THE TRANSIT AND CIRCLE DEPARTMENTS
The determination of time is a duty the importance of which readily
commends itself to the general public. It is easy to see that in any
civilized country it is very necessary to have an accurate standard
of time. Our railways and telegraphs make it quite impossible for
us to be content with the rough-and-ready sun-dial which satisfied
our forefathers. But it should be remembered that it was neither to
establish a 'longitude nought,' nor to create a system of standard
time, that Greenwich Observatory was founded in 1675. It was for
'The Rectifying the Tables of the Motions of the Heavens and the
Places of the Fixed Stars, in order to find out the so-much-desired
Longitude at Sea for the perfecting the Art of Navigation.'
The two related departments, therefore, those of the Transit and the
Circle, which are concerned in the work of making star-catalogues,
come next in order to the Time Department. Though both departments
deal with the same instrument, the transit circle, they are at
present placed at opposite ends of the Observatory domain; the
Circle Department being lodged in the upper computing room of the
old building; the Transit Department in the south wing of the New
Observatory in the south ground.
It may be asked why, if this were the purpose of the Observatory
at its foundation, two and a quarter centuries ago; if, as was
the case, the work was set on foot from the beginning and was
carried out with every possible care, how comes it that it is still
the fundamental work of the Observatory, and, instead of being
completed, has assumed greater proportions at the present day than
ever before?
The answer to this inquiry may be found in a special application of
the old proverb, originally directed against the discontent of man:
'The more he has, the more he wants.' For, however paradoxical it
may seem, it is true that the fuller a star-catalogue is, and the
more accurate the places of the stars that it contains, the greater
is the need for a yet fuller catalogue, with places more accurate
still.
It is worth while following up this paradox in some detail, as
it affords a very instructive example of the way in which a work
started on purely utilitarian grounds extends itself till it crosses
the undefined boundary and enters the region of pure science.
We have no idea who made the earliest census of the sky. It is
written for us in no book; it is not even engraved on any monument.
And yet no small portion of it is in our hands to-day, and,
strangest of all, we are able to fix fairly closely the time at
which it was made, and the latitude in which its compiler lived. The
catalogue is very unlike our star-catalogues of to-day. The places
of the stars are very roughly indicated; and yet this catalogue has
left a more enduring mark than all those that have succeeded it. The
catalogue simply consists of the star names.
An old lady who had attended a University Extension lecture
on astronomy was heard to exclaim: 'What wonderful men these
astronomers are! I can understand how they can find out how far
off the stars are, how big they are, and what they weigh--that is
all easy enough; and I think I can see how they find out what they
are made of. But there is one thing that I can't understand--I
don't know how they can find out what are their names!' This same
difficulty, though with a much deeper meaning than the old lady in
her simplicity was able to grasp, has occurred to many students of
astronomy. Many have wished to know what was the meaning of, and
whence were derived, the sonorous names which are found attached to
all the brighter stars on our celestial globes: Adhara, Alderamin,
Betelgeuse, Denebola, Schedar, Zubeneschamal, and many more. The
explanation lies here. Some 5000 years ago, a man, or college of
men, living in latitude 40° north, in order that they might better
remember the stars, associated certain groups of them with certain
fancied figures, and the individual star names are simply Arabic
words designed to indicate whereabouts in its peculiar figure or
constellation that special star was situated. Thus Adhara means
'back,' and is the name of the bright star in the back of the great
Dog. Alderamin means 'right arm,' and is the brightest star in the
right arm of Cepheus, the king. Betelgeuse is 'giant's shoulder,'
the giant being Orion; Denebola is 'lion's tail.' Schedar is the
star on the 'breast' of Cassiopeia, and Zubeneschamal is 'northern
claw,' that is, of the Scorpion. So far is clear enough. The names
of the stars for the most part explain themselves; but whence the
constellations derived their names, how it was that so many snakes
and fishes and centaurs were pictured out in the sky, is a much more
difficult problem, and one which does not concern us here.
One point, however, these old constellations do tell us, and tell
us plainly. They show that the axis of the earth, which, as the
earth travels round the sun, moves parallel with itself, yet, in
the course of ages, itself rotates so as in a period of some 26,000
years to trace out a circle amongst the stars. This is the cause of
what is called 'precession,' and explains how it is that the star
we call the pole-star to-day was not always the pole-star, nor will
always remain so. We learn this fact from the circumstance that
the old constellations do not cover the entire celestial sphere.
They leave a great circular space of 40° radius unmapped in the
southern heavens. This simply means that the originators of the
constellations lived in 40° north latitude, and stars within 40° of
their south pole never rose above their horizon, and consequently
were never seen, and could not be mapped, by them. In like manner,
the star census taken at Greenwich Observatory does not include
the whole sky, but leaves a space some 52° in radius round our
south pole. Since the latitude of Greenwich is nearly 52° north,
stars within that distance of the south pole do not rise above our
horizon, and are never seen here. But if we compare the vacant space
left by the old original constellations with the vacant space left
by a Greenwich catalogue of to-day, we see that the centre of the
first space, which must have been the south pole of that time, is
a long way from the centre of the second space--our south pole of
to-day. The difference tells us how far the pole has moved since
those old forgotten astronomers did their work. We know the rate
at which the pole appears to move, by comparing our more modern
catalogues one with another; and so we are able to fix pretty nearly
the time when lived those old first census-takers of the stars,
whose names have perished so completely, but whose work has proved
so immortal.
These old workers gave us the constellation groupings and names
which still remain to us, and are still in common, every-day use.
Their work affords us the most striking illustration of the result
of precession, but precession itself was not recognized till nearly
3000 years after their day, when a marvellous genius, Hipparchus,
established the fact, and 'built himself an everlasting name' by
the creation of a catalogue of over 1000 stars prepared on modern
principles. That catalogue formed the basis of one which survives to
us at the present time, and was made some 1750 years ago by Claudius
Ptolemy, the great astronomer of Alexandria, whose work, which still
bears the proud name of _Almagest_, 'The Greatest,' remained for
fourteen centuries the one universal astronomical text-book.
A modern catalogue contains, like that of Ptolemy, four columns
of entry. The first gives the star's designation; the second
an indication of its brightness; the third and fourth the
determinations of its place. These are expressed in two directions,
which, in modern catalogues, not in Ptolemy's, correspond on the
celestial sphere to longitude and latitude on the terrestrial.
Distance north or south of the celestial equator is termed
'declination,' corresponding to terrestrial latitude. Distance in
a direction parallel to the equator is termed 'right ascension,'
corresponding to terrestrial longitude. For geographical purposes
we conceive the earth to be encircled by two imaginary lines at
right angles to each other--the one, the equator, marked out for us
by the earth itself; the other, 'longitude nought,' the meridian
of Greenwich, fixed for us by general consent, after the lapse of
centuries, by a kind of historical evolution. On the celestial globe
in like manner we have two fundamental lines--one, the celestial
equator, marked out for us by nature; the other at right angles to
it, and passing through the poles of the sky, adopted as a matter
of convenience. But a difficulty at once confronts us--Where can
we fix our 'right ascension nought'? What star has the right to be
considered the Greenwich of the sky?
The difficulty is met in the following manner: For six months of
the year, the summer months, the sun is north of the celestial
equator; for the other six months of the year, the months of winter,
it is south of it. It crosses the equator, therefore, twice in
the year--once when moving northward at the spring equinox; once
when moving southward at the equinox of autumn. The point where it
crosses the equator at the first of these times is taken as the
fundamental point of the heavens, and the first sign of the zodiac,
Aries the Ram, is said to begin here, and it is called, therefore,
'the first point of Aries.'
One of the very first facts noticed in the very early days of
astronomy was that, as the stars seemed to move across the sky night
by night, they seemed to move in one solid piece, as if they were
lamps rigidly fixed in one and the same solid vault. Of course it
has long been perfectly understood that this apparent movement was
not in the least due to any motion of the stars, but simply to the
rotation of the earth on its axis. This rotation is the smoothest,
most constant, and regular movement of which we know. It follows,
therefore, that the interval of time between the passage of one star
across the meridian of Greenwich and that of any other given star
is always the same. This interval of time is simply the difference
of their right ascension. If we are able, then, to turn our transit
instrument to the sun, and to a number of stars, each in its proper
turn, and by pressing the tapping-piece on the instrument as the
sun or star comes up to each of the ten wires in succession, to
record the times of its transit on the chronograph, we shall have
practically determined their right ascensions--one of the two
elements of their places.
The other element, that of declination, is found in a different
manner. The celestial equator, like the terrestrial, is 90° from the
pole. The bright star Polaris is not exactly at the north pole, but
describes a small circle round it. Twice in the twenty-four hours
it transits across the meridian--once when going from east to west
it passes above the pole, once when going from west to east below
the pole. The mean between these two altitudes of Polaris above the
horizon gives the position of the true pole.
[Illustration: THE TRANSIT CIRCLE.]
A complete transit observation of a star consists therefore of two
operations. The observer, as we have already described, sees a
star entering the field of the telescope, and as it swims forward,
he presses the galvanic button, which sends a signal to the
chronograph as the star comes up to each of the ten vertical wires
in succession. But, beside the ten wires, there are others. Two
vertical wires lie outside the ten of which we have already spoken,
and there is also a horizontal wire. The latter can be moved by a
graduated screw-head just above the eye-piece, and as the star comes
in succession to these two vertical wires, this horizontal wire is
moved by the screw-head, so as to meet the star at the moment it
is crossing the vertical wire, and the observer presses a second
little button, which records the position of the horizontal wire on
a small paper-covered drum. Then, the transit over, the observer
leaves the telescope and comes round to the outside of the west
pier. Here he finds seven large microscopes, which pierce the whole
thickness of the pier, and are directed towards the circumference
of a large wheel which is rigidly attached to the telescope and
revolves with it. This wheel is six feet in diameter, and has a
silver circle upon both faces. Each circle is divided extremely
carefully into 4320 divisions--these divisions, therefore, being
about the one-twentieth of an inch apart. There are, therefore,
twelve divisions to every degree (12 × 360 = 4320), and each
division equals five minutes of arc. The lowest microscope is the
least powerful, and shows a large part of the circle, enabling the
observer to see at once to what degree and division of a degree
the microscope is pointing. The other six microscopes are very
carefully placed 60° apart--as equally placed as they possibly can
be. These microscopes are all fitted with movable wires--wires moved
by a very fine and delicate screw; the screw-head having divisions
upon it so that the exact amount of its movement can be told. Each
of the six screw-heads will read to the one five-thousandth part
of a division of the circle; in other words, to the one hundred
thousandth part of an inch. Using all six microscopes, and taking
their mean, we are able to _read_ to the one-hundredth of a second
of arc. If, therefore, the observations could be made with perfect
certainty down to the extremest nicety of reading which the
instrument supplies, we should be able to read the declination of a
star to this degree of refinement. It may be added that a halfpenny,
at a distance of three miles, appears to be one second of arc in
diameter; at three hundred miles it would be one-hundredth of a
second. It need scarcely be said that we cannot observe with quite
such refinement of exactness as this would indicate. Nevertheless,
this exactness is one after which the observer is constantly
striving, and tenths, even hundredths, of seconds of arc are
quantities which the astronomer cannot now neglect.
The observer has then to read the heads of all these seven
microscopes on the pier side, and also two positions of the
horizontal wire on the screw-head at the eye-piece. The following
morning he will also read off from the chronograph-sheet the times
when he made the ten taps as the star passed each of the ten
vertical wires. There are, therefore, nine entries to make for one
position of a star in declination, and ten for one position of a
star in right ascension. The observer will also have to read the
barometer to get the pressure of the air at the time of observation,
and one thermometer inside the transit room, and another outside,
to get the temperature of the air. In some cases thermometers at
different heights in the room are also read. A complete observation
of a single star means, therefore, the entry of two-and-twenty
different numbers.
It may be asked, What is the use of reading the barometer and
thermometer? The answer to the question can only be given by
contradicting a statement made above, that the true pole lay midway
between the position of the telescope when pointing to the pole-star
at its upper transit, and its position when pointing to it at its
lower transit. The pole being very high in the heavens in this
country, there are a great number of stars that, like the pole-star,
cross the meridian twice in the twenty-four hours--once when they
pass above the pole, moving from east to west, once when they pass
below it, moving from west to east As the real distance of a star
from the true pole does not alter, it follows that we ought to get
the position of the pole from the mean of the two transits of any of
these stars, and they ought all to exactly agree with each other.
But they do not. So, too, I said that the stars all appeared to move
as in a single piece. If, then, we constructed an instrument with
its axis parallel to the axis of the earth, and fixed a telescope to
it, pointing to any particular star, if we turn the telescope round
as fast from east to west as the earth itself is turning from west
to east--if we built an equatorial, that is to say--we ought to find
that the star once in the centre of the field would remain there. As
a matter of fact, when the star got near the horizon it would soon
be a long way from the centre of the field.
Sir George Airy, the seventh Astronomer Royal, makes, with reference
to this very point, the following remarks:
'Perhaps you may be surprised to hear me say the rule is
established as true, and yet there is a departure from it.
This is the way we go on in science, as in everything else;
we have to make out that something is true, then we find out
under certain circumstances that it is not quite true; and
then we have to consider and find out how the departure can be
explained.'
In this particular case, the disturbing cause is found in the
action of our own atmosphere. The rays of light from the star are
bent out of a perfectly straight course as they pass through the
various layers of that atmosphere, layers which necessarily become
denser the closer we get to the actual surface of the earth. Every
celestial body therefore appears to be a little higher in the sky
than it really is. This action is most noticeable at the horizon,
where it amounts to about half a degree. As both sun and moon are
about half a degree in diameter, it follows that when they have
really just entirely sunk below the horizon they appear to be just
entirely above it. It happens, in consequence, on rare occasions,
that an eclipse of the moon will take place when both sun and moon
are together seen above the horizon.
It was a great matter to discover this effect of refraction. It
was soon seen that it was not constant, that it varied with both
temperature and pressure. It is, indeed, the most troublesome of all
the hindrances to exact observation with which the astronomer has
to contend; partly because of its large amount--half a degree, as
has been already said, in the extreme case--and partly because it is
difficult in many cases to determine its exact effect.
The double observation with the transit circle gives us, then, the
place in the sky where the star _appeared_ to be at the moment of
observation, not its true place; to find that true place we have
to calculate how much refraction had displaced the star at the
particular height in the sky, and at the particular temperature and
atmospheric pressure at which the observation was made.
[Illustration: THE MURAL CIRCLE.]
The transit circle is a comparatively recent instrument. In earlier
times the two observations of right ascension and declination were
entrusted to perfectly separate instruments. The transit instrument
was mounted as the transit circle is, between two solid stone piers,
and moved in precisely the same way. But the great six-foot wheel,
which was made as stiff as it possibly could be, was mounted on
the face of a great stone pier or wall, from which circumstance it
was called the 'mural circle,' and a light telescope was attached
to it which turned about its centre. This arrangement had a double
disadvantage--that the two observations had to be made separately,
and the mural circle, not being a symmetrical instrument, was
liable to small errors which it was difficult to detect. Thus, being
supported on one side only, a flexure or bending outwards of either
telescope or circle, or both, might be feared.
It was for this reason that Pond set up a pair of mural circles,
one on the east side of its supporting pier and the other on
the west.[3] His plan was not only to have each star observed
simultaneously in the two instruments, a plan by which, at the cost
of some additional labour, he would have got rid, to a large extent,
of the individual errors of the two separate instruments, inasmuch
as, on the whole, it might have been expected that the errors of
the two instruments would have been very nearly equal in amount,
but of opposite character. The differences, too, between the two
instruments would have afforded the means for tracing these small
errors to their respective causes, and so ascertaining the laws to
which they were subject.
[3] The second circle was intended for the Cape Observatory, but
Pond obtained leave to retain it. In 1851 it was transferred to the
Observatory of Queen's College, Belfast.
Pond went further still. He added to the mural circle a simple
instrument, the extreme value of which every astronomer recognizes
to-day--the mercury trough. Not only was the star to be observed by
both circles when the two telescopes were pointing directly to it,
it was also to be observed by reflection; the telescopes were to
be pointed down towards a basin of mercury, in which the image of
the star would be seen reflected. The mercury being a liquid, its
surface is perfectly horizontal; and, since the law of reflection
is that the angle of incidence is equal to the angle of reflection,
it follows that the telescope, when pointed down toward the mercury
trough, points just at as great an angle below the horizon as, when
it is set directly on the star, it points above it. If the circle,
therefore, be carefully read at both settings, half the difference
between the two readings will give the angular elevation of the
star above the horizon. A combination, therefore, of all four
observations, that is to say, one reflection and one direct with
each of the telescopes, would give an exceedingly exact value for
the star's altitude. The conception of this method gives a striking
idea of Pond's thoroughness and skill as a practical observer, and
it is a distinct blot upon Airy's justly high reputation in the same
line that he discontinued the system upon his accession to office.
However, in 1851, as already mentioned, Airy substituted for the
two separate instruments--the transit and mural circle--the transit
circle, which, unlike the mural circle, is equally supported on both
sides. This, however, does not free it from the liability to some
minute flexure in the direction of its length, from the weight of
its two ends, and the mercury trough is used for the detection of
such bending, should it exist. The present practice is to observe
a star both by reflection and directly in the course of the same
transit. The observer sets the telescope carefully before ever the
star comes into the field of view, and reads his seven microscopes.
Then he climbs up a narrow wooden staircase and watches the star
transit nearly half across the field. Then comes a rush, the
observer swings himself down the ladder, unclamps the telescope,
turns it rapidly up to the star itself, clamps it again, flings
himself on his back on a bench below the telescope, and does it so
quickly that he is able to observe the star across the second half
of the field. There is no time for dawdling, no room for making any
mistakes; the stars never forgive; 'they haste not, they rest not;'
and if the unfortunate observer is too slow, or makes some slip in
his second setting, the star, cold and inexorable, takes no pity,
and moves regardless on.
It will be seen that a considerable amount of work is involved
in taking a single observation of a star-place. But in making a
star-catalogue it is always deemed necessary to obtain at least
three observations of each star; and many are observed much more
frequently.
A modern star-catalogue contains, like Ptolemy's, four columns. It
contains also several more. Of these the principal are devoted to
the effect of precession. As precession is caused by a movement
of the earth's axis making the pole of the sky seem to describe a
circle in the heavens, it follows that the celestial poles, and the
celestial equator with them are slowly, but continually, changing
their place with respect to the stars, and therefore that the
declinations of the stars are always undergoing change, and as the
equator changes, the point where the sun crosses it in spring--the
first point of Aries--changes also, and with it the stars' right
ascensions.
To make one determination of a star's place comparable with
another made at another time, it is clear that we must correct for
the effects of precession in the interval of time between the two
observations, and for the effects of refraction. But observations
made with the transit circle must also be corrected for errors
in the instrument itself. The astronomer will see to it that his
instrument is made and is set up as perfectly as possible. The
pivots on which it turns must be exactly on the same level; they
must point exactly east and west, and the axis of the telescope
must be exactly at right angles to the line joining the pivots
in all positions of the instrument. These conditions are very
nearly fulfilled, but never absolutely. Day by day, therefore, the
astronomer has to ascertain just how much his instrument is in error
in each of these three matters. Were his instrument absolutely
without error to-day, he could not assume that it would remain so,
nor, if he had measured the amount of its errors yesterday, would it
be safe to assume that those errors would not change to-day.
In the examination of these sources of error the mercury trough
comes again into use. The transit circle is turned directly
downwards, and the mercury trough brought below it. A light is
so arranged as to illuminate the field of the telescope, and the
observer, looking in, sees the ten transit wires and the one
declination wire, and also sees their images reflected from the
surface of the mercury. If the telescope be pointing _exactly_
down towards the surface of the mercury, then the image of the
declination wire will fall exactly on the declination wire
itself, and by reading the circle we can tell where the zenith
point of the circle is. Similarly, if the pivots of the telescope
are precisely on the same level, the centre wire of the right
ascension series would coincide with its reflected image. A third
point is determined by looking through the eye-piece of the north
collimator telescope--that is to say, the telescope mounted in a
horizontal position at the north end of the room--at the spider
lines in the focus of the south collimator. In order to get this
view, the transit telescope has either to be lifted up out of its
usual position, or else the middle of the tube has to be opened.
The spider lines in the north collimator are then made to coincide
with the image of the wires of the south collimator. The transit
telescope is then turned first to one collimator, then to the other,
and the central wire of the right ascension series is turned till it
coincides with the wire of the collimator; the amount by which it
has to be moved giving an index of the error of collimation; that is
to say, of the deviation of the optical axis of the telescope from
perpendicularity to the line joining the pivots.
I have said enough to show that the making of an observation is
a small matter as compared with those corrections which have to
be made to it afterwards, before it is available for use. But I
have only mentioned some of the reductions and corrections which
have to be made. There are several more, and it is a just pride of
Greenwich that her third ruler, Bradley, as has been already told
in the notice of his life, discovered two of the most important.
The one, aberration, is due to the fact that light, though it moves
so swiftly--186,000 miles per second--yet does not move with an
infinitely greater velocity than that of the earth. The other,
nutation, might be called a correction to precession, inasmuch as,
moved by the moon's attraction, the earth's axis does not swing
round smoothly, but with a slight nodding or staggering motion.
But when these observations of the places of a star have been made,
and have been properly 'reduced,' even then we do not find an
exact correspondence between two different determinations. Little
differences still remain. Some of these are to be accounted for by
changes in the actual crust of the earth, which, solid and stable as
we think it, is yet always in motion. Professor Milne, our greatest
authority on earth movements, says, 'The earth is so elastic that
a comparatively small impetus will set it vibrating; why, even two
hills tip together when there is a heavy load of moisture in the
valley between them. And then, when the moisture evaporates in a
hot sun, they tip away from each other.' So there is a perceptible
rocking to and fro even of the huge stone piers of a transit
circle, as seasons of rain and drought, heat and cold, follow each
other. More than that, the earth is so sensitive to pressure that
it was found, at the Oxford University Observatory, that there was
a distinct swaying shown by a horizontal pendulum when the whole
of a party of seventy-six undergraduates stood on one side of the
instrument and close up to it, from the position it had when the
party stood ninety feet away. More wonderful still, a comparison of
the star-places, obtained at a number of observatories, including
Greenwich, has shown that the earth is continually changing her axis
of rotation. And so the star-places determined at Greenwich have
shown that the north pole of the earth, 2600 miles away, moves about
in an irregular curve about thirty feet in radius.
Nothing is stable, nothing is immovable, nothing is constant. The
astronomer even finds that his own presence near the instrument is
sufficient to disturb it.
The great interest attaching to transit-circle work is this
striving after ever greater and greater precision, with the result
of bringing out fresh little discordances, which, at first sight,
appear purely accidental, but which, under further scrutiny, show
themselves to be subject to some law. Then comes the hunt for this
new unknown law. Its discovery follows. It explains much, but when
it is allowed for, though the observations now come much closer
together, little deviations still remain, to form the subject of a
fresh inquiry. Astronomy has well been called the exact science, and
yet exactitude ever eludes its pursuer.
If it be asked, 'What is the use of this ever-increasing refinement
of observation?' no better answer can be given than the words of
Sir John Herschel in one of his Presidential addresses to the Royal
Astronomical Society:--
'If we ask to what end magnificent establishments are maintained
by States and sovereigns, furnished with masterpieces of art,
and placed under the direction of men of first-rate talent and
high-minded enthusiasm, sought out for those qualities among the
foremost in the ranks of science, if we demand, _cui bono?_
for what good a Bradley has toiled, or a Maskelyne or a Piazzi
has worn out his venerable age in watching?--the answer is,
Not to settle mere speculative points in the doctrine of the
universe; not to cater for the pride of man by refined inquiries
into the remoter mysteries of nature; not to trace the path
of our system through space, or its history through past and
future eternities. These, indeed, are noble ends, and which I
am far from any thought of depreciating; the mind swells in
their contemplation, and attains in their pursuit an expansion
and a hardihood which fit it for the boldest enterprise. But
the direct practical utility of such labours is fully worthy of
their speculative grandeur. The stars are the landmarks of the
universe; and, amidst the endless and complicated fluctuations
of our system, seem placed by its Creator as guides and records,
not merely to elevate our minds by the contemplation of what
is vast, but to teach us to direct our actions by reference to
what is immutable in His works. It is, indeed, hardly possible
to over-appreciate their value in this point of view. Every
well-determined star, from the moment its place is registered,
becomes to the astronomer, the geographer, the navigator, the
surveyor, a point of departure which can never deceive or fail
him, the same for ever and in all places; of a delicacy so
extreme as to be a test for every instrument yet invented by
man, yet equally adapted for the most ordinary purposes; as
available for regulating a town clock as for conducting a navy
to the Indies; as effective for mapping down the intricacies of
a petty barony as for adjusting the boundaries of Transatlantic
empires. When once its place has been thoroughly ascertained
and carefully recorded, the brazen circle with which that
useful work was done may moulder, the marble pillar may totter
on its base, and the astronomer himself survive only in the
gratitude of posterity; but the record remains, and transfuses
all its own exactness into every determination which takes it
for a groundwork, giving to inferior instruments--nay, even to
temporary contrivances, and to the observations of a few weeks
or days--all the precision attained originally at the cost of so
much time, labour, and expense.'
But for these strictly utilitarian purposes a comparatively small
number of stars would meet our every requisite. In the narrow sense
of which Sir John Herschel is here speaking, we have no use for
anything beyond the smallest of catalogues; and if the question
before us is, 'Why are we continually extending our catalogues?' the
following words of a more recent writer[4] on the subject will set
forth the real explanation:--
'A word in conclusion, suggested by the history of
star-catalogues. We have no difficulty in understanding that it
is necessary to study the planets, and a reasonable number of
the brighter stars, for the purpose of determining the figure
of the earth, and the positions of points upon its surface; but
the use for a catalogue of ten thousand stars, such as La Caille
compiled, is not just so apparent. Nay, what did Ptolemy want
with a thousand stars, or Tamerlane's grandson, born, reared,
and destined to die amidst a horde of savages, however splendid
in their trappings? There is not, and there never was, any
real, practical use for the great volumes of star-catalogues
that weigh down the shelves of our libraries. The navigator
and explorer need never see them at all. Why, then, were these
pages compiled? Why have astronomers, from Hipparchus's time
to the present, spent their lives in the weary routine-work of
observing the places of tiny points in the stellar depths? Does
it not seem that there is something in the mind of man that
impels him to seek after knowledge--truly--for its own sake?
something heaven-born, heaven-nurtured, God-given ... that there
is something in man common to him and his Creator, and therefore
eternal ... in beautiful accord with the plain statement that
"God made man in His own image?"'
[4] Mr. Thomas Lindsay, _Transactions of the Astronomical and
Physical Society of Toronto_, 1899, p. 17.