A Laboratory Manual in Astronomy (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1899 edition. Excerpt: ...owing in part to accident. (A. E. T.) 113. Time at any hour of the day.--The shadow on the sundial gives apparent time at any hour of the day, and theoretically considered time may be derived from any altitude or any altitude and azimuth of the sun. We find in practice, however, that these angles give more accurate results if measured at certain hours and dates; since the effect of an error in altitude or azimuth varies largely according to the value of declination and zenith distance (see Obs. 1), or according to declination and azimuth. Observation 1.--S. C. O., Friday, Oct. 9, 1896. A little before the sun set, four measures of its altitude were made with the Circles, giving a mean value of 4 51', or 4 41', when corrected for refraction (Chauvenet, Table I). According, then, to the method given in full in Section 108, we find the sun's hour-angle to be 5h 9m 288, whence standard time is obtained as follows: Sun's hour-angle = apparent time = 5h 9m 28s Equation of time = 12 59 Local mean time = 4 56 29 Northampton east of 75th meridian = 9 27 Standard time = 4 47 2 The true standard time derived from the Ephemeris and sidereal clock was 388 earlier. By referring to the Formula ( 40 (6)) employed in finding the hour-angle above, we see that when the second angle of the numerator, ---, is small, a slight variation in the zenith distance appears with enlarged effect in the final time. Thus, in the present example, although--is 18, a change of 5' in, either way, changes the time by nearly half a minute. Observation 2.--S. C. O., Saturday, June 19,1897. This afternoon Circles No. 3, having been placed on the second meridian stone on the south, were brought accurately into the meridian with the help of a small carpenter's square...