less obliquely in the northern hemisphere than when the earth was in Libra. Let six months elapse the earth is now in Cancer and the sun in Capricornus; and he is overhead, 231° south of the equator. His rays strike less obliquely in the southern hemisphere than before, but in the northern hemisphere more obliquely. These six months have changed the direction of the sun's rays on every part of the earth's surface. This accounts for the difference in temperature between summer and winter. V. The Equinoxes. At the equinoxes one half

of

each hemisphere is illuminated: hence the name Equinox (œquus, equal, and nox, night). At these points of the orbit the days and nights are equal over the entire earth,* each being twelve hours in length.

VI. Northern and southern hemispheres unequally illuminated.-While one half of the earth is constantly illuminated, at all points in the orbit except the equinoxes the proportion of the northern or southern hemisphere which is in daylight or darkness varies. When more than half of a hemisphere is in the light, its days are longer than the nights, and vice versa.

VII. The seasons and the comparative length of days and nights in the South Temperate Zone, at any specified time, are the reverse of those in the North Temperate Zone, except at the Equinoxes, where the days and nights are of equal length.

* Except a small space at each pole.

VIII. The earth at the Summer Solstice.—When the earth is at the summer solstice, about the 21st of June, the sun is overhead 234° 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 231° 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 2310 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 231 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 231 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 231° 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 2310 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. The 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.

XIII. 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 favorable position 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

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 tempera-® ture 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.