ible. Stars at right angles to
the earth's line of motion would be most affected, and these would be
all displaced by the full amount of 20 seconds of arc. Stars in
intermediate directions would be displaced by intermediate amounts.

But the line of the earth's motion is approximately a circle round the
sun, hence the direction of its advance is constantly though slowly
changing, and in one year it goes through all the points of the compass.
The stars, being displaced always in the line of advance, must similarly
appear to describe little closed curves, always a quadrant in advance of
the earth, completing their orbits once a year. Those near the pole of
the ecliptic will describe circles, being always at right angles to the
motion. Those in the plane of the ecliptic (near the zodiac) will be
sometimes at right angles to the motion, but at other times will be
approached or receded from; hence these will oscillate like pendulums
once a year; and intermediate stars will have intermediate motions--that
is to say, will describe ellipses of varying excentricity, but all
completed in a year, and all with the major axis 20". This agreed very
closely with what was observed.

The main details were thus clearly and simply explained by the
hypothesis of a finite velocity for light, "the successive propagation
of light in time." This time there was no room for hesitation, and
astronomers hailed the discovery with enthusiasm.

Not yet, however, did Bradley rest. The finite velocity of light
explained the major part of the irregularities he had observed, but not
the whole. The more carefully he measured the amount of the deviation,
the less completely accurate became its explanation.

There clearly was a small outstanding error or discrepancy; the stars
were still subject to an unexplained displacement--not, indeed, a
displacement that repeated itself every year, but one that went through
a cycle of changes in a longer period.

The displacement was only about half that of aberration, and having a
longer period was rather more difficult to detect securely. But the
major difficulty was the fact that the two sorts of disturbances were
co-existent, and the skill of disentangling them, and exhibiting the
true and complete cause of each inequality, was very brilliant.

For nineteen years did Bradley observe this minor displacement, and in
that time he saw it go through a complete cycle. Its cause was now clear
to him; the nineteen-ye