bodies in motion, whose apparent change of place depends as well on their own motion, as on that of the spectator.

We shall first consider only that apparent change which takes place in those which are at rest, and which is owing wholly to the motion of the earth; and shew that the sun, when seen from the earth, will appear to move in the same manner, whether it revolves round the earth, or whether the earth revolves round the sun. Let us suppose the earth at rest, without any motion of its own, and let the sun be supposed to revolve round it in the orbit ABCD, plate 4, fig. 1; and let EFGH be a circle in the concave sphere of the starry heavens; as the sun moves in the order of the letters ABCD in its orbit, it will appear to a spectator on the earth to have described the circle EFHG. When the sun is at A, it will appear as if it was among the fixed stars that are at E; when it is at B, it will appear among the fixed stars at F; when at C, among those at H; and when it is at D, it will appear among the fixed stars at G. Indeed, the fixed stars and the sun are not seen at the same time; but we have shewn, that we may tell in what part of the heavens the sun is, or what fixed stars it is near, by knowing those which are opposite to it, or come to the south at midnight. Therefore, if we find that any set of stars, as those at G, for instance, come to the south at midnight, we may be sure that they are opposite to the sun ; and, consequently, if we could see the stars in that

part of the heaven where the sun is, we should find them to be those at F.

Secondly, let us suppose that S is the sun, having no motion of its own; that it rests within the orbit ABCD, in which we shall now suppose the earth to move, in the order of the letters ABCD. Upon this supposition, when the earth is at A, the sun will ap pear in that part of the heavens where the stars, H, are; when the earth is at B, the sun will appear in that part of the heavens where the stars, G, arę ; when the earth is at C, the sun will appear in that part of the heavens where the stars, E, are; and as the earth revolves round the sun in the orbit ABCD, the sun will appear to a spectator on the earth to describe the circle GHEF.

Thus, whether the earth be at rest, and the sun revolves in the orbit ABCD; or the sun be at rest, and the earth revolves in the same orbit, a spectator on the earth will see the sun describe the same circle EFGH, in the concave sphere of the heavens.

Hence, if the plane of the earth's orbit be imagined to be extended to the heavens, it would cut the starry firmament in that very circle, in which a spectator in the sun would see the earth revolye every year; while an inhabitant of the earth would observe the sun to go through the same circle, and in the same space of time, that the solar spectator would see the earth describe it."

The inhabitants of all the other planets will observe just such motions in the sun as we do, and for the very same reasons; and the sun will be seen

from every planet to describe the same circle, and in the same space of time, that a spectator in the sun would observe the planet to do. For example, an inhabitant of Jupiter would think that the sun revolved round him, describing a circle in the heavens in the space of twelve years: this circle would not be the same with our ecliptic, nor would the sun appear to pass through the same stars which he does to us. On the same account, the sun, seen from Saturn, will appear to move in another circle, distinct from either of the former; and will not seem to finish his period in less time than thirty years. Now, as it is impossible that the sun 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 arise from the motions of the respective planets.

One phenomenon arising from the annual motion of the earth, which has already been slightly touched upon, may now be more fully explained; for as from this motion the sun appears to move from west to east in the heavens, if a star rises or sets along with the sun at any time, it will, in the course of a few days, rise or set before it; because the sun's apparent place in the heavens will be removed to the eastward of that star. Hence, those stars, which at one time of the year set with the sun, and therefore do not appear at all, shall, at another time of the year, rise when the sun sets, and shine all the night. And as any one star shifts its place with respect to the sun, and in consequence of that with respect to the hour of the night, so do all the rest. Hence it is

that all those stars, which, at one time of the year, appear on any one side of the pole star in the even ing, shall in half-a-year appear on the contrary side thereof.

OF PHENOMENA OCCASIONED BY THE ANNUAL AND DIURNAL MOTIONS OF THE EARTH.

First, of those that arise from the diurnal motion. As the earth is of a spherical figure, that part of it, which comes at any time under the confined view of an observer, will seem to be extended like a plane; and the heavens will appear as a concave spherical superficies, divided by the aforesaid plane into two equal parts; one of which is visible, the other concealed from us by the opacity of the earth. er Now the earth, by its revolution round its axis, carries the spectator and the aforesaid plane from west to east; therefore, all those bodies to the east, which could not be seen because they were below the plane of the horizon, will become visible, or rise above it, when, by the rotation of the earth, the horizon sinks as it were below them. On the other hand, the opposite part of the plane towards the west, rising above the stars on that side, will hide them from the spectator, and they will appear to set, or go below the horizon.

As the earth, together with the horizon of the spectator, continues moving to the east, and about

*See page 68 of these Essays.

the same axis, all such bodies as are separated from the earth, and which do not partake of that motion, will seem to move uniformly in the same time, but in an opposite direction; that is, from east to west; excepting the celestial poles, which will appear to be at rest. Therefore, when we say that the whole concave sphere of the heavens appears to turn round upon the axis of the world, whilst the earth is performing one rotation upon its own axis, we must be understood to except the two poles of the world; for these do not partake of this apparent motion.

It is, therefore, on account of the revolution of the earth round its axis, that the spectator imagines the whole starry firmament, and every point of the Keaven, excepting the two celestial poles, to revolve about the earth from east to west every twenty-four hours; each point describing a greater or lesser circle, as it is more or less remote from one of the celestial poles.

The earth is made to revolve on its axis, in order to give alternate night and day to every part of its

surface.

Although every place on the surface is illuminated by all the stars which are above the horizon of that place, yet, when the sun is above the horizon, his light is so strong, that it quite extinguishes the faint light of the stars, and produces day. When the sun goes below the horizon, or, more properly, when our horizon gets above the sun, the stars give their light, and we are in that state which is called night.