The elements of astronomy; a textbook . is faster or slower than the time it ought to indicate. The method ordinarily em-ployed by astronomers is bymeans of the Transit Instru-ment, which is an instrumentprecisely like the meridiancircle (Art. 49) without thecircle and its reading micro-scopes. As the instrument(Fig. 14) is turned upon itsaxis, the vertical wire in thecentre of the reticle exact-ly follows the meridian, whenthe instrument is in perfectadjustment. If, then, we knowthe instant shown by theclock when a known star is crossing this wire, we have atonce the means of determining the
The elements of astronomy; a textbook . is faster or slower than the time it ought to indicate. The method ordinarily em-ployed by astronomers is bymeans of the Transit Instru-ment, which is an instrumentprecisely like the meridiancircle (Art. 49) without thecircle and its reading micro-scopes. As the instrument(Fig. 14) is turned upon itsaxis, the vertical wire in thecentre of the reticle exact-ly follows the meridian, whenthe instrument is in perfectadjustment. If, then, we knowthe instant shown by theclock when a known star is crossing this wire, we have atonce the means of determining the error of the clock, becausethe sidereal time at that moment is equal to the stars rightascension (Art. 37). The difference between the right ascen-sion of the star as given in the almanac and the time shownby the face of the clock at the moment of transit gives directlythe error of the sidereal clock. The observation of only a single star would give the error of theclock pretty closely, but it is much better and usual to observe a num-. Fig. 14.— The Transit Instrument. § 58] DETERMINATION OF TIME. 37 ber of stars (from 8 to 10), reversing the instrument upon its pivotsonce at least during the operation. With a good instrument a skilledobserver can thus determine the clock error within about a thirtiethof a second of time, provided proper means are taken to allow for hispersonal equation. If instead of observing a star we observe the sun with this instru-ment, the time shown by the (solar) clock ought to be noon plus orminus the equation of time for the day as given in the almanac. Butfor various reasons transit observations of the sun are less accuratethan those of the stars, and it is better to deduce the mean solar time,when needed, from the sidereal by means of the almanac data. (Fora fuller description of the transit instrument and its adjustments seeAppendix, Art. 544.) 59. Personal Equation. — It is found that every observer hashis own peculiarities of time-observat
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