessary changes in the description of the figure and inthe names of the angles. Let S be still a celestial body, but let C be the centre of thesun, and L that of the earth. The angle p will then repre-sent the heliocentric parallax, and the angle 8LG the angulardistance of the body from the sun, as measured from theearths centre, or, as it is called, the bodys elongation. Theangle P will be the greatest value of the heliocentric parallax,taken when the bodys elongation from the sun is 90°, and iscalled the annual parallax. We shall then find, from the for-mulae of Art. 54, that the animal parallax has for its sine theratio of the distance of the earth from the sun to that of thebody from the sun, and that the parallax for any other posi-tion is the product of the annual parallax by the sine of thebodys elongation. DIP OF THE HORIZON. 57 DIP OF THE 57. The dip of the horizon is the angular depression of thevisible horizon below the celestial £. Dhorizon. In Fig. 22, let EG be aportion of the earths surface, and 0the earths centre. Let a radius ofthe earth, CA, be prolonged to somepoint, D, beyond the surface, and letan observer be supposed to be at thepoint D. At the point D let the lineBD be drawn perpendicular to theline CD, and also the line DH, tan- gent to the earths surface at some point H. If these twolines be revolved about the line CD, DB will generate theplane of the celestial horizon (since we have seen that aljplanes passed perpendicular to the radius will, when indefi-nitely extended, mark out the same great circle on the celestialsphere), and DH will generate the surface of a cone, which willtouch the earth in a small circle. If we disregard for the pres-ent the effect of the earths atmosphere, this small circle wil
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectastrono, bookyear1901
