n. We shall not pursue the calculations relative to other stars;
suffice it here to say that the movement of the earth has been found
adequate to account for the phenomena, and thus the doctrine of the
aberration of light is demonstrated.

It remains to allude to one point of the utmost interest and importance.
We have seen that the magnitude of the aberration can be measured by
astronomical observation. The amount of this aberration depends upon the
velocity of light, and on the velocity with which the earth's motion is
performed. We can measure the velocity of light by independent
measurements, in the manner already explained in Chapter XII. We are
thus enabled to calculate what the velocity of the earth must be, for
there is only one particular velocity for the earth which, when combined
with the measured velocity of light, will give the measured value of
aberration. The velocity of the earth being thus ascertained, and the
length of the year being known, it is easy to find the circumference of
the earth's path, and therefore its radius; that is, the distance from
the earth to the sun.

Here is indeed a singular result, and one which shows how profoundly the
various phenomena of science are interwoven. We make experiments in our
laboratory, and find the velocity of light. We observe the fixed stars,
and measure the aberration. We combine these results, and deduce
therefrom the distance from the earth to the sun! Although this method
of finding the sun's distance is one of very great elegance, and admits
of a certain amount of precision, yet it cannot be relied upon as a
perfectly unimpeachable method of deducing the great constant. A perfect
method must be based on the operations of mere surveying, and ought not
to involve recondite physical considerations. We cannot, however, fail
to regard the discovery of aberration by Bradley as a most pleasing and
beautiful achievement, for it not only greatly improves the calculations
of practical astronomy, but links together several physical phenomena of
the greatest interest.

CHAPTER XXVI.

THE ASTRONOMICAL SIGNIFICANCE OF HEAT.

Heat and Astronomy--Distribution of Heat--The Presence of Heat in
the Earth--Heat in other Celestial Bodies--Varieties of
Temperature--The Law of Cooling--The Heat of the Sun--Can its
Temperature be Measured?--Radiation connected with the Sun's
Bulk--Can the Sun be Exhausting his Resources?--No marked Change