"If it were not for this, the earth would still have her zones of seasons; a part of the earth would have endless summer, a part endless spring, and the rest unbroken winter, but the alternation of seasons at the same place would be unknown. The axis of the earth is now inclined about twenty-three degrees, twenty-seven minutes, twenty-three seconds to the plane of the earth's orbit, and as this axis maintains constantly the same position, being parallel in one part of the earth's orbit to its position in any other part of its orbit, during one part of the year the north pole is turned twenty-three and a half degrees toward the sun, while in the opposite part of the year the south pole is in like manner brought into the light and heat. This causes the sun to appear to move to and fro, north and south, twenty-three degrees, twenty-seven minutes, and twenty-three seconds from the equator in either direction. The tropics, or turning-places, mark the limits of the sun's northern and southern journey. Everywhere between the tropics the sun, at some period of the year, passes through the zenith, that is, exactly overhead at noon. North and south of the tropics the sun seems to rise higher in summer and to sink lower in winter. In summer the sun at midday is about forty-seven degrees nearer the zenith than in winter. Within the polar circles, which are the same distance from the poles as the tropics from the equator, the heat of the sun is entirely withdrawn during a portion of the year, and during another portion of about equal length the sun does not set. The extremes of temperature, caused by the inclination of the earth's axis and its revolution around the sun, are very great. In the northern part of Minnesota, the temperature rises in summer to one hundred degrees, and in winter sinks to fifty degrees below zero, giving thus an alternation of one hundred and fifty degrees. "In this connection you may also remember that the sun is nearer the earth in one part of its orbit than in another part. This difference amounts to about 3,000,000 miles. The sun also remains eight days longer north of the equator than south of it. Our summer, therefore, is eight days longer than the summer of the southern hemispheres, and our winters are correspondingly shorter. These differences tend, however, to balance each other, for while the southern summer is shorter, the sun at that time is nearer, and while our summer is longer, the sun is more distant. Peter, you may explain to us the effect upon temperature caused by the division of the earth's surface into land and water." "I learned while studying physical geography that the temperature is more even upon the sea than upon the land. But why, I do not know."