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'"VIII. The earth at the Summer Solstice.—When the earth is at the summer solstice, about the 21st of June, the sun is overhead 23£° north of the equator, and if its vertical rays could leave a golden line on the surface of the earth as it revolves, they would mark the Tropic of Cancer. The sun is at its furthest northern declination, ascends the highest it is ever seen above our horizon, and rises and sets 23^° north of the east and west points. It seems now to stand still in its northern and southern course, and hence the name Solstice (sol, the sun, sto, to stand). The days in the north temperate zone are longer than the nights. It is our summer, and the 21st of June is the longest day of the year. In the south temperate zone it is winter, and the shortest day of the year. The circle that separates day from night extends 23£° beyond the north pole, and if the sun's rays could in like manner leave a golden line on that day, they would trace on the earth the Arctic Circle. It is the noon of the long six months polar day. The reverse is true at the Antarctic Circle, and it is there the midnight of the long six months polar night.
IX. The earth at the Autumnal Equinox.—The earth crosses the aphelion point the 1st of July, when it is at its furthest distance from the sun, * - which is then said to be in apogee. The sun each day rising and setting a trifle further toward the south, passes through a lower circuit in the heavens. We reach the autumnal equinox the 22d of Sep
tember. The sun being now on the equinoctial, if its vertical rays could leave a line of golden light, they would mark on the earth the circle of the equator. It is autumn in the north temperate zone and spring in the south temperate zone. The days and nights are equal over the whole earth, the sun rising at 6 A. M. and setting at 6 p. M., exactly in the east and west where the equinoctial intersects the horizon.
X. The earth at the Winter Solstice.—The sun after passing the equinoctial—"crossing the line," as it is called—sinks lower toward the southern horizon each day. We reach the winter solstice the 21st of December. The sun is now directly overhead 232° south of the equator, and if its rays could leave a line of golden light they would mark on the earth's surface the Tropic of Capricorn. It is at its furthest southern declination, and rises and sets 23§° south of the east and west points. It is our winter, and the 21st of December is the shortest day of the year. In the south temperate zone it is summer, and the longest day of the year. The circle that separates day from night extends 23£° beyond the south pole, and if the sun's rays in like manner could leave a line of golden light they would mark the Antarctic Circle. It is there the noon of the long six months polar day. At the Arctic Circle the reverse is true; the rays fall 23^° short of the north pole, and it is there the midnight of the long six months polar night. Here again the sun appears to us to stand still a day or two before retracing its course, and it is therefore called the Winter Solstice.
XI. TJie earth at the Vernal Equinox.—The earth reaches its perihelion about the 31st of December. It is then nearest the sun, which is therefore said to be in perigee. The sun rises and sets each day further and further north, and climbs up higher in the heavens at midday. Our days gradually increase in length, and our nights shorten in the same proportion. On the 21st of March* the sun reaches the equinoctial, at the vernal equinox. He is overhead at the equator, and the days and nights are again equal. It is our spring, but in the south temperate zone it is autumn.
XII. The yearly path finished.—The earth moves on in its orbit through the spring and summer months. The sun continues its northerly course, ascending each day higher in the heavens, and its rays becoming less and less oblique. On the 21st of June it again reaches its furthest northern declination, and the earth is at the summer solstice. We have thus traced the yearly path, and noticed the course of the changing seasons, with the length of the days and nights. The same series has been repeated through all the ages of the past, and will be till time shall be no more.
XTII. Distance of the earth from the sun varies.—
* The precise time of the equinoxes and solstices varies each year, but within a small limit.
We notice, from what we have just seen, that we are nearer the sun by 3,000,000 miles in winter than in summer. The obliqueness with which the rays strike the north temperate zone at that time prevents our receiving any special benefit from this favorablejposition of the earth.
XIV. Southern summer.—The inhabitants of the south temperate zone have their summer while the earth is in perihelion, and the sun's rays are about ^5 warmer than when in aphelion, our summer-time. This will perhaps partly account for the extreme heat of their season. Herschel tells us that he has found the temperature of the surface soil of South Africa 159° F. Captain Sturt, in speaking of the extreme heat of Australia, says that matches accidentally dropped on the ground were immediately ignited. The southern winters, for a similar reason, are colder; and this makes the average yearly temperature about the same as ours.
XV. Extremes of heat and cold not at the solstices.— We notice that we do not have our greatest heat at the time of the summer solstice, nor our greatest cold at the winter solstice. After the 21st of June, the earth, already warmed by the genial spring days, continues to receive more heat from the sun by day than it radiates by night: thus its temperature still increases. On the other hand, after the 21st of December the earth continues to become colder, because it loses more heat during the night than it receives during the day.
XVI. Summer longer than winter.—As the sun is not in the centre of the earth's orbit, but at one of its foci, that portion of the orbit which the earth passes through in going from the vernal to the autumnal equinox comprises more than one-half the entire ecliptic. On this account the summer is longer than the winter. The difference is still further enhanced by the variation in the earth's velocity at aphelion and perihelion. The annexed table gives the mean duration of the seasons:
Seasons. Days. Seasons. Days.
Spring 92.9 Autumn 89.7
Summer 93.6 Winter 89.0
The difference of time in the earth's stay in the two portions of the ecliptic, as will be seen from the above, is 7.8 days.
XVII. Varying velocity of the earth.—We can see, by looking at the plate, that the velocity of the earth must vary in different portions of its orbit. When passing from the vernal equinox to aphelion, the attraction of the sun tends to check its speed; from that point to the autumnal equinox, the attraction is partly in the direction of its motion, and so increases its velocity. The same principle applies when going to and from perihelion.
XVIII. Curious appearance of the sun at the north pole.—" To a person standing at the north pole, the sun appears to sweep horizontally around the sky every twenty-four hours, without any perceptible