ASTRONOMY BEFORE THE TELESCOPE
There was thus a real science of astronomy before we have any history
of it. Some important discoveries had been made, and the first step
had been taken towards cataloguing the fixed stars. It was certainly
known to some of the students of the heavens, though perhaps only to a
few, that the Earth was a sphere, freely suspended in space, and
surrounded on all sides by the starry heavens, amongst which moved the
Sun, Moon, and the five planets. The general character of the Sun's
movement was also known; namely, that he not only moved day by day from
east to west, as the stars do, but also had a second motion inclined at
an angle to the first, and in the opposite direction, which he
accomplished in the course of a year.
To this sum of knowledge, no doubt, several nations had contributed.
We do not know to what race we owe the constellations, but there are
evidences of an elementary acquaintance with astronomy on the part of
the Chinese, the Babylonians, the Egyptians, and the Jews. But in the
second stage of the development of the science the entire credit for
the progress made belongs to the Greeks.
The Greeks, as a race, appear to have been very little apt at
originating ideas, but they possessed, beyond all other races, the
power of developing and perfecting crude ideas which they had obtained
from other sources, {21} and when once their attention was drawn to the
movements of the heavenly bodies, they devoted themselves with striking
ingenuity and success to devising theories to account for the
appearances presented, to working out methods of computation, and,
last, to devising instruments for observing the places of the
luminaries in which they were interested.
In the brief space available it is only possible to refer to two or
three of the men whose commanding intellects did so much to help on the
development of the science. EUDOXUS of Knidus, in Asia Minor (408-355
B.C.), was, so far as we know, the first to attempt to represent the
movements of the heavenly bodies by a simple mathematical process. His
root idea was something like this. The Earth was in the centre of the
universe, and it was surrounded, at a great distance from us, by a
number of invisible transparent shells, or spheres. Each of these
spheres rotated with perfect uniformity, though the speed of rotation
differed for different spheres. One sphere carried the stars, and
rotated from east to west in about 23 h. 56 m. The Sun was carried by
another sphere, which rotated from west to east in a year, but the
pivots, or poles, of this sphere were carried by a second, rotating
exactly like the sphere of the stars. This explained how it is that
the ecliptic--that is to say, the apparent path of the Sun amongst the
stars--is inclined 23-½° to the equator of the sky, so that the Sun is
23-½° north of the equator at midsummer and 23-½° south of the equator
at midwinter, for the poles of the sphere peculiar to the Sun were
supposed to be 23-½° from the poles of the sphere peculiar to the
stars. Then the Moon had three spheres; that which actually carried
the Moon having its poles 5° from the poles of the sphere peculiar to
the {22} Sun. These poles were carried by a sphere placed like the
sphere of the Sun, but rotating in 27 days; and this, again, had its
poles in the sphere of the stars. The sphere carrying the Moon
afforded the explanation of the wavy motion of the Moon to and fro
across the ecliptic in the course of a month, for at one time in the
month the Moon is 5° north of the ecliptic, at another time 5° south.
The motions of the planets were more difficult to represent, because
they not only have a general daily motion from east to west, like the
stars, and a general motion from west to east along the ecliptic, like
the Sun and Moon, but from time to time they turn back on their course
in the ecliptic, and "retrograde." But the introduction of a third and
fourth sphere enabled the motions of most of the planets to be fairly
represented. There were thus twenty-seven spheres in all--four for
each of the five planets, three for the Moon, three for the Sun
(including one not mentioned in the foregoing summary), and one for the
stars. These spheres were not, however, supposed to be solid
structures really existing; the theory was simply a means for
representing the observed motions of the heavenly bodies by
computations based upon a series of uniform movements in concentric
circles.
But this assumption that each heavenly body moves in its path at a
uniform rate was soon seen to be contrary to fact. A reference to the
almanac will show at once that the Sun's movement is not uniform. Thus
for the year 1910-11 the solstices and equinoxes fell as given on the
next page:
{23}
_Epoch Time Interval_
Winter Solstice 1910 Dec. 22 d. 5 h. 12 m. P.M. 89 d. 0 h. 42 m.
Spring Equinox 1911 Mar. 21 " 5 " 54 " P.M. 92 " 19 " 41 "
Summer Solstice 191l June 22 " 1 " 35 " P.M. 93 " 14 " 43 "
Autumn Equinox 1911 Sept. 24 " 4 " 18 " A.M. 89 " 18 " 36 "
Winter Solstice 1911 Dec. 22 " 10 " 54 " P.M.
so that the winter half of the year is shorter than the summer half;
the Sun moves more quickly over the half of its orbit which is south of
the equator than over the half which is north of it.
The motion of the Moon is more irregular still, as we can see by taking
out from the almanac the times of new and full moon:
_New Moon Interval to Full Moon_
Dec. 1910 1 d. 9 h. 10.7 m. P.M. 14 d. 13 h. 54.4 m.
" " 31 " 4 " 21.2 " P.M. 14 " 6 " 4.8 "
Jan. 1911 30 " 9 " 44.7 " A.M. 14 " 0 " 52.8 "
March " 1 " 0 " 31.1 " A.M. 13 " 23 " 27.4 "
" " 30 " 0 " 37.8 " P.M. 14 " 1 " 58.8 "
April " 28 " 10 " 25.0 " P.M. 14 " 7 " 44.7 "
May " 28 " 6 " 24.4 " A.M. 14 " 15 " 26.3 "
June " 26 " 1 " 19.7 " P.M. 14 " 23 " 33.7 "
July " 25 " 8 " 12.0 " P.M. 15 " 6 " 42.7 "
Aug. " 24 " 4 " 14.3 " A.M. 15 " 11 " 42.4 "
Sept. " 22 " 2 " 37.4 " P.M. 15 " 13 " 33.7 "
Oct. " 22 " 4 " 9.3 " A.M. 15 " 11 " 38.8 "
Nov. " 20 " 8 " 49.4 " P.M. 15 " 6 " 2.5 "
Dec. " 20 " 3 " 40.3 " P.M. 14 " 21 " 49.4 "
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_Full Moon Interval to New Moon_
Dec. 1910 16 d 11 h. 5.1 m. A.M. 15 d. 5 h. 16.1 m.
Jan. 1911 14 " 10 " 26.0 " P.M. 15 " 11 " 18.7 "
Feb. " 13 " 10 " 37.5 " A.M. 15 " 13 " 53.6 "
March " 14 " 11 " 58.5 " P.M. 15 " 12 " 39.3 "
April " 13 " 2 " 36.6 " P.M. 15 " 7 " 48.4 "
May " 13 " 6 " 9.7 " A.M. 15 " 0 " 14.7 "
June " 11 " 9 " 50.7 " P.M. 14 " 15 " 29.0 "
July " 11 " 0 " 53.4 " P.M. 14 " 7 " 18.6 "
Aug. " 10 " 2 " 54.7 " A.M. 14 " 1 " 19.6 "
Sept. " 8 " 3 " 56.7 " P.M. 13 " 22 " 40.7 "
Oct. " 8 " 4 " 11.1 " A.M. 13 " 23 " 58.2 "
Nov. " 6 " 3 " 48.1 " P.M. 14 " 5 " 1.3 "
Dec. " 6 " 2 " 51.9 " A.M. 14 " 12 " 48.4 "
Jan. 1912 4 " 1 " 99.7 " P.M. 14 " 21 " 40.3 "
The astronomer who dealt with this difficulty was HIPPARCHUS (about
190-120 B.C.), who was born at Nicæa, in Bithynia, but made most of his
astronomical observations in Rhodes. He attempted to explain these
irregularities in the motions of the Sun and Moon by supposing that
though they really moved uniformly in their orbits, yet the centre of
their orbits was not the centre of the Earth, but was situated a little
distance from it. This point was called "+the excentric+," and the
line from the excentric to the Earth was called "+the line of apsides+."
But when he tried to deal with the movements of the planets, he found
that there were not enough good observations available for him to build
up any satisfactory theory. He therefore devoted himself to the work
of making systematic determinations of the places of the planets that
he might put his successors in a better position to deal with the
problem than he was. His great successor was CLAUDIUS PTOLEMY of {25}
Alexandria, who carried the work of astronomical observation from about
A.D. 127 to 150. He was, however, much greater as a mathematician than
as an observer, and he worked out a very elaborate scheme, by which he
was able to represent the motions of the planets with considerable
accuracy. The system was an extremely complex one, but its principle
may be represented as follows: If we suppose that a planet is moving
round the Earth in a circle at a uniform rate, and we tried to compute
the place of the planet on this assumption for regular intervals of
time, we should find that the planet gradually got further and further
away from the predicted place. Then after a certain time the error
would reach a maximum, and begin to diminish, until the error vanished
and the planet was in the predicted place at the proper time. The
error would then begin to fall in the opposite direction, and would
increase as before to a maximum, subsequently diminishing again to
zero. This state of things might be met by supposing that the planet
was not itself carried by the circle round the earth, but by an
+epicycle+--_i.e._ a circle travelling upon the first circle--and by
judiciously choosing the size of the epicycle and the time of
revolution the bulk of the errors in the planet's place might be
represented. But still there would be smaller errors going through
their own period, and these, again, would have to be met by imagining
that the first epicycle carried a second, and it might be that the
second carried a third, and so on.
The Ptolemaic system was more complicated than this brief summary would
suggest, but it is not possible here to do more than indicate the
general principles upon which it was founded, and the numerous other
systems or modifications of them produced in the {26} five centuries
from Eudoxus to Ptolemy must be left unnoticed. The point to be borne
in mind is that one fundamental assumption underlay them all, an
assumption fundamental to all science--the assumption that like causes
must always produce like effects. It was apparent to the ancient
astronomers that the stars--that is to say, the great majority of the
heavenly bodies--do move round the Earth in circles, and with a perfect
uniformity of motion, and it seemed inevitable that, if one body moved
round another, it should thus move. For if the revolving body came
nearer to the centre at one time and receded at another, if it moved
faster at one time and slower at another, then, the cause remaining the
same, the effect seemed to be different. Any complexity introduced by
superposing one epicycle upon another seemed preferable to abandoning
this great fundamental principle of the perfect uniformity of the
actings of Nature.
For more than 1300 years the Ptolemaic system remained without serious
challenge, and the next great name that it is necessary to notice is
that of COPERNICUS (1473-1543). Copernicus was a canon of Frauenburg,
and led the quiet, retired life of a student. The great work which
made him immortal, _De Revolutionibus_, was the result of many years'
meditation and work, and was not printed until he was on his deathbed.
In this work Copernicus showed that he was one of those great thinkers
who are able to look beyond the mere appearance of things and to grasp
the reality of the unseen. Copernicus realised that the appearance
would be just the same whether the whole starry vault rotated every
twenty-four hours round an immovable Earth from east to west or the
Earth rotated from west to east in the midst of the starry sphere; and,
as the {27} stars are at an immeasurable distance, the latter
conception was much the simpler. Extending the idea of the Earth's
motion further, the supposition that, instead of the Sun revolving
round a fixed Earth in a year, the Earth revolved round a fixed Sun,
made at once an immense simplification in the planetary motions. The
reason became obvious why Mercury and Venus were seen first on one side
of the Sun and then on the other, and why neither of them could move
very far from the Sun; their orbits were within the orbit of the Earth.
The stationary points and retrogressions of the planets were also
explained; for, as the Earth was a planet, and as the planets moved in
orbits of different sizes, the outer planets taking a longer time to
complete a revolution than the inner, it followed, of necessity, that
the Earth in her motion would from time to time be passed by the two
inner planets, and would overtake the three outer. The chief of the
Ptolemaic epicycles were done away with, and all the planets moved
continuously in the same direction round the Sun. But no planet's
motion could be represented by uniform motion in a single circle, and
Copernicus had still to make use of systems of epicycles to account for
the deviations from regularity in the planetary motions round the Sun.
The Earth having been abandoned as the centre of the universe, a
further sacrifice had to be made: the principle of uniform motion in a
circle, which had seemed so necessary and inevitable, had also to be
given up.
For the time came when the instruments for measuring the positions of
the stars and planets had been much improved, largely due to TYCHO
BRAHE (1546-1601), a Dane of noble birth, who was the keenest and most
careful observer that astronomy had yet produced. {28} His
observations enabled his friend and pupil, JOHANN KEPLER, (1571-1630),
to subject the planetary movements to a far more searching examination
than had yet been attempted, and he discovered that the Sun is in the
plane of the orbit of each of the planets, and also in its +line of
apsides+--that is to say, the line joining the two points of the orbit
which are respectively nearest and furthest from the Sun. Copernicus
had not been aware of either of these two relations, but their
discovery greatly strengthened the Copernican theory.
Then for many years Kepler tried one expedient after another in order
to find a combination of circular motions which would satisfy the
problem before him, until at length he was led to discard the circle
and try a different curve--the oval or ellipse. Now the property of a
circle is that every point of it is situated at the same distance from
the centre, but in an ellipse there are two points within it, the
"foci," and the sum of the distances of any point on the circumference
from these two foci is constant. If the two foci are at a great
distance from each other, then the ellipse is very long and narrow; if
the foci are close together, the ellipse differs very little from a
circle; and if we imagine that the two foci actually coincide, the
ellipse becomes a circle. When Kepler tried motion in an ellipse
instead of motion in a circle, he found that it represented correctly
the motions of all the planets without any need for epicycles, and that
in each case the Sun occupied one of the foci. And though the planet
did not move at a uniform speed in the ellipse, yet its motion was
governed by a uniform law, for the straight line joining the planet to
the Sun, the "+radius vector+," passed over equal areas of space in
equal periods of time.
{29}
These two discoveries are known as Kepler's First and Second Laws. His
Third Law connects all the planets together. It was known that the
outer planets not only take longer to revolve round the Sun than the
inner, but that their actual motion in space is slower, and Kepler
found that this actual speed of motion is inversely as the square root
of its distance from the Sun; or, if the square of the speed of a
planet be multiplied by its distance from the Sun, we get the same
result in each case. This is usually expressed by saying that the cube
of the distance is proportional to the square of the time of
revolution. Thus the varying rate of motion of each planet in its
orbit is not only subject to a single law, but the very different
speeds of the different planets are also all subject to a law that is
the same for all.
Thus the whole of the complicated machinery of Ptolemy had been reduced
to three simple laws, which at the same time represented the facts of
observation much better than any possible development of the Ptolemaic
mechanism. On his discovery of his third law Kepler had written: "The
book is written to be read either now or by posterity--I care not
which; it may well wait a century for a reader, as God has waited 6000
years for an observer." Twelve years after his death, on Christmas Day
1642 (old style), near Grantham, in Lincolnshire, the predestined
"reader" was born. The inner meaning of Kepler's three laws was
brought to light by ISAAC NEWTON.
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