Imagine the sun to be an electric lamp fixed by strings at the center of a very large hoop, laid flatwise, but somewhat tilted. The hoop represents the path taken by the earth round the sun, and the earth is represented by a spherical spinning-top moving along a groove in the upper side of the hoop. Then picture the top making a circuit of the hoop once in a year.
Twice during the year it will be level, or nearly level, with the lamp, and its upper and lower halves be lit equally. These periods are like our spring and autumn. When the top is at the highest point of the hoop, its lower half is lighted more strongly than its upper half. This position corresponds to midsummer in the southern hemisphere of the earth ; and to midwinter in the northern hemisphere. On the other hand, the northern hemisphere has reached midsummer, and the southern hemisphere midwinter, when the top has got to the lowest point of its path.