Elements of astronomy ..with explanatory notes, and questions for examination . the effects of refraction andparallax in the above particulars 1 Why is the knowledge of parallax important to theastronomer ? 66 THE EARTH VIEWED ASTRONOMICALLY. CHAPTER V. OP THE MEASUREMENT OF TIME. 101. Transit Instrument. Having now acquireda knowledge of the circles of the celestial sphere, andthe manner of fixing the positions of celestial bodies inthe sky, we are prepared to investigate more minutelythe rotation of the earth on its axis. We have discov-ered the fact of the rotation, but have not yet ascerta
Elements of astronomy ..with explanatory notes, and questions for examination . the effects of refraction andparallax in the above particulars 1 Why is the knowledge of parallax important to theastronomer ? 66 THE EARTH VIEWED ASTRONOMICALLY. CHAPTER V. OP THE MEASUREMENT OF TIME. 101. Transit Instrument. Having now acquireda knowledge of the circles of the celestial sphere, andthe manner of fixing the positions of celestial bodies inthe sky, we are prepared to investigate more minutelythe rotation of the earth on its axis. We have discov-ered the fact of the rotation, but have not yet ascertainedwhether the earth moves faster at one time than at an-other. This point, however, is readily ascertained by the CMP. Taking the figure as drawn, we have first, the latitudes of the twostations which gives us the angle PCP1, consequently in the isosceles tri-angle PJCP we have the two lines PC, P2C, each a radius of the earth, andthe included angle to find the other angles and the side PP1. Now thezenith distances of the moon, as seen from both stations, can be measured $. they are the angles ZPL and Z-P1!.1; therefore we know their supplementsto wit, LPC and L1P1C. Taking away from these respectively, the anglesCPP1 and CP*P, we have remaining the angles LPP1 and LPT. Con-sequently in the triangle MPT we have the side PP1, and all the angles tofind the other two sides MP and MP1. Now taking the triangle MPC wehave the side MP (just found) CP a radius of the earth, and the angle MPC the supplement of the moons zenith distance, to find the other partsone of which namely CMP is the parallax. In this manner the parallaxof Mars, was obtained by Lacaille and Wargentin, the former being sta-tioned at the Cape of Good Hope, the latter at Stockholm. If the parallaxat any altitude is obtained, the horizontal parallax can be derived from it;the parallax varying as the sine of the zenith distances. For instance, thesine of MPZ : the parallax CMP : : the sine of ninety degrees : the zen
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