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Places in north latitude, as they are carried round by the turning of the earth, are then in sunlight during more than half their course; those in south latitude less.

The result as it appears to us is that the sun is more than half the time above the horizon, and that we have the hot weather of summer, while in the southern hemisphere the days are short, and the season is winter.

During our winter months the case is reversed. The southern hemisphere is then tipped toward the sun, and the northern hemisphere away from it. Consequently, summer and long days are the order in the southern, and the reverse in the northern hemisphere.

We most naturally define the year as the interval of time in which the earth revolves around the sun. From what we have said, there are two ways of ascertaining its length. One is to find the interval between two passages of the sun past the same star. The other is to find the interval between two passages of the sun past the same equinox, that is, across the equator. If the latter were fixed among the stars the two intervals would be equal. But it was found by the ancient astronomers, from observations extending through several centuries, that these two methods did not give the same length of year. It took the sun about eleven minutes longer to make the circuit of the stars than to make the circuit of the equinoxes. This shows that the equinoxes steadily shift their position among the stars from year to year. This shift is called the precession of the equinoxes. It