The elements of astronomy; a textbook . Fig. 121. forIn 493. Method of Calculating the Local Time from the SunsAltitude (supplementary to Arts. 60 and 69).—In Fig. 122 thecircle NZM is the meridian, P being the pole, Z the zenith, and 364 APPENDIX. [§493 EQ the equator. S is the sun, whose altitude SH has beenmeasured. This altitude (properly corrected for semi-diameter,dip, refraction, and parallax), and subtracted from 90°, gives ZS, the suns zenithdistance, as one side ofthe triangle. The sec-ond side ZP is the com-plement of NP, whichis the observers lati-tude. Finally, sinceAS is the decl
The elements of astronomy; a textbook . Fig. 121. forIn 493. Method of Calculating the Local Time from the SunsAltitude (supplementary to Arts. 60 and 69).—In Fig. 122 thecircle NZM is the meridian, P being the pole, Z the zenith, and 364 APPENDIX. [§493 EQ the equator. S is the sun, whose altitude SH has beenmeasured. This altitude (properly corrected for semi-diameter,dip, refraction, and parallax), and subtracted from 90°, gives ZS, the suns zenithdistance, as one side ofthe triangle. The sec-ond side ZP is the com-plement of NP, whichis the observers lati-tude. Finally, sinceAS is the declinationof the sun (given in thealmanac) the third sidePS is the complementof the declination. We, therefore, know the three sides of thespherical triangle, ZPS, and can find either of its angles. Theangle at P is the one we want, — the suns hour-angle (); , the apparent time. The trigonometrical formula ordinarily used in computing it is. Determination of Time by the Suns Altitude. sin J P = Jrin*r«+(«-»)1ain*r»-(»-8n,V cos cos 8 in which z is the suns zenith distance, 8 its declination, and thelatitude of the observer. We may add that the angle PZS is thesuns azimuth at the time of the observation. The third angle, ZSP,is called the parallactic angle for reasons we cannot here stop toexplain. The observation should be made not near noon, but when the sunis as near to the prime vertical (Art. 17) as possible, because when theangle at Z is nearly 90° any uncertainty in the side PZ (which de-pends on the ships latitude) will produce the least possible error incomputing the hour-angle. The apparent time, corrected for the equation of time (whichis given in the almanac), gives the local mean time, and the 493] THEORY OF THE FOUCAULT PENDULUM. 365 difference between this local time and the Greenwich time(furnished by the chronometer) is the longitude. 494. Theory of the Foucault Pendulum (supplemen
Size: 2046px × 1222px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjec, booksubjectastronomy
