John Louis Emil Dreyer

“ For a place not on the meridian of Greenwich the sidereal time at noon must be corrected by the addition or subtraction of 9" - 8565 for each hour of longitude, according as the place is west or east of Greenwich.
While it has for obvious reasons become customary in all civilized countries to commence the ordinary or civil day at midnight, astronomers count the day from noon, being the transit of the mean sun across the meridian, in strict conformity with the rule as to the beginning of the sidereal day. The hours of the astronomical day are also counted from to 24. ”

“ Clocks and chrono meters regulated to sidereal time are only used by astronomers, to whom they are indispensable, as the sidereal time at any moment is equal to the right ascension of any star just then passing the meridian. ”

“ For nearly two hundred years, until the application of the pendulum to clocks became general, astronomers could place little or no reliance on their clocks, and consequently it was always necessary to fix the moment of an observation by a simultaneous time determination. ”

“ In the United States, where the large extent of the country in longitude makes it impossible to use the time of one meridian, four standard meridians were adopted in 1883, viz., 75, 90, 105, 120 west of Greenwich, so that clocks showing "Eastern, Central, Mountain, or Pacific time" are exactly five, six, seven, or eight hours slower than a Greenwich mean-time clock. ”

“ Bessel also suggested what appears to be the right explanation, viz., the co-operation of two senses in observing transits by eye and ear, the ear having to count the beats of the clock while the eye com pares the distance of the star from the spider line at the last beat before the transit with the distance at the first beat after it, thus estimating the fraction of second at which the transit took place. It can easily be conceived that one person may first hear and then see, while to another these sensations take place in the reverse order ”

“ The method of equal altitudes gives very simply the clock error equal to the right ascension minus half the sum of the clock times corresponding to the observed equal altitudes on both sides of the meridian. ”

“ The intervals are determined by observing the time taken by a star of known declination to pass from one wire to the other, the pole star being best on account of its slow motion. [6] The instrument is provided with a clamping apparatus, by which the observer, after having beforehand set to the approximate declination of a star, can clamp the axis so that the telescope cannot be moved except very slowly by a handle pushing the end of a fine screw against the clamp arm, which at the other side is pressed by a strong spring. ”

“ The observations made in each position of the instrument are separately reduced with an assumed approximate value of the error of collimation, and two equations are thus derived from which the clock error and correction to the assumed collimation error are found. This use of the transit or alt-azimuth out of the meridian throws considerably more work on the computer than the meridian observations do, and it is therefore never resorted to except when an observer during field operations is pressed for time. ”

“ But in order to compare ob servations made at different places on the surface of the earth a knowledge of their difference of longitude becomes necessary, as the local time varies proportionally with the longitude, one hour corresponding to 15. Longitude can be determined either geodetically or astronomically. The first method supposes the earth to be a spheroid of known dimensions. Starting from a point of departure of which the latitude has been determined, the azimuth from the meridian (as determined astronomically) and the distance of some other station are measured. ”

“ This instrument enabled the observer to determine simultaneously right ascension and declination, but it does not appear to have been much used for right ascension during the 17th century, the method of equal altitudes by portable quadrants or distance measures with a sextant being preferred (see Observatory and Time) . These methods were, how ever, very inconvenient, which induced Roemer (q.v.) to invent the transit instrument about 1690. ”

“ From being very untrustworthy, astronomical clocks are now made to great perfection by the application of the pendulum and by its compensation, while the invention of chronometers has placed a portable and equally trust worthy timekeeper in the hands of travellers. ”

“ It is evident that the success of a determination of longitude depends to a very great extent on the accurate determination of time at the two stations, and great care must therefore be taken to determine the instrumental errors repeatedly during a night's work. ”

“ The periodic errors of the screw must be investigated and taken into account, and care must be taken that the microscopes are placed and kept at such a distance from the circle that one revolution will correspond to 1, the excess or defect (error of run) being determined from time to time by measuring standard intervals of 2 or 5 on the circle.
The telescope consists of two slightly conical tubes screwed to the central cube of the axis. ”

“ As the altitude changes very slowly near the meridian, this method is most advantageous if the star be taken near the prime vertical, while it is also easy to see that the greater the latitude the more uncertain the result. If a number of altitudes of the same object are observed, it is not necessary to deduce the clock error separately from each observa tion, but a correction may be applied to the mean of the zenith distances. ”

“ The great majority of American and Continental astronomers have, however, expressed themselves very strongly against this change; and, even if it should be made in the British Nautical Almanac, it appears very doubtful whether the other great ephemerides will adopt it, the more so as astronomers have hitherto felt no in convenience from the difference between the astronomical and the civil day. ”

“ The currents thus made register the transits automatically on a chronograph, while the observer, viewing the apparatus through his telescope, can observe the transits in the usual manner either oy eye and ear or by chrono graph, thus immediately finding his personal error. On the Continent these contrivances have frequently been used to educate pupils learning to observe, and experience has repeatedly shown that a considerable personal error can be generally somewhat diminished through practice. ”

“ It is owing to the uncertainty which necessarily attaches to the rate of a chronometer during long journeys, especially by land, where they are exposed to shaking and more or less violent motion, that it is desirable to employ a great number. ”

“ The level is applied again, and the mean of the two results is used in the reductions. In case the collimation error of the instrument is not accurately known, the instrument should be reversed and another observation of the same kind taken. ”

“ The value now accepted from the telegraphic determination is 5h 8m 12s·09. The probable errors of the results for PulkowaAltona and Altona-Greenwich were supposed to be S- 039 and S- 042. It is of course only natural that the uncertainty of the results for the trans-Atlantic longitude should be much greater, considering the length of time which elapsed between the rating of the chronometers at the observatories of Boston (Cambridge, Massachusetts) and Liverpool. The difficulty of the method con sists in determining the "travelling rate." ”

“ Putting , we get Hansen's formula, which gives the correction , which is more convenient for a greater number of observations. The daily aberration is always deducted from c, as it is also multiplied by sec 8 (being 0". 31 cos < sec 8) . ”