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HISTORY OF PHYSICAL ASTRONOMY.

CHAPTER XVIII.

Early Methods of observing the Celestial Bodies_Instruments of the Greek As-tronomers_Accurate Principles of Observation first employed by the Astronomers of

the Alexandrian School.—Improvements effected by Hipparchus.—Ptolemy substitutesthe Quadrant for the Complete Circle.—Arabian Astronomers.—The Method bywhich they indicated the Time of an Observation..—Revival of Practical Astronomy in

Europe_Labours of Waltherus.—Tycho Brahe.—Landgrave of Hesse.—Hevelius..—

Close of the Tychonic School of Observation.—Observatory of Copenhagen estab-lished.—The Pendulum applied to clocks by Huyghens.—The Royal Society ofLondon, and the Academy of Sciences of Paris , established.—Invention of the Micro-meter.—Application of the Telescope to divided instruments_Observatories of Paris

and Greenwich established.—Labours of Roemer.—Transit Instrument invented.—The use of Circular Instruments for taking Altitudes introduced.—Labours of Flam-steed and Halley—Royal Observatory of Paris.—Commencement of the era ofaccurate observation. — Bradley.—Lacaille.—Mayer.—Maskelyne.—Pond. — Airy.—Reduction of Planetary and Lunar Observations.—Present state of Practical As-tronomy.

The history of astronomy does not exhibit a more interesting picturethan that which represents the progress of the art of observation, from therude essays of early ages to the refinement and precision which cha-racterise its present state of advancement. The Chaldeans, to whom theorigin of astronomy is usually ascribed, do not seem to have attained anyexcellence in this important department of the science. Their observa-tions of eclipses of the moon, as cited by Ptolemy in the Syntaxis , are asrude as can possibly be imagined. The time is expressed only in hours,and the quantity eclipsed in terms of the half and quarter of the moon'sdiameter. Herodotus states that the Greeks were indebted to the Baby-lonians for the pole, the gnomon, and the division of the day into twelvehours. The pole seems to have been a concave hemispherical sun-dial,having a vertical style in the centre, by means of which the interval in-cluded between sunrise and sunset, for each day throughout the year, wasdivided into twelve equal parts. The construction of such an instrumentdoes not require any acquaintance with the principles of trigonometry ordialling; it merely implies a knowledge of the uniform motion of thecelestial sphere. The gnomon, besides being an imperfect instrument forastronomical purposes, is limited in its application. It is probable that,by the use of it, the Chaldeans succeeded in obtaining an approximation tothe length of the solar year, but there is not the smallest reason to sup-pose that they employed it in determining any other of the fundamentalfacts of astronomy. Indeed, they do not seem to have made observationsat all for the purpose of forming materials to serve as the groundwork offuture reasoning. They simply contented themselves with noting themore remarkable phenomena as they occurred, and hence deducing a fewrough conclusions of a general nature. It is manifest that so long asastronomy continued to be cultivated in this manner, it could not attain ahigh state of perfection.

The Greek philosophers were too much preoccupied with the idea ofarriving at final conclusions on all subjects by the mere force of abstractdiscussion, to devote their attention to the prosecution of astronomicalobservations. It would appear, however, that by comparing together theChaldean records of eclipses, extending over a long succession of ages,some of the earlier of the Greek mathematicians ascertained with con-