CELESTIAL MEASUREMENT
Had the telescope never been invented our knowledge of astronomy would
be trifling indeed.
Prior to the year 1610, when Galileo first turned the new instrument
upon the sky, all that men knew of the starry realms was gathered from
observation with their own eyes unaided by any artificial means. In such
researches they had been very much at a disadvantage. The sun and moon,
in their opinion, were no doubt the largest bodies in the heavens, for
the mere reason that they looked so! The mighty solar disturbances,
which are now such common-places to us, were then quite undreamed of.
The moon displayed a patchy surface, and that was all; her craters and
ring-mountains were surprises as yet in store for men. Nothing of course
was known about the surfaces of the planets. These objects had indeed no
particular characteristics to distinguish them from the great host of
the stars, except that they continually changed their positions in the
sky while the rest did not. The stars themselves were considered as
fixed inalterably upon the vault of heaven. The sun, moon, and planets
apparently moved about in the intermediate space, supported in their
courses by strange and fanciful devices. The idea of satellites was as
yet unknown. Comets were regarded as celestial portents, and meteors as
small conflagrations taking place in the upper air.
In the entire absence of any knowledge with regard to the actual sizes
and distances of the various celestial bodies, men naturally considered
them as small; and, concluding that they were comparatively near,
assigned to them in consequence a permanent connection with terrestrial
affairs. Thus arose the quaint and erroneous beliefs of astrology,
according to which the events which took place upon our earth were
considered to depend upon the various positions in which the planets,
for instance, found themselves from time to time.
It must, however, be acknowledged that the study of astrology,
fallacious though its conclusions were, indirectly performed a great
service to astronomy by reason of the accurate observations and diligent
study of the stars which it entailed.
We will now inquire into the means by which the distances and sizes of
the celestial orbs have been ascertained, and see how it was that the
ancients were so entirely in the dark in this matter.
There are two distinct methods of finding out the distance at which any
object happens to be situated from us.
One method is by actual measurement.
The other is by moving oneself a little to the right or left, and
observing whether the distant object appears in any degree altered in
position by our own change of place.
One of the best illustrations of this relative change of position which
objects undergo as a result of our own change of place, is to observe
the landscape from the window of a moving railway carriage. As we are
borne rapidly along we notice that the telegraph posts which are set
close to the line appear to fly past us in the contrary direction; the
trees, houses, and other things beyond go by too, but not so fast;
objects a good way off displace slowly; while some spire, or tall
landmark, in the far distance appears to remain unmoved during a
comparatively long time.
Actual change of position on our own part is found indeed to be
invariably accompanied by an apparent displacement of the objects about
us, such apparent displacement as a result of our own change of position
being known as "parallax." The dependence between the two is so
mathematically exact, that if we know the amount of our own change of
place, and if we observe the amount of the consequent displacement of
any object, we are enabled to calculate its precise distance from us.
Thus it comes to pass that distances can be measured without the
necessity of moving over them; and the breadth of a river, for instance,
or the distance from us of a ship at sea, can be found merely by such
means.
It is by the application of this principle to the wider field of the sky
that we are able to ascertain the distance of celestial bodies. We have
noted that it requires a goodly change of place on our own part to shift
the position in which some object in the far distance is seen by us. To
two persons separated by, say, a few hundred yards, a ship upon the
horizon will appear pretty much in the same direction. They would
require, in fact, to be much farther apart in order to displace it
sufficiently for the purpose of estimating their distance from it. It
is the same with regard to the moon. Two observers, standing upon our
earth, will require to be some thousands of miles apart in order to see
the position of our satellite sufficiently altered with regard to the
starry background, to give the necessary data upon which to ground their
calculations.
The change of position thus offered by one side of the earth's surface
at a time is, however, not sufficient to displace any but the nearest
celestial bodies. When we have occasion to go farther afield we have to
seek a greater change of place. This we can get as a consequence of the
earth's movement around the sun. Observations, taken several days apart,
will show the effect of the earth's change of place during the interval
upon the positions of the other bodies of our system. But when we desire
to sound the depths of space beyond, and to reach out to measure the
distance of the nearest star, we find ourselves at once thrown upon the
greatest change of place which we can possibly hope for; and this we get
during the long journey of many millions of miles which our earth
performs around the sun during the course of each year. But even this
last change of place, great as it seems in comparison with terrestrial
measurements, is insufficient to show anything more than the tiniest
displacements in a paltry forty-three out of the entire host of the
stars.
We can thus realise at what a disadvantage the ancients were. The
measuring instruments at their command were utterly inadequate to detect
such small displacements. It was reserved for the telescope to reveal
them; and even then it required the great telescopes of recent times to
show the slight changes in the position of the nearer stars, which were
caused by the earth's being at one time at one end of its orbit, and
some six months later at the other end--stations separated from each
other by a gulf of about one hundred and eighty-six millions of miles.
The actual distances of certain celestial bodies being thus
ascertainable, it becomes a matter of no great difficulty to determine
the actual sizes of the measurable ones. It is a matter of everyday
experience that the size which any object appears to have, depends
exactly upon the distance it is from us. The farther off it is the
smaller it looks; the nearer it is the bigger. If, then, an object which
lies at a known distance from us looks such and such a size, we can of
course ascertain its real dimensions. Take the moon, for instance. As we
have already shown, we are able to ascertain its distance. We observe
also that it looks a certain size. It is therefore only a matter of
calculation to find what its actual dimensions should be, in order that
it may look that size at that distance away. Similarly we can ascertain
the real dimensions of the sun. The planets, appearing to us as points
of light, seem at first to offer a difficulty; but, by means of the
telescope, we can bring them, as it were, so much nearer to us, that
their broad expanses may be seen. We fail, however, signally with regard
to the stars; for they are so very distant, and therefore such tiny
points of light, that our mightiest telescopes cannot magnify them
sufficiently to show any breadth of surface.
Instead of saying that an object looks a certain breadth across, such
as a yard or a foot, a statement which would really mean nothing,
astronomers speak of it as measuring a certain angle. Such angles are
estimated in what are called "degrees of arc"; each degree being divided
into sixty minutes, and each minute again into sixty seconds. Popularly
considered the moon and sun _look_ about the same size, or, as an
astronomer would put it, they measure about the same angle. This is an
angle, roughly, of thirty-two minutes of arc; that is to say, slightly
more than half a degree. The broad expanse of surface which a celestial
body shows to us, whether to the naked eye, as in the case of the sun
and moon, or in the telescope, as in the case of other members of our
system, is technically known as its "disc."