Читать книгу The Circle of Knowledge: A Classified, Simplified, Visualized Book of Answers - Various - Страница 125

The Earth revolves constantly around the Sun, and at the same time rotates upon an axis inclined twenty-three and one-half degrees towards the plane of its orbit. In consequence of the inclination of the axis, the declination of the Sun, or its angular distance from the Equator, varies with the advance of the Earth in its orbit, causing periodical variations in the length of day and night, and, consequently, in temperature.

Vernal Equinox. On the twentieth of March, at mid-day, the Sun is vertical at the Equator. Rising directly in the east it ascends the heavens to the zenith, and, descending, sets directly in the west.

The illuminated hemisphere extends from pole to pole, and embraces half of every parallel of latitude; hence every point on the Earth’s surface is under the rays of the Sun during half of the diurnal rotation; the days and nights are equal all over the globe; and the heating power of the Sun is the same in both the northern and the southern hemisphere.

Summer Solstice. As the Earth advances in its orbit the vertical Sun declines northward; and on the twenty-first of June, at the Summer Solstice, it is over the northern Tropic, twenty-three and one-half degrees from the Equator.

The illuminated hemisphere, extending ninety degrees on each side of the parallel of the vertical Sun, reaches twenty-three and one-half degrees beyond the North Pole; but, at the south, it barely touches the Antarctic circle. It embraces more than half of each parallel north of the Equator, hence throughout the northern hemisphere the day is longer than the night, the difference in their duration increasing with the latitude; and all points within the Arctic circle are in the light during the entire rotation.

In the southern hemisphere, less than half of each parallel being illuminated, the night is longer than the day, and within the Antarctic circle there is constant night. The heating power of the Sun is now at the maximum in the northern hemisphere, while in the southern it is at the minimum.

Autumnal Equinox. On the twenty-second of September, the distribution of light and heat upon the two hemispheres is the same as at the Vernal, and at the Winter Solstice, on the twenty-second of December, it is the reverse of that at the Summer Solstice.

WHAT CAUSES THE SEASONS AND DAY AND NIGHT

FIGURE ILLUSTRATING THE CHANGE OF SEASONS THROUGHOUT THE YEAR

The change of seasons is caused by the revolution of the earth around the sun, and the inclinations of the planes of the equator and ecliptic. These causes also account for the difference in the length of the days and nights and the difference in the height of the midday sun. The exact duration of the seasons we get by observing the dates of equinoxes and solstices.

FIGURE SHOWING THE CAUSE OF DAY AND NIGHT

The revolution of the earth gives us the length of the year; its rotation on its axis, the length of the day and night, by causing the risings and settings and daily apparent motion of the sun and stars.