These alterations are very small in
amount, and they take place with extreme slowness, but they are in
incessant progress, and their amount admits of being accurately
calculated. At the present time, and for thousands of years past, as
well as for thousands of years to come, the eccentricity of the
earth's orbit is diminishing, and consequently the orbit described by
the earth each year is becoming more nearly circular. We must,
however, remember that under all circumstances the length of the
longest axis of the ellipse is unaltered, and consequently the size
of the track which the earth describes around the sun is gradually
increasing. In other words, it may be said that during the present
ages the average distance between the earth and the sun is waxing
greater in consequence of the perturbations which the earth
experiences from the attraction of the other planets. We have,
however, already seen that the efficiency of the solar attraction for
disturbing the moon's movement depends on the distance between the
earth and the sun. As therefore the average distance between the
earth and the sun is increasing, at all events during the thousands
of years over which our observations extend, it follows that the
ability of the sun for disturbing the moon must be gradually
diminishing.
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