conceive the solar system as we know it to be, that we are wont
to forget how very different it is from what it seems. Yet one
needs but to glance up at the sky, and then to glance about one
at the solid earth, to grant, on a moment's reflection, that the
geocentric idea is of all others the most natural; and that to
conceive the sun as the actual Centre of the solar system is an
idea which must look for support to some other evidence than that
which ordinary observation can give. Such was the view of most of
the ancient philosophers, and such continued to be the opinion of
the majority of mankind long after the time of Copernicus. We
must not forget that even so great an observing astronomer as
Tycho Brahe, so late as the seventeenth century, declined to
accept the heliocentric theory, though admitting that all the
planets except the earth revolve about the sun. We shall see that
before the Alexandrian school lost its influence a geocentric
scheme had been evolved which fully explained all the apparent
motions of the heavenly bodies. All this, then, makes us but
wonder the more that the genius of an Aristarchus could give
precedence to scientific induction as against the seemingly clear
evidence of the senses.

What, then, was the line of scientific induction that led
Aristarchus to this wonderful goal? Fortunately, we are able to
answer that query, at least in part. Aristarchus gained his
evidence through some wonderful measurements. First, he measured
the disks of the sun and the moon. This, of course, could in
itself give him no clew to the distance of these bodies, and
therefore no clew as to their relative size; but in attempting to
obtain such a clew he hit upon a wonderful yet altogether simple
experiment. It occurred to him that when the moon is precisely
dichotomized-- that is to say, precisely at the half-the line of
vision from the earth to the moon must be precisely at right
angles with the line of light passing from the sun to the moon.
At this moment, then, the imaginary lines joining the sun, the
moon, and the earth, make a right angle triangle. But the
properties of the right-angle triangle had long been studied and
were well under stood. One acute angle of such a triangle
determines the figure of the triangle itself. We have already
seen that Thales, the very earliest of the Greek philosophers,
measured the distance of a ship at sea by the application of this
principle. Now Aristarchus sights the sun in place of Thales'
ship, and, sighting the moon at the same time, measures the angle
and establishes the shape of his right-angle triangle. This does
not tell him the distance of the sun, to be sure, for he does not
know the length of his base-line--that is to say, of the line
between the moon and the earth. But it does establish the
relation of that base-line to the other lines of the triangle; in
other words, it tells him the distance of the sun in terms of the
moon's distance. As Aristarchus strikes the angle, it shows that
the sun is eighteen times as distant as the moon. Now, by
comparing the apparent size of the sun with the apparent size of
the moon--which, as we have seen, Aristarchus has already
measured--he is able to tell us that, the sun is "more than 5832
times, and less than 8000" times larger than the moon; though his
measurements, taken by themselves, give no clew to the actual
bulk of either body. These conclusions, be it understood, are
absolutely valid inferences--nay, demonstrations--from the
measurements involved, provided only that these measurements have
been correct. Unfortunately, the angle of the triangle we have
just seen measured is exceedingly difficult to determine with
accuracy, while at the same time, as a moment's reflection will
show, it is so large an angle that a very slight deviation from
the truth will greatly affect the distance at which its line
joins the other side of the triangle. Then again, it is virtually
impossible to tell the precise moment when the moon is at half,
as the line it gives is not so sharp that we can fix it with
absolute accuracy. There is, moreover, another element of error
due to the refraction of light by the earth's atmosphere. The
experiment was probably made when the sun was near the horizon,
at which time, as we now know, but as Aristarchus probably did
not suspect, the apparent displacement of the sun's position is
considerable; and this displacement, 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


<< previous page | next page >>

Jump to page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | 81 | 82 | 83 | 84 | 85 | 86 | 87 | 88 | 89 |