cement, it will be observed, is in the direction to lessen the
angle in question.

In point of fact, Aristarchus estimated the angle at eighty-seven
degrees. Had his instrument been more precise, and had he been able
to take account of all the elements of error, he would have found
it eighty-seven degrees and fifty-two minutes. The difference of
measurement seems slight; but it sufficed to make the computations
differ absurdly from the truth. The sun is really not merely eighteen
times but more than two hundred times the distance of the moon, as
Wendelein discovered on repeating the experiment of Aristarchus about
two thousand years later. Yet this discrepancy does not in the least
take away from the validity of the method which Aristarchus employed.
Moreover, his conclusion, stated in general terms, was perfectly
correct: the sun is many times more distant than the moon and vastly
larger than that body. Granted, then, that the moon is, as Aristarchus
correctly believed, considerably less in size than the earth, the
sun must be enormously larger than the earth; and this is the vital
inference which, more than any other, must have seemed to Aristarchus
to confirm the suspicion that the sun and not the earth is the centre
of the planetary system. It seemed to him inherently improbable that an
enormously large body like the sun should revolve about a small one such
as the earth. And again, it seemed inconceivable that a body so distant
as the sun should whirl through space so rapidly as to make the circuit
of its orbit in twenty-four hours. But, on the other hand, that a
small body like the earth should revolve about the gigantic sun seemed
inherently probable. This proposition granted, the rotation of the earth
on its axis follows as a necessary consequence in explanation of the
seeming motion of the stars. Here, then, was the heliocentric doctrine
reduced to a virtual demonstration by Aristarchus of Samos, somewhere
about the middle of the third century B.C.

It must be understood that in following out the steps of reasoning by
which we suppose Aristarchus to have reached so remarkable a conclusion,
we have to some extent guessed at the processes of thought-development;
for no line of explication written by the astronomer himself on this
particular point has come down to us. There does exist, however, as we
have already stated, a very remarkable treatise by Aristarchus on the
Size and Distance of the Sun and the Moon, w