on the 21st of December; after which it will decrease until the centre of the sun returns to the equator on the 21st of March.

By ascertaining the position of the centre of the sun's disk from day to day, by means of its right ascension and declination (42), and tracing its course upon the surface of a celestial globe, its path is proved to be a great circle of the heavens, inclined to the equator at an angle of 23° 28′.

126 The ecliptic. This great circle in which the centre of the disk of the sun thus appears to move, completing its revolution in it in a year, is called the ECLIPTIC, because solar and lunar eclipses can never take place except when the moon is in or very near it.

127. The equinoxial points. -The ecliptic intersects the celestial equator at two points diametrically opposite to each other, dividing the equator, and being divided by it into equal parts. These are called the EQUINOXIAL POINTS, because, when the centre of the solar disk arrives at them, being then in the celestial equator, the sun will be equal times above and below the horizon (119), and the days and nights will be equal.

128. The vernal and autumnal equinoxes.-The equinoxial point at which the sun passes from the south to the north of the celestial equator is called the VERNAL, and that at which it passes from the north to the south is called the AUTUMNAL, equinoxial point. The TIMES at which the centre of the sun is found at these points are called, respectively, the VERNAL and AUTUMNAL

EQUINOXES.

The vernal equinox, therefore, takes place on the 21st of March, and the autumnal on the 21st of September.

129. The seasons.-That semicircle of the ecliptic through which the sun moves from the vernal to the autumnal equinox is north of the celestial equator; and during that interval the sun will therefore be longer above than below the horizon, and will pass the meridian above the equator in places having north latitude. The days, therefore, during that half-year will be longer than the nights.

That semicircle through which the centre of the sun moves from the autumnal to the vernal equinox being south of the celestial equator, the sun, for like reasons, will during that half-year be longer below than above the horizon, and the days will be shorter than the nights, the sun rising to a point of the meridian below the equator.

The three months which succeed the vernal equinox are called SPRING, and those which precede it WINTER; the three months which precede the autumnal equinox are called SUMMER, and those which succeed it AUTUMN.

130. The solstices.-Those points of the ecliptic which are midway between the equinoxial points are the most distant from the celestial equator. The arcs of the ecliptic between these points and the equinoxial points are therefore 90°. These are called the SOLSTITIAL POINTS, and the times at which the centre of the solar disk passes through them are called the SOLSTICES.

The summer solstice, therefore, takes place on the 21st of June and the winter solstice on the 21st of December.

This distance of the summer solstitial point north, and of the winter solstitial point south of the celestial equator is 23° 28'.

The more distant the centre of the sun is from the celestial equator, the more unequal will be the days and nights (108), and consequently the longest day will be the day of the summer solstice, and the shortest the day of the winter solstice.

It will be evident that the seasons must be reversed in southern latitudes, since there the visible celestial pole will be the south pole. The summer solstice and the vernal equinox of the northern, are the winter solstice and autumnal equinox of the southern hemisphere. Nevertheless, as the most densely inhabited and civilised parts of the globe are in the northern hemisphere, the names in reference to the local phenomena are usually preserved.

131. The zodiac.-The apparent motions of the planets are included within a space of the celestial sphere extending a few degrees north and south of the ecliptic. The zone of the heavens included within these limits is called the ZODIAC.

132. The signs of the zodiac.-The circle of the zodiac is divided into twelve equal parts, called SIGNS, each of which therefore measures 30°. They are named from principal constellations, or groups of stars, which are placed in or near them. Beginning from the vernal equinoxial point they are as follows:

Thus, the position of the vernal equinoxial point is the FIRST POINT OF ARIES, and that of the autumnal the FIRST POINT OF LIBRA. The summer solstitial point is at the FIRST POINT OF CANCER, and the winter at the FIRST POINT OF CAPRICORN.

133. The tropics.-The points of the ecliptic at which the centre of the sun is most distant from the celestial equator are also called the TROPICS,—the northern being the TROPIC OF CANCER, and the southern the TROPIC OF CAPRICORN.

This term TROPIC is also applied in geography to those parts of the earth whose distances from the terrestrial equator are equal to the greatest distance of the centre of the solar disk from the celestial equator. The NORTHERN TROPIC is, therefore, a parallel of latitude 23° 28′ north, and the SOUTHERN TROPIC a parallel of latitude 23° 28' south of the terrestrial equator.

134. Celestial latitude and longitude.—The terms latitude and longitude, as applied to objects on the heavens, have a signification different from that given to them when applied to places upon the earth. The latitude of an object on the heavens means its distance from the ecliptic, measured in a direction perpendicular to the ecliptic; and its longitude is the arc of the ecliptic, between the first point of Aries and the circle which measures its latitude, taken, like the right ascension, according to the order of the signs.

Thus since the centre of the sun is always on the ecliptic, its latitude is always o°. At the vernal equinox its longitude is o°, at the summer solstice it is 90°, at the autumnal equinox 180°, and at the winter solstice 270°.

135. Annual motion of the earth.-The apparent annual motion of the sun, described above, is a phenomenon which can only proceed from one or other of two causes. It may arise from a real annual revolution of the sun round the earth at rest, or from a real revolution of the earth round the sun at rest. Either of these causes would explain, in an equally satisfactory manner, all the circumstances attending the apparent annual motion of the sun around the firmament. There is nothing in the appearance of the sun itself which could give a greater probability to either of these hypotheses than to the other. If, therefore, we are to choose between them, we must seek the grounds of choice in some other circumstances.

It was not until the revival of letters that the annual motion of the earth was admitted. Its apparent stability and repose were until then universally maintained. An opinion so long and so deeply rooted must have had some natural and intelligible grounds. These grounds, undoubtedly, are to be found only in the general impression, that if the globe moved, and especially if its motion had so enormous a velocity as must be imputed to it, on the supposition that it moves annually round the sun, we must in some way or other be sensible of such movement.

All the reasons, however, why we are unconscious of the real rotation of the earth upon its axis (102) are equally applicable to show why we must be unconscious of the progressive motion of the earth in its annual course round the sun. The motion of the globe through space being perfectly smooth and uniform, we can have no sensible means of knowing it, except those which we possess in the case of a boat moving smoothly along a river: that is, by

looking abroad at some external objects which do not participate in the motion imputed to the earth. Now, when we do look abroad at such objects, we find that they appear to move exactly as stationary objects would appear to move, seen from a movable station. It is plain, then, if it be true that the earth really has the annual motion round the sun which is contended for, that we cannot expect to be conscious of this motion from anything which can be observed on our own bodies or those which surround us on the surface of the earth: we must look for it elsewhere.

But it will be contended that the apparent motion of the sun, even upon the argument just stated, may equally be explained by the motion of the earth round the sun, or the motion of the sun round the earth; and that, therefore, this appearance can still prove nothing positively on this question. We have, however, other proofs, of a very decisive character.

Newton showed that it was a general law of nature, and part, in fact, of the principle of gravitation, that any two globes placed at a distance from each other, if they are in the first instance quiescent and free, must move with an accelerated motion to their common centre of gravity, where they will meet and coalesce; but if they be projected in a direction not passing through this centre of gravity, they will both of them revolve in orbits around that point periodically.

Now the common centre of gravity of the earth and sun, owing to the immense preponderance of the mass of the sun (M.309), is placed at a point very near the centre of the sun. Round that, point, therefore, the earth must, according to this principle, revolve.

136. Motion of light proves the annual motion of the earth. -Since the principle of gravitation itself might be considered as more or less hypothetical, it has been considered desirable to find other independent and more direct proofs of a phenomenon, so fundamentally important and so contrary to the first impressions of mankind, as the revolution of the earth and the quiescence of the sun. A remarkable evidence of this motion has been accordingly discovered in a vast body of apparently complicated phenomena which are the immediate effects of such a motion, which could not be explained if the earth were at rest and the sun in motion, and which would be inexplicable on any other supposition save the revolution of the earth round the sun.

It has been ascertained that light is propagated through space with a certain great but definite velocity of about 184,000 miles per second. That light has this velocity is proved by the body of optical phenomena which cannot be explained without imputing to it such a motion, and which are perfectly explicable if such a

H

motion be admitted. Independently of this, another demonstration that light moves with this velocity is supplied by an astronomical phenomenon which will be noticed in a subsequent part of this volume.

137. Aberration of light. Assuming, then, the velocity of light, and that the earth is in motion in an orbit round the sun with a velocity of about 19 miles per second, which must be its speed if it move at all, as will hereafter appear, an effect would be produced upon the apparent places of all celestial objects by the combination of these two motions which we shall now explain.

It has been stated that the apparent direction of a visible object is the direction from which the visual ray enters the eye. Now this will depend on the actual direction of the ray, if the eye which receives it be quiescent; but if the eye be in motion, the same effect is produced upon the organ of sense as if the ray, besides the motion which is proper to it, had another motion equal and contrary to that of the eye. Thus, if light moving from the north to the south with a velocity of 184,000 miles per second be struck by an eye moving from west to east with the same velocity, the effect produced by the light upon the organ will be the same as if the eye, being at rest, were struck by the light having a motion compounded of two equal motions, one from north to south, and the other from east to west. The direction of this compound effect would, by the principles of the composition of motion (M.172), be equivalent to a motion from the direction of the north-east. The object from which the light comes would, therefore, be apparently displaced, and would be seen at a point beyond that which it really occupies in the direction in which the eye of the observer is moved. This displacement is called accordingly the ABERRATION OF LIGHT.

This may be made still more evident by the following mode of illustration. Let 0, fig. 36, be the object from which light comes in the direction o o e". Let e be the place of the eye of the observer when the light is at o, and let the eye be supposed to move from e to e" in the same time that the light moves from o to e". Let a straight tube be imagined to be directed from the eye at e to the light at o, so that the light shall be in the centre of its opening, while the tube moves with the eye from o e to o" e" maintaining constantly the same direction, and remaining parallel to itself: the light in moving from o to e", will pass along its axis, and will arrive at e' when the eye arrives at that point. Now it is evident that in this case the direction in which the object would be visible, would be the direction of the axis of the tube, so that, instead of appearing in the direction o o, which is its true direction, it would appear in the direction o o' advanced from o in the direction of the motion e e' with which the observer is affected.