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ftlong with the sun on the 16th of April, b. c. 44, rose with the sun at Rome several days earlier in the age of Meton, and do not now rise with the sun at Rome until several days later. This is caused by the precession of the equinoxes.
2. The time of the risings and settings of the fixed stars is different on the same day in places whose latitude is different. Thus, in the year when the Pleiades rose along with the sun at Rome on the 16th of April, they did not rise along with the sun at Athens until the 22d of April.
Too little attention was paid to these considerations by the Roman writers ; and consequently we not unfrequently discover that they combined the observations of astronomers who lived at times and places remote from them and from each other — that calculations made for the latitude of Athens, or of Rhodes, or of Alexandria, 300 years before, were adopted at once and transferred to their calendars without change or modification.
Another source of confusion is a want of precision in specifying the different kinds of risings and settings, which ought always to be most carefully distinguished from each other by appropriate scientific terms.
The risings and settings of the fixed stars, when considered with reference to the surPs place in his orbit, may be arranged under eight heads : —
(a) When a star rises at sunrise.
(6) When a star rises at sunset.
(V) When a star sets at sunrise.
(d) When a star sets at sunset.
(a) When a star rises shortly before the sun so as to be just visible in the morning twilight as it ascends above the horizon before its rays are overpowered by the light of the more brilliant luminary.
(/8) When a star rises shortly after sunset so as to be just visible in the evening twilight as it ascends above the horizon,
(7) When a star sets shortly before sunrise so as to be just visible in the morning twilight as it sinks below the horizon.
(5) When a star sets shortly after sunset so as to be just visible in the evening twilight as it sinks below the horizon.
The names by which these, taken in order, are discriminated by the Greek astronomers Geminus (Isagog. cap. xi.) and Ptolemy (Math, Syntax, viii. 4) are the following : —
morning rising (7>) '
(a) 'ettitoa^ e&ta a\f]Qivi^ G. — 5E<^a (rvvaya-a.Kf]Qivi]y P. — Ortus Matutinus Verus. True
eorirepia aA?j0iz>T7, G. iKfj, P. — Ortus Vespertinus Verus. True evening rising.
(c) kvffis eaa aXfiQivt}) G. — eE«a <rvy/cara-Svcris d\7]6ivf}9 P. — Occasus Matutinus Verus. True morning setting.
(d) Avffis ecr/repia aXf]Qivi]^ G. — 'EtrTrep/a (Tuy/caTaSucm aAT^tvrj, P. — Occasus Vespertinus Verus. True evening setting.
(a) 'ETrtToA-Jj e^a <£cuj/oju,€j/t], G. — 'E<p'a irpoava-TO/vJj ^aivo/^ei/T], P. — Ortus Matutinus Apparens s. Ortus Heliacus. Heliacal rising, i. e. First visible rising of a star in the morning twilight.
(/3) 'ETnToA-fy ecnrepla, (paivojAevTi, G. — 'Etrvrepto: tVamToA); fyw.vo^vf], P. — Orius Vespertinus Apparens, Last visible rising of a star after gunset.
(7) Averts e&fa <£cui/Q/^en}, G. — 'Ewa
cVrj, P. — Occasus Matutinus Apparens, First visible setting of a star before sunrise.
(§) Aucrts IcTTrepia ffjaivofjifvr], G. — 'Ecnrepia eiriKardo'vo-is ^cuw/^eV??, P. — Occasus Vespertinus Apparens s. Occasus Heliacus. Heliacal .setting, i. e. Last visible setting of a star in the evening twilight.
With regard to the above technicalities we must observe
1. That Geminus (I. c.) draws a distinction between the words aj/aroArj and eTriroA^. By az/aroA'/i he understands the rising of a star considered simply with reference to its elevation above the horizon, which takes place once in twenty-four hours in consequence of the diurnal motion ; by e7TiroA?7, the rising of the star considered with reference to its distance from the sun, which depends upon the sun's place in the ecliptic. As to the settings of the stars, he would make Sutm the correlative of avaroKi] and icpfyis of eiriroX-fj ; but to this last definition he does not himself adhere, since he constantly employs fivcrts to denote the setting of a star, when considered with reference to its distance from the sun. Ptolemy, while he includes all the risings and settings under the general designation of <f>ao*as airXavtav, endeavours to introduce an improved nomenclature, by varying the preposition according as the star rises or sets along with (ffvv\ or before (irp&) or after (err/) the sun, but pays no regard to the rule of Geminus with respect to aj/aroAT) and eTriroX-f].
2. Two terms, in addition to those set down above, are commonly employed by writers on these topics, the cosmical rising and setting (Ortus Cosmicus^ Occasus (7.), and the aciiony- chal rising and setting (Ortus Acronyclius^ Oc~ casus A.). • -
The epithet Cosmicus, as applied to this subject, first occurs in a note of Servius on Virg. Georg. i. 218, "ortus et cccasiis duo sunt: unus riXia.n6s^ id est, solans ; et alter Ko<r/j,iK6s, id est, mundanus : Unde fit ut ea signa quae cum sole oriuntur a nobig non possint videri ; et ea, quae vidcmus, quantum ad solis rationem pertinet, videantur occidere.11 Modem astronomers have for the most part (see Petavius, Varr. Diss. p. 3, ed. 1630) adopted the phrase Ortus Casmicus to indicate the rising marked (a), that is, the Ortus Matutinus Verus, and Qc-casus Cosmicus to indicate the setting marked (c), that is, the Occasus Matutinus Verus, but Ideler (Historisclie Untersuclmngen, &c. p. 311), while he' interprets Ortus Cosmicus in the sense usually received, applies Occasus Cosmicus to the setting marked (7), that is, to the Occasus Matutinus
Again, the epithet dnp6vv)(os appears to be first used by Theophrastus (De Signis Pluv. et Vent. cap. i. § 2) \vhere dvaroKai a/cptW%oi are alone mentioned, and are distinctly explained to mean the rising of a star at sunset, that is, the Ortus Vespertinus Verus marked (6), and in this sense the phrase Ortus Acronyclius is found in the treatises of Petavius and others who employ also the expression Occasus Acronyclius to indicate the setting marked (tZ), that is, the Occasus Vssp&rtinuLs Verus. Ideler concurs in the latter, but interprets Ortus Acronyclius to mean the rising marked (/3), that is, the Ortus Vespertinus Apparens. This view is certainly at variance \vith the words of Theophrastus, which are quite explicit and are corroborated by Julius Firmicus (ii. 8) ; but on the