The Seasons. The variation of the seasons and the difference of the length of days and nights in those seasons, result from the same cause. In moving round the sun, the axis of the earth is not perpendicular to the plane of its orbit - in other words, its axis does not move round the sun in an upright position, but slanting, or oblique. This may be understood more clearly by carrying a small globe round a candle, which is to represent the sun. You must consider the ecliptic drawn on the small globe as representing the plane of the earth's orbit; and the equator, which crosses the ecliptic in two places, shows the degree of obliquity of the axis of the earth in that orbit, which is nearly 23 1/2 degrees. The points in which the ecliptic intersects the equator are called nodes. The globe at A is situated as it is in the midst of summer, or what is called the summer solstice, which is on the 21st of June. The north pole is then inclined towards the sun, and the northern hemisphere enjoys much more of his rays than the southern. The sun now shines over the whole of the north frigid zone, and, notwithstanding the earth's diurnal revolution, it will continue to shine upon it as long as it remains in this situation, whilst the south frigid zone is at the same time completely in obscurity.
Let the earth now set off from its position in the summer solstice, and carry it round the sun; observe that the axis must be always inclined in the same direction, and the north pole point to the same spot in the heavens. There is a fixed star situated near that spot, which is hence called the north polar star. The earth at B has gone through one quarter of its orbit, and is arrived at that point at which the ecliptic cuts or crosses the equator, and which is called the autumnal equinox. The sun now shines from one pole to the other. At this period in the year, the days and nights are equal of every part of the earth; but the next step she takes in her orbit involves the north pole in total darkness, whilst it illumines that of the south. This change was gradually preparing as the earth moved from summer to autumn. The instant the earth passes the autumnal equinox, the long night of the north pole commences, and the south pole begins to enjoy the light of the sun. As the earth proceeds in her orbit, the days shorten and the nights lengthen throughout the northern hemisphere, until it arrives at the winter solstice, on the 21st of December, when the north frigid zone is entirely in darkness, and the southern enjoys uninterrupted daylight. Exactly half of the equator, it will be observed, is enlightened in every position, and consequently the day is there always equal to the night.
Observe, that the inhabitants of the torrid zone have much more heat than we have, as the sun's rays fall perpendicularly on them, while they shine obliquely on the temperate, and almost horizontally on the frigid zone; for during their long day, the sun moves round at no great elevation above their horizon, without either rising or setting.
To a person placed in the temperate zone, the sun's rays will shine neither so obliquely as at the poles, nor so vertically as at the equator; but will fall upon him more obliquely in autumn and in winter than in summer. Therefore, the inhabitants of the earth between the polar circles and the equator will not have merely one day and one night in the year, as happens at the pole; nor will they have equal days and equal nights, as at the equator, but their days and nights will vary in length at different times of the year, according as their respective poles incline towards or from the 6un, and the difference will be greater in proportion to their distance from the equator. During the other half of her orbit, the same effect takes place in the southern hemisphere as we have just remarked in the northern. When the earth arrives at the vernal equinox, D, where the ecliptic again cuts the equator, on the 22nd of March, she is situated with respect to the sun exactly in the same position as in the autumnal equinox ; excepting that it is now autumn in the southern hemisphere, while it is spring time with us; for the half of the globe, which is enlightened, extends exactly from one pole to the other. On the two days of the equinox the sun is visible at both poles, but only half of it is seen from either, the other half being concealed by the horizon.