Sect. II. Of Phenomena arising from the Revolution of the

Earth on its own axis.

DAY AND NIGHT,

99. Common experience shows, that when we are moving swiftly in one direction, surrounding objects appear to be moving in the opposite direction. This effect is no where more striking than in sailing near a shore or coast. It is difficult for a person in this situation for the first time, to realize that himself, and not the land, is in motion. So by the earth's motion on its axis from west to east, the sun and stars appear to move from east to west. The sun constantly shines upon one half the earth's surface; and by the regular motion of the earth on its axis, every place is successively brought into light and immersed in darkness. This occasions alternate day and night.

100. If the line NS (Pl. IV, fig. 4.) about which the earth turns, were always in the circle dividing the light from the dark hemisphere, the days would every where be of the same length, and just as long as the nights. For an inhabitant at the equator, at 0, and one on the same meridian towards the poles, as at I, would come into the light at the same time, would come to the meridian q Q, at the same time, and, on the other side, would immerge into darkness at the same time. And since the motion of the earth is uniform, they would be in the dark hemisphere just as long as in the light; that is, the night would be just as long as the day.

101. But this is not the position of the line NS, except when the sun is in the celestial equator. But as the ecliptic and the equator make an angle with each other of 23°, the sun cannot be in the celestial equator, except at the points where the equator cuts the

ecliptic, which are the beginning of the signs Aries and Libra. The sun enters these signs on the 20th March and 23d of September. Hence at these periods, and at no others, the days and nights are equal all over the world ; and on this account they are called equinoxes ; the first the vernal equinox, the second the autumnal. At these seasons, the sun rises exactly in the east at 6 o'clock, and sets exactly in the west at 6 o'clock.

102. But at other seasons, when the sun is not in the celestial equator, the line NS is not in the circle dividing the light from the dark hemisphere; but has more or less of the position as represented at sign Cancer (5) or Capricorn (13). (Pl. V, fig. 1.) Here it is plain that an inhabitant at the equator 0, does not come out of the dark hemisphere, or immerge into it, at the same time with an inhabitant on the same meridian towards the poles as ai

. I. But while the earth is at us, an inhabitant at I is in the light hemisphere longer than in the dark; that is, the day is longer than the night. But at go, an inhabitant at I is in the dark hemisphere longer than in the light. Whereas in all situations of the earth, day and night are equal at the equator.

103. It is plain from these figures, that when the days are longest in north latitude, they are shortest in south latitude, and vice versa. It is also plain, that as the sun has declination from the celestial equator either north or south, he shines over or beyond one pole, and not to the other. So that there is a region about one pole, which is a long time in the light hemisphere; and a region about the other pole, which for an equal length of time is in the dark hemisphere. At the poles, there is but one day and one night in a year, each of six months. The distance to which the sun shines beyond the poles is always equal to his own declination; and as his declination can be but 23°, he can never shine but 231° beyond a pole. Less circles surrounding the earth at

the distance of 23° from the poles are called polar circles. Less circles surrounding the earth at the distance of 23jo from the equator are called tropics ; the one on the north side, the tropic of Cancer ; the one on the south side, the tropic of Capricorn.

These terms are indiscriminately applied to these circles, as drawn on the earth, or in the heavens. The subject will show which are meant.

104. When the sun enters the signs & and wg, (which takes place June 21, and December 22, he is at his greatest declination, and in the tropics. At the first period which is called the summer solstice, days are longest and nights shortest in north latitude; and nights longest and days shortest in south latitude. At the latter period, which is called the winter solstice, directly the reverse is the case in each latitude.

105. During a year the earth turns on its axis once more than we have days. The reason of this is, that on account of the earth's motion in her orbit, she turns a little more than once on her axis between the time of noon one day, and noon the next day. For, (Pl. V, fig 2) if the earth be supposed at A on any particular day, and the place e be under the sun at noon, it is manifest that on the next day, when the earth comes to B, the place e will not be under the sun, when it has completed its revolution; but the earth must revolve through the space eo, before it is noon at e. So again on the next day, when the earth is at C, the earth must more than complete a second revolution by the space eo, before it is noon at e. These little excesses amount to a whole revolution of the earth on its axis in the course of a year. A complete revolution of the earth on its axis constitutes a siderial day; the time from noon to noon constitutes a solar or natural day. Siderial days are all of the same length; but solar days are not.

The mean difference in the length of a siderial and solar day is 3'56". The cause of the different lengths of solar or natural days will be explained, when we treat of equation of time.

For precisely the same reason that the earth turns on its axis once more in a year, than there are solar days, the moon must revolve once more round the earth, than it changes or fulls, in the course of a year. For between one change and another, the earth has advanced in her orbit ; and consequently the moon must more than complete ber revolution before she can be between the sun and earth. The time she occupies in describiog her orbit is the time of her periodical revolution; and the time between one change and another, or one full moon and another, is the period of her Synodical revolution,

SECT. III.

Of Phenomena arising from the Earth's motion round the Sun, together with the obliquity of the Ecliptic.

ART. 1. Aberration of Light. 106. It was stated above, (No. 36,) that light is progressive; that it is not transmitted from one body to another instantaneously. It is about sixteen minutes in crossing the earth's orbit; that is, it moves at the rate of about 200,000 miles a second. The earth also moves in its orbit at the rate of about 68,000 miles an hour; that is, nearly 190 miles a second. On account of these two motions, viz. of light and of the earth, we never see any of the heavenly bodies, especially the stars, in precisely the place they occupy, but a little to the eastward of their true places.

107. To illustrate this, (Pl. V, fig. 3,) suppose light falling upon the earth at a from a star in the line Were the earth stationary at a, the star would be seen by the direct ray c a, and would appear to be where it actually is. But while the direct ray ca is coming to

the earth, the earth has moved from a to b; consequently, the star will not be seen by the ray c a, but by the ray cb; and this in the direction bd, parallel to a c. Hence the star appears at d, instead of at c. This effect is called the aberration of light, and amounts to about 20% of a degree.

If the pupil find the preceding illustration difficult to be understood, it may perhaps be rendered more intelligible, if we suppose the line a c to be a long tube or telescope, fixed on the earth in the direction represented in the figure. It is obvious that if a star be seen through this tube or telescope, it must be seen at that place exactly to which the tube or telescope points. Let us suppose that a ray of light would come from c to the earth in the same time that the earth would move from a to b. If the ray should enter the tube or telescope in a direction towards a, it is obvious that on account of the motion of the telescope, the ray must strike upon its upper side and be lost before it comes to a. But if the ray enter at c, in the direction towards b, then the motion of the telescope will prevent it from striking its under side ; for this is continually sliding, as it were, from under the ray, till the ray reaches b. But when the ray reaches b, the telescope is in the position bd, and the eye looking through the telescope must of course see the star at d. Now the effect is just the same on the naked eye, as it would be through a telescope.

108. As the earth's orbit is elliptical, the earth must at one season of the year be nearer the sun, than at another. For instance (Pl. I, fig. 1,) the earth is nearer the sun at A, than when in the opposite point of its orbit at C. And as the heat and light from the sun are greater as the distance is less, it is plain the earth must receive a greater degree when at A, than when at C. This circumstance would occasion a variation in the temperature of the air, analagous to the seasons, were the sun always in the celestial equator ; that is, if the equator coincided with the ecliptic. But the seasons with us, in north latitude, are not in the least de