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to set their clocks by the sun. But owing to the obliquity of the ecliptic and the eccentricity of the earth's orbit around the sun, the intervals between successive passages of the sun are not exactly equal. The consequence is that, if a clock keeps exact time, the sun will sometimes pass the meridian before and sometimes after twelve by the clock. When this was understood, a distinction was made between apparent and mean time. Apparent time was the unequal time determined by the sun; mean time was that given by a clock keeping perfect time month after month. The difference between these two is called the equation of time. Its greatest amounts are reached every year about the first of November and the middle of February. At the former time, the sun passes the meridian sixteen minutes before the clock shows twelve; in February, fourteen or fifteen minutes after twelve.

To define mean time astronomers imagine a mean sun which always moves along the celestial equator so as to pass the meridian at exactly equal intervals of time, and which is sometimes ahead of the real sun and sometimes behind it. This imaginary or mean sun determines the time of day. The subject will perhaps be a little easier if we describe things as they appear, imagining the earth to be at rest while the mean sun revolves around it, crossing the meridian of every place in succession. We thus imagine noon to be constantly travelling around the world. In our latitudes, its speed is not far from a thousand feet per second; that is to say, if it is noon at a certain place where we stand, it will one second afterward be noon about one thousand feet farther west, in