ASTRONOMY.

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tigation of the quantity of the aberration of light from the eclipsesof the first satellite Ot Jupiter , lie found the whole quantity tobe exactly equal to that winch Dr, Bradley deduced from his ow nobservations, “ It is very curious,” observes M. de la Place,

“ that there should be such a perfect agreement in results whichhave been obtained by such very different methods.” “ Itfollows,” continues he, “ from this agreement that the velocity otlight is uniform through the whole space comprehended by theorbit of the .Earth . In fact, the velocity of light, as given by theaberration, is that which takes place at the circumference of theEarth ’s orbit, and which, being combined w ith the motion of theEarth , produces this pharnomenon. The velocity of light as givenbv the eclipses of the satellites of Jupiter , is determined by thetune which light employs to traverse the Earth ’s orbit; these twovelocities bein'* the same, the velocity is consequently uniformthrough the whole length of the Earth ’s orbit. It results alsofrom these eclipses, that the velocity of light is uniform throughthe whole diameter of Jupiter ’s orbit; for from the eccentricityof this orbit, the effect of the variations in the radii vectoresis very sensible in the eclipses of the satellites, and from obser-vations these are exactly conformable to the supposition of thevelocity of light being uniform.” Hence light travels throughequal spaces in equal times.

PART II.

Astronomical Operations and Calculations.

Sect. I. Of Drawing a Meridian Line .

(£12.) I. Upon a plain hoard, set parallel to tiie horizon, de-scribe a circle A11E, as in Plate XXIII. Fig. 21. And upon thecentre, C, erect a stile or gnomon, exactly perpendicular to it,and so high, that the top of the shadow thereof may fall upon thecircumference of the circle about the middle of the forenoon.Mark the point B exactly where the top of the shadow falls inthe forenoon ; and mark the point F where the top of the shadowfalls on the circumference, in the afternoon. Then through thecentre C, draw the line ACD, bisecting the arc BF. Then ADis the meridian required. It is proper to draw several concentriccircles, and to make observations with them all, that they mayconfirm one another. If the Sun happens to he clouded in one,it may be clear in another. It is best to make these observationsabout the solstices, when the Sun does not alter his declinationsensibly ; and the summer-solstice is to be preferred. The Sun Isevidently highest when in the meridian ; and at equal distancestherefrom has equal altitudes. Therefore, when ti e distancesDB, DE, are equal, the shadowsCB, CF will be equal, and there-fore the altitudes equal. And vice versa.

(213.) II. Hang up two thieads ami plummets, AB, CD,(PlateXXIII. Fig. 19,) at a good distance, in vessels of water, to keepthem steady; of which CD is moveable towards the left and right,upon a pin C. Wait till the pole-star, E, and the star Ailioth,F, (in the great bear’s rump, come into the same plumb-line, AB,to an eye placed at I. At that instant move the thread CD sothat it may hide the thread AB, and the pole-star E from the eyeat I. Tlien the plane ABCI) is the plane of the meridian; andwhere it intersects the horizontal plane, is the meridian-line. Thesame may be done with the star called Cassiopeia’s hip. Totake away the star’s rays, look through a small hole in a thinplate. 'Phis must be performed in a calm place.

(214.) If it be wished to have a meridian drawn in some otherplace, let the threads and plummets AB, CD, remain ; and hangup two others ab, cd, in the place proposed, as in Fig. 20, lettingub, be moveable upon a pin at a. Then wait till any star, as G,conies into the plane ABCD to the eye, at II ; and at that instant,move the thread ab, till the same star G fall in the plane abed,to the eye at h ; then abed is the plane of the meridiau. This isbest done by help of an assistant.

(215.) 111. Having a dock or watch, with minutes and se-conds, find the northing of the star, Ailioth, E\ (Fig. 19 ,) by theastronomical tables ; and wait till the pole-star F. is in a plumb-linew ith F. At that instant, set the clock to the said time of north-ing. And next day at 12 o’clock, draw a meridian-line, by theshadow of a plumb-line bung in the Sun . Or tind the time ofsouthing of any other star, as G, and the clock remaining as be-fore, when he shews the tune of southing, plate the tlireads ub, cd,

VOL. 1.— no. 21.

(Fig. 20,) so that the line G/i may pass through them both. Themabed will he in the plane of the meridian.

(21(5.) IV. On an equinoctial day fix a pin CD, (Fig. 23,) eitherupright or any way inclined upon any plane, so that the top Dmay project a shadow on the plane: mark the tip of the shadowat any' two or three distant intervals of the day, as at the pointsA, B, K, through which draw AK, then any line perpendicular tothis, as EF or GH, will be in the plane of the meridian. 'Ekesame easy method will he sufficiently exact a day or two before orafter the equinox.

Sect 11. Of Finding Time, and the Equation of Time.

(217.) Having drawn a meridian-line as directed in last article,the time when the Sun , or any othei celestial body, is exactly inthe meridian, may be found by a common quadiant, placing theedge of it along the line, and observing when the Sun or otherluminary' can be seen exactly through its two sights, and notingexactly the time ; which, supposing the luminary viewed to bethe Sun , will be precisely noon, or 12 o’clock ; but as the appa-rent diameter of the Sun is pretty large, it ought to be knownexactly when his centre is m the meridian, which will be someshort space after his western limb has arrived at it, and before hiseastern limb comes thither. It will be proper, therefore, toobserve exactly the time of the two limbs being seen through thesights of the quadrant; and the half of the dillerence betweenthese times, added to the one or subtracted from the other, willgive the exact time when the Sun ’s centre is in the meridian.

(218.) The same method is equally applicable to tne Moon ,but not to the stars, which have no sensible diameter. It is found,by observation, that the stars appear to go round the Earth in 23hours, 56 minutes, 4 seconds, and the Sun in 24 hours, at a meanrate; so that the stars gain 3 minutes 56 seconds upon the Sun every day, which amounts to one diurnal revolution in a year;and therefore, in 365 days, as measured by the returns of the Sun to tiie meridian, there are 366 days as measured by the stars re-turning to it; the former are called solar days, and the latter*sidereal.

(219.) I.et P <y> S in Fig. 8, Plate XXIV. be the equinoctialcohire, <y> IIGK the equator, PS the axis, qp ABC =& the northern

halt otthe ecliptic next the eye, =a=FED < Y' the southern half on theopposite side. If the Earth had no annual motion, the Sun wouldappear at rest among the stars, as suppose at <y>; then any meridianot the Earth, for example, the meridian of Ixmdon, Edinburgh ,Paris, or any either place, will revolve from the Sun or any starprecisely in the time that the Earth turns round its axis, and becausethe diurnal motion is always equable, the days would be alwaysexactly of the same length; but, as already shew n, if the Earth move in an orbit round the Sun, the Sun would apparently moveamong the stars in some plane. Suppose the Earth to moveequably in the plane of the equator, (hen the Sun would appearto move equably in lie equator from qp to =^, and back on theother side from = 2 = to again ; and the solar days would be stillequal among themselves, but longer than the sidereal, for when.,the Earth has revolved on its axis the sidereal day is completed,and the meridian has come to the star at <y, but the Sun havingadvanced toward the Earth ’s diurnal motion must be continueda little longer before that meridian comes to the Sun at y, hencethe solar clay is longer than the sidereal, and w hile the meridianby the diurnal motion revolves from y to y again tiie Sun lias ad-vanced an equal portion toward z, hence the solar days, thoughlonger than the sidereal, would be equal among themselves, andthe same as shewn by a well-regulated clock c but in reality theEarth ’s orbit is inclined to the plane of the equator, so that its ap-parent motion will be in the ecliptic from qp to A, IS, C, &c. Wcwill suppose it to move equably in this path while a lictitious Sun may be supposed to mine in the equator as above described;both setting out from qp at the same instant;. let tiie lictitiousSun be any where between qp and G, as at II in the meridianPAIIS, the arc <y> A, because it rises obliquely, or because itsubtends a right angle at II, is greater than the arc <y> II, thereforethe real Sun moving equably will not be at A, but short of it esat m, hence the meridian of a place w ill come to the real Sun , 01 -meridian P m S, before it-arrives at the lictitious Sun, or meridianPHS. This is the equation of time so far as it depends on theobliquity of the ecliptic. When the fictitious Sun is at G, thereal Sun is at B; here the time is the same by both, hence the4 N

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