The way in which he was led to the detection of the nutation
is strikingly illustrative of the extreme care with which Bradley
conducted his observations. He found that in the course of a
twelvemonth, when the star had completed the movement which was due
to aberration, it did not return exactly to the same position which
it had previously occupied. At first he thought this must be due to
some instrumental error, but after closer examination and repeated
study of the effect as manifested by many different stars, he came to
the conclusion that its origin must be sought in some quite different
source. The fact is that a certain change takes place in the
apparent position of the stars which is not due to the movement of
the star itself, but is rather to be attributed to changes in the
points from which the star's positions are measured.
We may explain the matter in this way. As the earth is not a sphere,
but has protuberant parts at the equator, the attraction of the moon
exercises on those protuberant parts a pulling effect which
continually changes the direction of the earth's axis, and
consequently the position of the pole must be in a state of incessant
fluctuation. The pole to which the earth's axis points on the sky
is, therefore, slowly changing.
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