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

year,

that all those stars, which, at one time of the appear on any one side of the pole star in the evening, 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.

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 hea→ ven, 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.

Now, as the earth is an opake spherical body, at a great distance from the sun, one-half of it will always be illuminated thereby, while the other half will remain in darkness.

The circle which distinguishes or divides the illuminated face of the earth from the dark side, and is the boundary between light and darkness, is generally called the terminator. A line drawn from the centre of the sun to the centre of the earth, is perpendicular to the plane of this circle.

It is plain that, when any given place on the globe first gets into the enlightened hemisphere, the sun is just risen to that part; when it gets half-way, or to its greatest distance from the terminator, it is then noon; and when it leaves the enlightened hemisphere, it is then sun-set.

"

Here it will be necessary to premise a few considerations: First, that on account of the immense distance of the sun from the earth, the rays which proceed from it may be considered as parallel to each other. Secondly, that only one-half of a globe can be illuminated by parallel rays; and, therefore, only one-half of the earth will be enlightened by the sun at one time.

These considerations will be rendered more forcibie, by an attentive survey of fig. 1, plate 5 ; in which S represents the sun, from whom we suppose parallel rays to flow in all directions. At A, B, C, are represented three different positions of the globe of the earth, the bright part being that which is illuminated by the rays proceeding from the sun; the shaded

F

part, the portion of the globe which is in darkness; of course, the line TT is the terminator, or boundary of light and darkness.

In the globe at C, the poles coincide with the terminator.

In the globe at A, the north pole N is in the enlightened portion, and the south pole in the dark hemisphere; while, in the opposite globe, at B, the southern pole S is in the illuminated part, and the north pole in obscurity.

It is evident, that it is day in any given place on the globe, so long as that place continues in the enlightened hemisphere; but when, by the diurnal rotation of the earth on its axis, it is carried into the dark hemisphere, it becomes night to that place.

The length of the day and the night depend, therefore, on the position of the terminator, with respect to the axis of the earth.

If the poles of the earth be situated in the terminator, as at C, every parallel will be divided into two equal parts; and as the uniform motion of the earth causes any given place to describe equal parts of its parallel in equal times, the day and the night would be equal on every parallel of latitude; that is, all over the globe, except at the poles, where the sun would neither rise nor set, but continue in the horizon.

But if, as at A and B, the axis be not in the plane of the terminator, the terminator will divide the equator into two equal parts, but all the circles parallel to it into unequal parts; those circles that are