HISTORY OF PHYSICAL ASTRONOMY.
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ever, have not been realised. Notwithstanding a great amount of attentionand skill bestowed upon it for many years, the results have not beenfound to exceed in accuracy those obtained by the aid of the mural circle.The Astronomer lloyal, in consequence, has recently contrived a new instru-ment, termed the reflex zenith telescope, which appears to be adapted forgiving results of great accuracy. The adjustment of this ingenious instru-ment, with respect to the zenith, is effected not by means of the plumb-line, which Mr. Airy considers to have been the main cause of the failureof the former instrument, but by reflexion from a surface of mercury.This instrument is now being constructed, and will soon be ready for use.
The volume which issues annually from the Greenwich Observatory,exhibits an interesting picture of the activity which reigns in that establishment. The sun, moon, and planets, and the stars of the NauticalAlmanac, are observed as they pass the meridian on every practicableoccasion. A considerable number of other stars is also observed on themeridian for particular purposes. Then there are the extra-meridionalobservations with the equatorials, comprehending occasional observationsof comets, determinations of the diameters of the planets, and micro-metrical measurements of double stars; besides observations from timeto time of eclipses, occultations, and other such phenomena, with detachedtelescopes. The observations of the planets for the year are all reduced,and a comparison is instituted between the results thus obtained, and thecorresponding results calculated from the most esteemed tables of theseveral bodies. Not only are the errors in the geocentric longitude andecliptic polar distance of each planet thus regularly tabulated, but theequations between the errors in geocentric longitude and the heliocentricerrors of the earth and planet are also regularly formed. The observa-tions, in fact, are so exhibited as to be immediately available for correctingthe elements of the planets, or otherwise perfecting the theory of theirmovements*. The stars observed during each year are also reduced totheir mean places, forming the materials for occasional catalogues, whichprove of inestimable value, both as a guide to the practical astronomerand as a groundwork of research to the theorist. The same system ofcomplete reduction is applied to all the other observations. In the caseof the occultations of stars by the moon, the results in every instanceare employed in forming air equation for the rectification of the lunartheory. So with respect to the occultations and transits of Jupiter ’ssatellites, the times of actual occurrence are compared with the timesindicated by the Nautical Almanac, and the errors of the ephemeris,or rather of the tables of the satellites, thereby clearly exhibited tothe eye.
It will be obvious to any person who bestows even a passing glanceupon the annual volume of the Greenwich Observations, that the office of
* The error in the geocentric longitude of a planet may arise from either of the fol-lowing causes :—1st, an error in the earth’s longitude; 2nd, an error in the radius vectorof the earth’s orbit; 3rd, an error in the heliocentric longitude of the planet; 4th, an errorin the radius vector of the planet’s orbit. Hence an equation is formed between eacherror of geocentric longitude and the four errors above mentioned, represented algebrai-cally as unknown quantities. In the annual volume of the Greemeich Observations, thenumerical values of the coefficients of the different terms in each equation are regularlycalculated, but this is the farthest step that is practicable. The discussion of the equa-tions can only be undertaken, with any hopes of success, when they have accumulated inlarge numbers, and when the observations to which they relate have extended over aconsiderable period of time.
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