| XXX | (458) | XXX |
453 ASTRO N O M Y. Mr'Romeur firfl: difcovered, the motion of light is there- ftar pafles over the meridian on any day with the fun, by proved to be progreffive, and not inftantaneous, as will pafs over the fame meridian on the next day when ■was formerly believed. It is eafy to compute in what the fun is almoft a degree (hort of it; that is, 3 minutes time .the earth moves from A x.q B for the chord of 60 56 feconds fooner. If the year contained only 360 degrees of any circle is equal to the femidiameter of that days, as the ecliptic does 360 degrees, the fun’s appacircle ; and as the earth goes through all the 360 degrees rent place, fo far as his motion is equable, would change of its orbit in a year, it goes through 60 of thofe degrees a degrfee every day ; and then the fydereal days would be in about 61 davs. Therefore, if on any given day, fup- juft four minutes ftiorter than the folar. pofe the firil of June, the earth is at A, on the firfl: of In Plate XLII. fig. 3. let ABCDEFGHIKLM be Auguft it will be at 2?; the chord, or ftraight line the earth’s orbit, in which it goes round the fun every being equal to DS the radius of the earth’s orbit, the year, according to the order of the letters, that is, from fame with' AS its didance from the fun. weft to e'aft; and turns round its axis the fame way from As the earth moves from T) to C, through the fide the fun to the fun again every 24 hours. Let S be the AB of its orbit, it is conftantly meeting the light of Ju- fun, and R a fixed ftar, at fuch an immenfe diftance, that piter's fatellites fooner, which occafions an apparent ac- the diameter of the earth’s orbit bears no fenfible proceleration of their eclipfes ; and as it moves through the portion to that diftance. Let be any particular meother half H of its orbit, from C to D, it is receding ridian of the earth and N a given point or place upon from their light, which occafions an apparent retardation that meridian. When the earth is at A, the fun .S'hides of their eclipfes, beeaufe their light is then longer be- the ftar R, which would always be hid if the earth nefore it overtakes the earth. ver removed from A; and confequently, as the earth That thefe accelerations of the immerfions of Jupi- turns round its axis, the point N would always come ter’s fatellites into his ihadow, as the earth approaches round to the fun and ftar at the fame time. But when towards Jupiter, and the retardations of their emerfions the earth has advanced, fuppofe a twelfth part of its out of his (hadovv, as the earth is going from him, are orbit from A to B, its motion round its axis will bring not occafionedby any inequality arifing trom the motions the point N a twelfth part of a natural day, or two of the fatellites in excentric orbits, is plain, becauie it hours, fooner to the ftar than to the fun ; for the angle affefts them all alike, in whatever parts of their orbits ArBn is equal to the angle ASB ; and therefore any they are eclipfed. Befides, they go often round their ftar,. which comes to the meridian at noon with the fun orbits every year, and their motions are no way commen- when the eaith is at A, will come to the meridian at furate to the earth’s. Therefore, a phenomenon not to 10 in the forenoon when the earth is at B. When the be accounted for from the real motions of the fatellites, earth comes to C, the point N will have the ftar on its but fo eafily deducible from the earth’s motion, and fo meridian at 8 in the morning, ;r four hours fooner than anfwerable ihereto, muft be allowed to refult from it. it comes round to the fun; foi it muft revolve from 2V to n This affords one very good proof of the earth’s annual before it has the fun in its meridian.' When the eartht motion. comes to D, the point N will have the ftar on its meridian at 6 in the morning, but that point muft revolve fix hours more from 2Vt<5 », before it has mid-day by the Chap. X. Of Solar and Sydereal Time. fun : For now the angle ASD is a right angle, and fo is NDh ; that is, the earth has advanced 90 degrees in The fixed ftars appear to go round the earth in 23 its orbit, and muft turn 90 degreee on its axis to carry hours 56 minutes 4 feconds, and the fun in 24 hours; the point N from the ftar to the fun : For the ftar always fo that the ftars gain three minutes 56 feconds upon the comes to the meridian when N?n is parallel to RS A ; fun every day, which amounts to one diurnal revolu- becaufe DS is but a point in refpeft of RS. When the tion in a year; and therefore, in 365 miles, as mea- earth is at E, the ftar comes' to the meridian at 4 in fured by the returns of the fun to the meridian, there the morning; at F, at 2 in the morning; and at G, the are 366 days, as meafured by the ftars returning to it; earth having gone half round its orbit, 2V points to the the former are called fclar days, and the latter fydersal. ftar R at midnight, it being then dire&ly oppofite to the The diameter of the earth’s orbit is but a phyfical fun ; and therefore, by the earth’s diurnal motion, the point in proportion to the ciftance of the ftars ; for which ftar comes to the meridian 12 hours before thf fun. reafon, and the earth’s uniform motion on its axis, any When the earth is at H, the'ftar comes to the meridian given meridian wilt revolve from any ftar to the fame ftar at 10 in the evening; at /, it comes to the meridian 8,. again in every abfolute turn of the earth on its axis, that is, 16 hours before the fun; at K, 18 hours bewithout the leaft perceptible difference of time Ihewn by fore him ; at L, 20 hours ; at M, 22 ; ancf at A, equally with the fun again. a clock which goes exadlly true. If the earth had only a diurnal motion, without an an- Thus it is plain, that an abfolute turn of the earth on nual any given meridian would revolve from the fun to its axis (which is always completed when any particular the fun again in the fame quantity of time as from any meridian comes to be parallel to its fituation at any time of ftar to the fame ftar again, becaufe the fun would never the day before) never brings the fame meridian round from change his place with refpedt to the ftars. But as the the fun to the fun again ; but that the earth requires as earth advances almqft a degree eaftward in its orbit in much more than one turn on its axis to finilh a nautral at a the time that it turns, eaftward round its a^is^. whatever day, as it lias gone forward in that time j, which, mean
