Time Determinations 



The change in the sun's position with respect to the stars as 

 already noted, takes place along the ecliptic which is inclined to the 

 equator at an angle of about 23 27', intersecting it at the vernal 

 and the autumnal equinox. The result is that any point on 

 the surface of the earth makes one less annual passage under the sun 

 than under a given star. The number of sidereal days in a year is 

 therefore one more than the number of solar days to which we are 

 accustomed; or a sidereal unit of time is of shorter duration than a 

 solar unit in the ratio of 3 min. 56.55 sec. per diem, mean time. 



Sidereal and Mean Solar Time are identical at Greenwich 

 noon, Mar. 22nd, each year. To roughly translate solar to 

 sidereal time, add 2 hrs. per mo. thereafter and allow 4 min. per 

 diem, thus: on June 7th at noon we have 2 full months and 16 

 extra days or 5:04 p. m. 



O\\.ng to the irregularity, of the sun's motion, the variable 

 length of apparent solar days are equalized for our convenience by 

 the adoption of a uniform system of time measurement called: 



Mean Solar Time 



The zero point of this system is an imaginary body called the 

 "mean sun" which is supposed to move uniformly along the equator 

 keeping as nearly in the same right ascension with the actual 

 position of the sun as is consistent with perfect uniformity of 

 motion. * 



The mean solar time is the hour angle, or the right ascension 

 of the imaginary mean sun and differs numerically by the difference 

 of the hour angles or the Equation of Time. 



To Convert Apparent Time into Mean Time, consult 

 the ephemeris tables and select the tabular correction for the day of 

 the month upon which the observation is made. Where the column 

 is headed, "substract from mean time" it should also be con- 

 strued at "add to apparent time" and vice 



To Obtain Time with the Solar Attachment 



When an observation for time is to be made, proceed with the 

 meridian determination as previously direcu-il, but at the instant the 

 sun is finally centered in the (clamped) solar telescope, note the 

 local time to the nearest second on some reliable time piece. 



'Consult "Field Astronomy tor Enzinrfrs" Geo. C. Con>stotk, Tw/'r. <>( 

 /r/.v., 1910. p. 39. etc. 



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