THE SCIENCE OF
THE STARS
BY E. WALTER MAUNDER, F.R.A.S.
OF THE ROYAL OBSERVATORY, GREENWICH
AUTHOR OF "ASTRONOMY WITHOUT A TELESCOPE"
"THE ASTRONOMY OF THE BIBLE," ETC.
LONDON: T. C. & E. C. JACK
67 LONG ACRE, W.C., AND EDINBURGH
NEW YORK: DODGE PUBLISHING CO.
{vii}
CONTENTS
CHAP.
I. ASTRONOMY BEFORE HISTORY
II. ASTRONOMY BEFORE THE TELESCOPE
III. THE LAW OF GRAVITATION
IV. ASTRONOMICAL MEASUREMENTS
V. THE MEMBERS OF THE SOLAR SYSTEM
VI. THE SYSTEM OF THE STARS
INDEX
{9}
THE SCIENCE OF THE STARS
CHAPTER I
ASTRONOMY BEFORE HISTORY
The plan of the present series requires each volume to be complete in
about eighty small pages. But no adequate account of the achievements
of astronomy can possibly be given within limits so narrow, for so
small a space would not suffice for a mere catalogue of the results
which have been obtained; and in most cases the result alone would be
almost meaningless unless some explanation were offered of the way in
which it had been reached. All, therefore, that can be done in a work
of the present size is to take the student to the starting-point of
astronomy, show him the various roads of research which have opened out
from it, and give a brief indication of the character and general
direction of each.
That which distinguishes astronomy from all the other sciences is this:
it deals with objects that we cannot touch. The heavenly bodies are
beyond our reach; we cannot tamper with them, or subject them to any
form of experiment; we cannot bring them into our laboratories to
analyse or dissect them. We can only watch them and wait for such
indications as their {10} own movements may supply. But we are
confined to this earth of ours, and they are so remote; we are so
short-lived, and they are so long-enduring; that the difficulty of
finding out much about them might well seem insuperable.
Yet these difficulties have been so far overcome that astronomy is the
most advanced of all the sciences, the one in which our knowledge is
the most definite and certain. All science rests on sight and thought,
on ordered observation and reasoned deduction; but both sight and
thought were earlier trained to the service of astronomy than of the
other physical sciences.
It is here that the highest value of astronomy lies; in the discipline
that it has afforded to man's powers of observation and reflection; and
the real triumphs which it has achieved are not the bringing to light
of the beauties or the sensational dimensions and distances of the
heavenly bodies, but the vanquishing of difficulties which might well
have seemed superhuman. The true spirit of the science can be far
better exemplified by the presentation of some of these difficulties,
and of the methods by which they have been overcome, than by many
volumes of picturesque description or of eloquent rhapsody.
There was a time when men knew nothing of astronomy; like every other
science it began from zero. But it is not possible to suppose that
such a state of things lasted long, we know that there was a time when
men had noticed that there were two great lights in the sky--a greater
light that shone by day, a lesser light that shone by night--and there
were the stars also. And this, the earliest observation of primitive
astronomy, is preserved for us, expressed in the simplest possible
language, in the first chapter of the first book {11} of the sacred
writings handed down to us by the Hebrews.
This observation, that there are bodies above us giving light, and that
they are not all equally bright, is so simple, so inevitable, that men
must have made it as soon as they possessed any mental power at all.
But, once made, a number of questions must have intruded themselves:
"What are these lights? Where are they? How far are they off?"
Many different answers were early given to these questions. Some were
foolish; some, though intelligent, were mistaken; some, though wrong,
led eventually to the discovery of the truth. Many myths, many
legends, some full of beauty and interest, were invented. But in so
small a book as this it is only possible to glance at those lines of
thought which eventually led to the true solution.
As the greater light, the lesser light, and the stars were carefully
watched, it was seen not only that they shone, but that they appeared
to move; slowly, steadily, and without ceasing. The stars all moved
together like a column of soldiers on the march, not altering their
positions relative to each other. The lesser light, the Moon, moved
with the stars, and yet at the same time among them. The greater
light, the Sun, was not seen with the stars; the brightness of his
presence made the day, his absence brought the night, and it was only
during his absence that the stars were seen; they faded out of the sky
before he came up in the morning, and did not reappear again until
after he passed out of sight in the evening. But there came a time
when it was realised that there were stars shining in the sky all day
long as well as at night, and this discovery was one of the greatest
and most important ever made, {12} because it was the earliest
discovery of something quite unseen. Men laid hold of this fact, not
from the direct and immediate evidence of their senses, but from
reflection and reasoning. We do not know who made this discovery, nor
how long ago it was made, but from that time onward the eyes with which
men looked upon nature were not only the eyes of the body, but also the
eyes of the mind.
It followed from this that the Sun, like the Moon, not only moved with
the general host of the stars, but also among them. If an observer
looks out from any fixed station and watches the rising of some bright
star, night after night, he will notice that it always appears to rise
in the same place; so too with its setting. From any given observing
station the direction in which any particular star is observed to rise
or set is invariable.
Not so with the Sun. We are accustomed to say that the Sun rises in
the east and sets in the west. But the direction in which the Sun
rises in midwinter lies far to the south of the east point; the
direction in which he rises in midsummer lies as far to the north. The
Sun is therefore not only moving with the stars, but among them. This
gradual change in the position of the Sun in the sky was noticed in
many ancient nations at an early time. It is referred to in Job
xxxviii. 12: "Hast thou commanded the morning since thy days; and
caused the dayspring to know his place?"
And the apparent path of the Sun on one day is always parallel to its
path on the days preceding and following. When, therefore, the Sun
rises far to the south of east, he sets correspondingly far to the
south of west, and at noon he is low down in the south. His course
during the day is a short one, and the daylight {13} is much shorter
than the night, and the Sun at noon, being low down in the sky, has not
his full power. The cold and darkness of winter, therefore, follows
directly upon this position of the Sun. These conditions are reversed
when the Sun rises in the north-east. The night is short, the daylight
prolonged, and the Sun, being high in the heavens at noon, his heat is
felt to the full.
Thus the movements of the Sun are directly connected with the changes
of season upon the Earth. But the stars also are connected with those
seasons; for if we look out immediately after it has become dark after
sunset, we shall notice that the stars seen in the night of winter are
only in part those seen in the nights of summer.
In the northern part of the sky there are a number of stars which are
always visible whenever we look out, no matter at what time of the
night nor what part of the year. If we watch throughout the whole
night, we see that the whole heavens appear to be slowly
turning--turning, as if all were in a single piece--and the pivot about
which it is turning is high up in the northern sky. The stars,
therefore, are divided into two classes. Those near this invisible
pivot--the "Pole" of the Heavens, as we term it--move round it in
complete circles; they never pass out of sight, but even when lowest
they clear the horizon. The other stars move round the same pivot in
curved paths, which are evidently parts of circles, but circles of
which we do not see the whole. These stars rise on the eastern side of
the heavens and set on the western, and for a greater or less space of
time are lost to sight below the horizon. And some of these stars are
visible at one time of the year, others at another; some being seen
during the {14} whole of the long nights of winter, others throughout
the short nights of summer. This distinction again, and its connection
with the change of the seasons on the earth, was observed many ages
ago. It is alluded to in Job xxxviii. 32: "Canst thou lead forth the
Signs of the Zodiac in their season, or canst thou guide the Bear with
her train?" (R.V., Margin). The Signs of the Zodiac are taken as
representing the stars which rise and set, and therefore have each
their season for being "led forth," while the northern stars, which are
always visible, appearing to be "guided" in their continual movement
round the Pole of the sky in perfect circles, are represented by "the
Bear with her train."
The changes in position of the Sun, the greater light, must have
attracted attention in the very earliest ages, because these changes
are so closely connected with the changes of the seasons upon the
Earth, which affect men directly. The Moon, the lesser light, goes
through changes of position like the Sun, but these are not of the same
direct consequence to men, and probably much less notice was taken of
them. But there were changes of the Moon which men could not help
noticing--her changes of shape and brightness. One evening she may be
seen soon after the Sun has set, as a thin arch of light, low down in
the sunset sky. On the following evenings she is seen higher and
higher in the sky, and the bow of light increases, until by the
fourteenth day it is a perfect round. Then the Moon begins to diminish
and to disappear, until, on the twenty-ninth or thirtieth day after the
first observation, she is again seen in the west after sunset as a
narrow crescent. This succession of changes gave men an important
measure of time, and, in an age when artificial means of light were
difficult to procure, moonlight was of the greatest {15} value, and the
return of the moonlit portion of the month was eagerly looked for.
These early astronomical observations were simple and obvious, and of
great practical value. The day, month, and year were convenient
measures of time, and the power of determining, from the observation of
the Sun and of the stars, how far the year had progressed was most
important to farmers, as an indication when they should plough and sow
their land. Such observations had probably been made independently by
many men and in many nations, but in one place a greater advance had
been made. The Sun and Moon are both unmistakable, but one star is
very like another, and, for the most part, individual stars can only be
recognised by their positions relative to others. The stars were
therefore grouped together into +Constellations+ and associated with
certain fancied designs, and twelve of these designs were arranged in a
belt round the sky to mark the apparent path of the Sun in the course
of the year, these twelve being known as the "+Signs of the
Zodiac+"--the Ram, Bull, Twins, Crab, Lion, Virgin, Balance, Scorpion,
Archer, Goat, Water-pourer, and Fishes. In the rest of the sky some
thirty to thirty-six other groups, or constellations, were formed, the
Bear being the largest and brightest of the constellations of the
northern heavens.
But these ancient constellations do not cover the entire heavens; a
large area in the south is untouched by them. And this fact affords an
indication both of the time when and the place where the old stellar
groups were designed, for the region left untouched was the region
below the horizon of 40° North latitude, about 4600 years ago. It is
probable, therefore, that the ancient astronomers who carried out this
great work {16} lived about 2700 B.C., and in North latitude 37° or
38°. The indication is only rough, but the amount of uncertainty is
not very large; the constellations must be at least 4000 years old,
they cannot be more than 5000.
All this was done by prehistoric astronomers; though no record of the
actual carrying out of the work and no names of the men who did it have
come down to us. But it is clear from the fact that the Signs of the
Zodiac are arranged so as to mark out the annual path of the Sun, and
that they are twelve in number--there being twelve months in the
year--that those who designed the constellations already knew that
there are stars shining near the Sun in full daylight, and that they
had worked out some means for determining what stars the Sun is near at
any given time.
Another great discovery of which the date and the maker are equally
unknown is referred to in only one of the ancient records available to
us. It was seen that all along the eastern horizon, from north to
south, stars rise, and all along the western horizon, from north to
south, stars set. That is what was seen; it was the fact observed.
There is no hindrance anywhere to the movement of the stars--they have
a free passage under the Earth; the Earth is unsupported in space.
That is what was _thought_; it was the inference drawn. Or, as it is
written in Job xxvi. 7, "He (God) stretcheth out the north over empty
space, and hangeth the earth upon nothing."
The Earth therefore floats unsupported in the centre of an immense
star-spangled sphere. And what is the shape of the Earth? The natural
and correct inference is that it is spherical, and we find in some of
the early Greek writers the arguments which establish this inference as
clearly set forth as they would be to-day. {17} The same inference
followed, moreover, from the observation of a simple fact, namely, that
the stars as observed from any particular place all make the same angle
with the horizon as they rise in the east, and all set at the same
angle with it in the west; but if we go northward, we find that angle
steadily decreasing; if we go southward, we find it increasing. But if
the Earth is round like a globe, then it must have a definite size, and
that size can be measured. The discoveries noted above were made by
men whose names have been lost, but the name of the first person whom
we know to have measured the size of the Earth was ERATOSTHENES. He
found that the Sun was directly overhead at noon at midsummer at Syene
(the modern Assouan), in Egypt, but was 7° south of the "zenith"--the
point overhead--at Alexandria, and from this he computed the Earth to
be 250,000 stadia (a stadium = 606 feet) in circumference.
Another consequence of the careful watch upon the stars was the
discovery that five of them were planets; "wandering" stars; they did
not move all in one piece with the rest of the celestial host. In this
they resemble the Sun and Moon, and they further resemble the Moon in
that, though too small for any change of shape to be detected, they
change in brightness from time to time. But their movements are more
complicated than those of the other heavenly bodies. The Sun moves a
little slower than the stars, and so seems to travel amongst them from
west to east; the Moon moves much slower than the stars, so her motion
from west to east is more pronounced than that of the Sun. But the
five planets sometimes move slower than the stars, sometimes quicker,
and sometimes at the same rate. Two of the five, which we now know as
Mercury {18} and Venus, never move far from the Sun, sometimes being
seen in the east before he rises in the morning, and sometimes in the
west after he has set in the evening. Mercury is the closer to the
Sun, and moves more quickly; Venus goes through much the greater
changes of brightness. Jupiter and Saturn move nearly at the same
average rate as the stars, Saturn taking about thirteen days more than
a year to come again to the point of the sky opposite to the Sun, and
Jupiter about thirty-four days. Mars, the fifth planet, takes two
years and fifty days to accomplish the same journey.
These planetary movements were not, like those of the Sun and Moon and
stars, of great and obvious consequence to men. It was important to
men to know when they would have moonlight nights, to know when the
successive seasons of the year would return. But it was no help to men
to know when Venus was at her brightest more than when she was
invisible. She gave them no useful light, and she and her companion
planets returned at no definite seasons. Nevertheless, men began to
make ordered observations of the planets--observations that required
much more patience and perseverance than those of the other celestial
lights. And they set themselves with the greatest ingenuity to unravel
the secret of their complicated and seemingly capricious movements.
This was a yet higher development than anything that had gone before,
for men were devoting time, trouble, and patient thought, for long
series of years, to an inquiry which did not promise to bring them any
profit or advantage. Yet the profit which it actually did bring was of
the highest order. It developed men's mental powers; it led to the
devising of {19} instruments of precision for the observations; it led
to the foundation of mathematics, and thus lay at the root of all our
modern mechanical progress. It brought out, in a higher degree,
ordered observation and ordered thought.
{20}