John Louis Emil Dreyer

“ 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 tix the moment of an observation by a simultaneous time determination. For this purpose Tycho Brahe employed altitudes observed with quadrants; but he remarks that if the star is taken too near the meridian the altitude varies too slowly, and if too near the horizon the refraction (which at that time was very imperfectly known) introduces an element of uncertainty. ”

“ The observation of the time when a star has a certain azimuth may also be used for determining the clock error, as the hour-angle can be found from the declination, the latitude and the azimuth. As the azimuth changes most rapidly at the meridian, the observation is most advantageous there, besides which it is neither necessary to know the latitude nor the declination accurately. The observed time of transit over the meridian must be corrected for the deviations of the instrument in azimuth, level and collimation. ”

“ Ptolemy, in his Almagest, states nothing whatever as to how the time was found when the numerous astronomical phenomena which he records took place; but Hipparchus, in the only book we possess from his hand, gives a list of 44 stars scattered over the sky at intervals of right ascension equal to exactly one hour, so that one or more of them would be on the meridian at the commencement of every sidereal hour. ”

“ In transit, observations generally this personal error does not cause any inconvenience, so long as only one observer is employed at a time, and unless the amount of the error varies with the magnitude of the star (which is often the case) ; but when absolute time has to be determined, as in longitude work, the full amount of the personal equation between the two observers must be carefully ascertained and taken into account. And an observer’s error has often been found to vary very considerably not only from year to year but even within much shorter intervals ”

“ Clocks and chronometers 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. ”

“ That century also witnessed the combination of the transit instrument and the mural quadrant or circle in one instrument-the transit or meridian circle. While the necessity of following the sun, moon and planets as regularly as possible increased the daily work of observatories, other branches of astronomy were opened and demanded other observations. ”

“ 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 car having to count the beats of the clock while the eye compares 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 invention of the telescope early in the 17th century made it possible to observe eclipses of Jupiter's satellites; but there is to a great extent the same drawback attached to these as to lunar eclipses: that it is impossible to observe with sufficient accuracy the moments at which they occur.
Eclipses of the sun and occultations of stars by the moon were also much used for determining longitude before the invention of chronometers and the electric telegraph offered better means for fixing the longitude of observatories. ”

“ The transit instrument superseded the sextant and offered the advantage of furnishing the difference of right ascension directly; the clocks and chronometers were greatly improved; and lastly astronomers began early in the 19th century to treat their instruments, not as faultless apparatuses but as imperfect ones, Whose errors of construction had to be detected, studied and taken into account before the results of observations could be used to test the theory. ”

“ 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. ”

“ This applies principally to the great observatories, where the sun, moon, planets and a limited number of fixed stars are without interruption being observed, but even among these institutions hardly two are conducted on the same principles. Thus in Greenwich the instruments and observations are all treated according to strict rules laid down by the astronomer royal, while in Washington or Pulkowa each astronomer has to a certain extent his choice as to the treatment of the instrument and arrangement of the observations. ”

“ Hipparchus of Rhodes, the founder of modern astronomy, by repeating observations made at Alexandria, discovered the precession of the equinoxes, and investigated with considerable success the motions of the sun, moon and planets. His work was continued by more or less distinguished astronomers, until Ptolemy (in the 2nd century A.D.) gave the astronomy of Alexandria its final development. When science again began to be cultivated after the dark ages which followed, we find several observatories founded by Arabian princes ”

“ The simplest and best way to find the equation between two observers is to let one observe the transits of stars over half the wires in the telescope, and the other observe the transits over the remainder, each taking care to refocus the eyepiece for himself in order to avoid the above-mentioned source of error. The single transits reduced to the_ middle wire give immediately the equation; and, in order to eliminate errors in the assumed wire-intervals, each observer uses alternately the first and the second half of the wires. ”

“ Instead of vertical wires, the eye end may be fitted with Repsold’s self-registering micrometer with one movable wire to follow the star (see Micrometer) . 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 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. ”

“ When the progress of terrestrial magnetism and meteorology began to make regular observations necessary, the duty of taking these was often thrown on astronomical observatories, although in some cases separate institutions were created for the purpose. In this article the astronomical observatories will be chiefly considered.
Up to about 300 B.C. it can scarcely be said that an observatory existed anywhere, as the crude observations of the heavens then taken were only made by individuals and at intervals, employing the simplest possible apparatus. ”

“ 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 arc 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. ”

“ The application of spectrum analysis, photometry, &c., in astronomy has still more increased the number and variety of observations to be made, while the use of photography in work of precision has completely revolutionized many branches of practical astronomy. It has now become necessary for most observatories to devote themselves to one or two special fields of work. ”

“ In the spherical triangle ZPS between the zenith, the pole and a star the side ZP=90°−φ (φ being the latitude) , PS=90°− δ (δ being the declination) , and ZS or z=90° minus the observed altitude. The angle ZPS=t is the star’s hour-angle or, in time, the interval between the moment of observation and the meridian passage of the star. ”

“ 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 trustworthy timekeeper in the hands of travellers. ”

“ About the 21st of March of each year a sidereal clock agrees with a mean time clock, but it gains on the latter 3m 56·5s everyday, so that in the course of a year it has gained a whole day. For a place not on the meridian of Greenwich the sidereal time at noon must be corrected by. the addition or subtraction of 9·8565s for each hour of longitude, according as the place is west or east of Greenwich. ”

“ But, when philosophical speculation had exhausted its resources, and an accumulation of facts was found to be necessary before the knowledge of the construction of the universe could advance farther, the first observatory was founded at Alexandria, and continued in activity for about four hundred years, or until the middle or end of the 2nd century of the Christian era. ”

“ 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 measures of the angular distance between stars with a sextant being preferred. These methods were, however, very inconvenient, which induced Römer to invent the transit instrument about 1690. ”

“ Obs. were begun in 1829 with a transit instrument by Dollond of 5-in. ap. and a mural circle by jones. Thomas Maclear undertook to verify and extend the arc of meridian measured by N. L. de Lacaille in 1751-1753, which work occupied the obs. staff for a number of years. In 1849 a 7-in. refr. by Merz was mounted, and in 1855 a new meridian circle, a facsimile of the one at Greenwich, superseded the older instruments. ”

“ 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. ”

“ Nearly all the more important public observatories have now been connected in this way on the continent of Europe, chiefly at the instigation of the “Europaisehe Gradmessung,” while the determinations in connexion with the transits of Venus and those carried out in recent years by the American, French, British and Colonial governments have completed the circuit of the greater part of the globe. ”

“ Thus the obs. of Bradley (ob. 1762) form the foundation of modern stellar astronomy; but it was especially during the directorship of Airy (1835-1881) that the obs. rose to its present high state of efficiency. There are now two chief assistants, six assistants, and a staff of computers employed. The principal instruments now in use are: a meridian circle by Simms (and Ransomes and May as engineers) , erected in 1850, having a circle of 6-ft. diameter and a telescope of 8-in. ap., Lassell's 2-ft. refl., erected 1884; I3-in. phot. refr. with 10-in. vis. o.g. by Grubb; 28-in. refr. by Grubb ”

“ Differentiation of the formula shows that observations near the equinoxes are most advantageous, and that errors in the assumed ε and the observed δ will have no influence if the Δα. is observed at two epochs when the sun’s R.A. is A and 180°−A or as near thereto as possible. ”

“ The latter was comparatively little used (two satellites oi Saturn were discovered with it) , while the former served to discover about 2500 nebulae and clusters, 800 double stars, and two satellites of Uranus, as also to make the innumerable other obs. which have made the name of Herschel so celebrated. Sir J. Herschel used a 20-ft. reli. at Slough from 1825 to 1833, and from 1834 to 1838 at the Cape of Good Hope, to examine the nebulae and double stars of the whole of the visible heavens, discovering 2100 new nebulae and 5500 new double stars. ”

“ Mr W. Lassell's obs., from 1840 to 1861 at Starfield near Liverpool, lat. -l-53° 25' 28", long. 0 h. II m. 38-7 s. W.; contained refl. of 9- and 24-in. ap.; employed for obs. of the satellites of Saturn, Uranus and Neptune, and of nebulae. The 2-ft. refl. was used at Malta in 1852-1853. and a 4-ft. refi. was mounted in 1861, also at Malta, and used till 1864 for obs. of satellites and nebulae. The eighth satellite of Saturn., the two inner satellites of Uranus and the satellite of Neptune were discovered at Starfield by Mr Lassell. ”