The elements of astronomy; a textbook . about the earths orbit itself, exceptthat it all lies in one plane passing through the sun. It tells usnothing as to the orbits real form and size. By reducing the observations of the suns right ascensionand declination through the year to longitude and latitude(the latitude would always be exactly zero except for someslight perturbations), and combining these data with observa-tions of the suns ap-parent diameter, we can,however, ascertain theform of the earths orbitand the law of its motionin this orbit. The sizeof the orbit—its scale ofmiles — cannot
The elements of astronomy; a textbook . about the earths orbit itself, exceptthat it all lies in one plane passing through the sun. It tells usnothing as to the orbits real form and size. By reducing the observations of the suns right ascensionand declination through the year to longitude and latitude(the latitude would always be exactly zero except for someslight perturbations), and combining these data with observa-tions of the suns ap-parent diameter, we can,however, ascertain theform of the earths orbitand the law of its motionin this orbit. The sizeof the orbit—its scale ofmiles — cannot be fixeduntil we find the sunsdistance. 116. To find the Formof the Orbit, we proceedthus: Take a point, 8,for the sun, and drawthrough it a line, S°f (Fig. 21), directed towards the vernal equi-nox, from which longitudes are measured. Lay off from Slines indefinite in length, making angles with S°f equal to theearths longitude as seen from the sun1 on each of the days 1 This is 180° + the suns longitude as seen from the Fig. of the Form of the Earths Orbit. §116] DEFINITIONS OF ORBITAL ELLIPSE. 71 when observations were made. We shall thus get a sort of spider, showing the direction of the earth as seen from thesun on each of those days. Next as to the distances. While the apparent diameter ofthe sun does not tell us its absolute distance from the earth,unless we know this diameter in miles, yet the changes in theapparent diameter do inform us as to the relative distance atdifferent times, the distance being inversely proportional tothe suns apparent diameter (Art. 12). If then on this spider we lay off distances equal to the quotient obtained by dividingsome constant, say 10000, by the suns apparent diameterat each date, these distances will be proportional to the truedistance of the earth from the sun, and the curve joining thepoints thus obtained will be a true map of the earths orbit,though without any scale of miles. When the operation isp
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