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is usually called the north pole, and which is distant from the pole of the world only 1° 45′ in the present century. A secondary of the ecliptic passing through the pole of the world, passes through the first degree of Cancer; so that the axis of the earth declines towards the first degree of that sign.

From the two motions of the earth on her axis and round the sun, we have not only the succession of day and night, but also the different seasons of the year; as may be explained in the following manner.

By the rotation of the earth on her axis from west to east, all the heavenly bodies, of which the sun is one, appear to move from east to west, and every appearance is the same, whether we suppose that the sun and stars really move from east to west, while the earth remains quiescent, or that the sun and stars remain at rest, while the earth revolves from west to east on her axis.

If we suppose a broad plane touching the surface of the earth where we are, and extending to the concave heavens, this plane, which is our sensible horizon, will divide the heavens into two parts; one half of the heavenly bodies being above the plane and one half below it. Now if this plane, being fixed to the surface of the earth, move with it while it revolves from west to east, those heavenly bodies, which were below it towards the east, will gradually rise above it, and the sun among the rest; while those that were above it towards the west will gradually sink below it. While the sun is above the horizon it is day, while below it, it is night. In this manner all the stars appear to go round the earth from east to west, in 23h 56′ 4′′, the time of a complete rotation of the earth round her axis. The sun also would appear to go round in the same time, if he kept his place among the stars

without variation. But this is not the case. While the earth is moving in her orbit from west to east, about a degree every day, the sun will appear to move over the same space, among the stars, in the same time. If you now suppose the sun and any particular star to be elevated at the same moment above the horizon, when the earth has completed one rotation on her axis, the same star will rise again above the horizon, but the sun will not rise with it; because in the mean time he has advanced a degree farther to the eastward, and it will require 3′ 56′′ of time over and above a complete rotation of the earth to overtake the sun and raise him above the horizon. So that while the stars require 23h 56′ 4′′ to complete their apparent revolution, the sun requires 24 hours, or 3′ 56′′ more. Hence the stars rise so much sooner every night, as to rise 366 times in 365 natural days, making 366 sidereal days in the year, as there are so many complete rotations of the earth on her axis in the course of a year consisting of 365 days. This is the difference between sidereal and solar time.

The whole of this may be illustrated by the motion of the hands of a clock. When the minute-hand had completed a revolution, it would overtake the hour-hand, if it had no motion; but this having moved forward, in the time, a small space more time than a complete revolution is necessary to bring them together.

As the motion of the earth on her axis is equable, we derive from it an easy and certain method of regulating our clocks. If a telescope be placed in the meridian-having cross hairs in the focus, then observing when any star passes the middle hair, note the time by the clock, and if it be found to pass the

same hair, on the succeeding night 3′ 56′′ sooner by the clock, the clock goes regularly and is well adjusted. If the star come sooner than 3′ 56′′ to the hair, than what it did the night before, the clock loses time; but if later, she goes too fast, and must be regulated accordingly. And even without a telescope, if through a small hole in a thin plate of tin fixed to a post, you observe the time when a star disappears behind any obstacle at a distance, and find that, on the succeeding evening, it disappears 3′ 56′′ sooner than on the night before, the clock goes regularly; but if not, it may be regulated in the manner already mentioned.

SEASONS OF THE YEAR.

THE inclination of the axis of the earth to the axis of the ecliptic in an angle of 23° 28′ occasions the variety of our seasons in the course of the year. In order to understand this, we must remember that the sun can enlighten only one half of a planet at a time, and the circle which bounds the enlightened part of the disk is called the circle of illumination. This circle is every where 90 degrees from that point to which the sun is vertical on the surface of a planet. Now when the earth is in Libra as seen from the sun, and the sun appears to enter Aries, as he does on the 22d of March, he will appear to be in the equator, because the ecliptic, which is his annual course, cuts the equator at that point, and his rays will reach from pole to pole, and consequently all the parallels of latitude will be equally divided by the circle of illumination. Hence the days and nights will be equal, all over the earth, which constitutes that season which we call spring. As the sun apparently passes from west to east through Aries, Taurus, and Gemini, to the beginning of Cancer,

the circle of illumination gradually extends beyond the north pole, and falls short of the south pole, until it extend at last 23° 28′ beyond the north pole, and fall so much short of the south pole. This will cause all the parallels of latitude to be unequally divided by the circle of illumination, the greater part of them falling in the circle of illumination in the northern hemisphere, and the lesser part in the southern hemisphere. Hence, while the sun is advancing through these three signs, the days will be gradually increasing in length, and the nights growing shorter, in all northern latitudes, and the contrary in all southern latitudes. All this will be greatest, when the sun arrives at the beginning of Cancer, which will constitute the middle of summer on the north side of the equator, and the middle of winter to the south. The north frigid zone, included within the arctic circle, is now wholly within the circle of illumination, and the sun appears to go round without setting; while the south frigid zone is wholly in darkness, so that the sun does not rise to it but passes round below its horizon. Hence the sun never sets for six months at the north pole, nor rises at the south, but appears to go round in the horizon on the day of the vernal equinox, and every day gradually rises higher and higher until he have attained to the altitude of 23° 28', on the day of the summer solstice, which is accomplished in three months; in the next three months he gradually sinks lower in every revolution until he come to the horizon again. During these three months, while the sun passes through Cancer, Leo, and Virgo, to the beginning of Libra, where he arrives on the 22d of September, the days, in the northern latitudes, gradually grow shorter, and the nights longer, until they become of equal length, at the autumnal equi.

nox, when the sun is again in the equator, and all the parallels of latitude are equally divided by the circle of illumination from pole to pole. This season is called the autumn.

While the sun passes through Libra, Scorpio, and Sagittarius, to the beginning of Capricorn, where he arrives on the 22d of December, the same appearances take place in the southern hemisphere, as were found to take place, from the 22d of March to the 22d of June, in the northern hemisphere. The days grow gradually longer in all south latitudes, and shorter in the northern hemisphere; until it shall be the middle of winter in north latitudes, and the middle of summer in all south latitudes. At this time, the whole antarctic circle is included within the circle of illumination, and the arctic circle is excluded. The sun will not therefore set to the south pole, nor rise to the north, for six months together. In the remaining three months of the year, the sun appears to move through Capricorn, Aquarius, and Pisces, to the beginning of Aries, where he arrives on the 22d of March. During this time, the days in south latitudes decrease in length, and on the contrary increase in all north latitudes, until they shall be each twelve hours long, all over the globe.

PRECESSION OF THE EQUINOXES.

By the diurnal rotation of the earth round its axis, the centrifugal force of every particle on the surface of the earth will be increased, and its gravity lessened, in proportion to its distance from the axis of the earth. So that about the equator, the surface of the earth will rise higher from the center, than towards the poles; and the earth will assume the figure of an oblate spheroid, whose equatorial diameter will be longer than the polar diameter. The proportion between these di

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