Secondly, let us fuppofe that S is the fun, that it has no motion of it's own, that it refts within the orbit A B CD, in which we fhall now fuppofe the earth to move, in the order of the letters A B CD. Upon this fuppofition, when the earth is at A, the fun will appear in that part of the heavens where the stars H are; when the earth is at B, the fun will appear in that part of the heavens where the ftars G are; when the earth is at C, the fun will appear in that part of the heavens where the ftars E are; and as the earth revolves round the fun, in the orbit A B CD, the fun will appear to a fpectator on the earth to defcribe the circle G H EF.

Thus whether the earth be at reft, and the fun revolves in the orbit A B CD; or the fun be at rest, and the earth revolves in the fame orbit, a spectator on the earth will fee the fun defcribe the fame circle EFGH, in the concave fphere of the hea

vens.

Hence if the plane of the earth's orbit be imagined to be extended to the heavens, it would cut the ftarry firmament in that very circle, in which a fpectator in the fun would fee the earth revolve every year: while an inhabitant of the earth would observe the fun to go through the fame circle, and in the fame space of time that the folar fpectator would fee the earth defcribe it.

The inhabitants of all the other planets will obferve just fuch motions in the fun as we do, and for the very fame reafons; and the fun will be feen from every planet to defcribe the fame circle, and in the fame fpace of time, that a fpectator in the fun would obferve the planet to do. For example, an inhabitant of Jupiter would think that the fun revolved

*This is pleasingly illuftrated by the armillary fphere, where the fphere may be moved independent of the earth, and the earth of the fphere.

revolved round him, defcribing a circle in the heavens in the space of twelve years: this circle would not be the fame with our ecliptic, nor would the fun appear to pass through the fame ftars which he does to us. On the fame account, the fun, feen from Saturn, will appear to move in another circle, diftinct from either of the former; and will not feem to finish his period in lefs time than thirty years. Now as it is impoffible that the fun can have all these motions really in itself, we may safely affirm, that none of them are real, but that they are all apparent, and arife from the motions of the refpective planets.

One phenomenon arifing from the annual motion of the earth, which has already been flightly touched upon, may now be more fully explained; for as from this motion, the fun appears to move from weft to eaft in the heavens, if a ftar rifes or fets along with the fun at any time, it will in the course of a few days rife or fet before it, because the fun's apparent place in the heavens will be removed to the eastward of that ftar. Hence thofe ftars which at one time of the year fet with the fun, and therefore do not appear at all, fhall at another time of the year rife when the fun fets, and fhine all the night. And as any one ftar fhifts it's place with refpect to the fun, and in confequence of that with refpect to the hour of the night, fo do all the reft. Hence it is that all thofe ftars, which at one time of the year appear on any one fide of the pole ftar in the evening, fhall in half a year appear on the contrary fide thereof.

From what has been faid it follows in general, 1. That, in whatever regards the fun's place, only with respect to the point in the heavens in which it appears, it may be fuppofed to move in an infinitely great circle, called the ecliptic, whofe center is the obferver's eye.

VOL. III.

F

2. That

2. That the earth's true place in his orbit being known, from obfervation or calculation; fix figns added to or fubtracted from it, gives the fun's true place in the ecliptic. Therefore the theory of the fun's motions feen from the earth, is the fame with that of the earth's motions feen from the fun.

3. To an inhabitant on the earth, the plane of the ecliptic is that, whereto the annual motions of the planets in their orbits must be naturally compared. In the fame manner, as the plane of the equator is that whereto the pofition of the parallels, which the stars appear to describe in confequence of the earth's di urnal revolution, must also be compared.

In order to combine the fun's annual motion with it's diurnal, you are to observe, that if the plane of the ecliptic coincided with that of the equator, the fun would, by it's diurnal revolution, feem to defcribe every day the fame circle, viz. the equator; and could therefore have no declination. For in defcribing the ecliptic by it's annual revolution, the fun would then fucceffively answer to all the stars in the equator, confequently, it's diurnal revolution would be made in the fame circle as that of the ftars; but I have already obferved to you, that the fun appears to defcribe daily different parallels; it is evident, therefore, that the plane of the ecliptic does not coincide with that of the equator, but is inclined thereto.

If fo, the fun muft, in confequence of it's annual motion, defcribe a great circle NBTL N, fig. 4, pl. 2, reprefenting the ecliptic, and bifecting the equator EBZLE. The fun muft therefore appear fometimes towards one pole, fometimes towards the other pole.

1. Let us fuppofe that it is in B, one of the interfections of the equator and the ecliptic, it's diurnal revolution muft defcribe the equator, and have no declination; as the fun gradually advances

in

n the ecliptic from B towards A, it appears gradually to recede from the equator with an increafing northern declination, and to defcribe fmaller and smaller parallels, till it arrives at A, where it appears to defcribe the parallel AIVA.

2. The fun being arrived at L, three figns or 90 degrees from B, and three months after his departure therefrom, it is then in that point of the ecliptic the moft diftant from the equator at it's greatest northern declination, and defcribes the fmalleft parallel OT.

3. In the three following months the fun going from T to L draws nearer the equator, it's northern declination diminishes, it's parallels augment; fo that when arrived in L, the other interfection of the equator and the ecliptic, it then has no declination, and that day again defcribes the celeftial equator.

4. The fun after that paffing from L to N, enters the fouth part of the heavens, it's fouthern declination then increases, and it's parallels diminish, till being come to N, three figns from the point L, it's fouthern declination is then the greatest, and it defcribes it's leaft parallel N D.

5. The fun continuing it's courfe from N to B again draws nearer the equator, and it's fouthern declination diminishes; fo that when returned to B a year after it's departure from that point, it is again in the equator and without declination, and then begins a new courfe attended with the fame phenomena.

It is evident, therefore, that the fun moving continually in the ecliptic, the parallels it every day defcribes cannot be circles, but a kind of fpirals, fuch as the curves made by a thread wound about a fphere. For after a diurnal revolution, the fun does not come to the fame point from whence it departed, but according às it approached to, or receded

F 2

ceded from the equator, is either a little below or above that point.

The angle formed by the plane of the equator and ecliptic, is called the obliquity of the ecliptic.

The obliquity of the ecliptic is equal to the fun's greatest declination, namely, when in the tropc, and is about 23 degrees; confequently, the axis of the earth must be inclined to the ecliptic in an angle of 66 degrees. The confideration of this obliquity brings us to an explanation,

OF THE SEASONS OF THE YEAR.

It is our business under the prefent head to account for the phenomena of the feafons, thofe grateful viciffitudes on which fo much both of the bufinefs and happiness of man depends.

Before I explain the caufes of thofe changes that are termed the feafons of the year, it will be neceffary to premife a few confiderations: First, that on account of the immenfe distance of the fun from the earth, the rays which proceed from it may be confidered as parallel to each other. Secondly, that only one-half of a globe can be illuninated by parallel rays, and therefore only onehalf of the earth will be enlightened by the fun at one time. Thirdly, that we may call the line which divides light from darknefs, the termi

nator.

In the diagram, fig. 1, pl. 5, S reprefents the fun, from which we fuppofe parallel rays to flow in all directions; A, B, C, reprefent three different pofitions of the globe of the earth, the bright part being that which is illuminated by the rays proceeding from the fun; the dark part, the portion of the globe which is in obfcurity at these