THE ASTROGRAPHIC DEPARTMENT
The two last departments mentioned, the heliographic and
spectroscopic, lie clearly and unmistakably outside the terms of
the original warrant of the Observatory, though the progress of
science has led naturally and inevitably to their being included in
the Greenwich programme. But the Astrographic Department, though
it could no more have been conceived in the days of Charles II.
than the spectroscopic, does come within the terms of the warrant,
and is but an expansion of that work of 'Rectifying the Places of
the Fixed Stars,' which formed part of the programme enjoined upon
Flamsteed, the first Astronomer Royal, at the first foundation of
the Observatory, and which was so diligently carried out by him, the
first Greenwich catalogue, containing about 3000 stars, being due to
his labours.
[Illustration: 'CHART PLATE' OF THE PLEIADES.
(_From a photograph taken at the Royal Observatory, Greenwich, with
an exposure of forty minutes._)]
His immediate successors did much less in this field, though
Bradley's observations were published, long after his death, as a
catalogue of 3222 stars, in some aspects the most important ever
issued. Pond, the sixth Astronomer Royal, restored catalogue-making
to a prominent place in the Greenwich routine, and his precedent
is sedulously followed to-day. But each of these was confined to
about 3000 stars. The necessity has long been felt for a much
ampler census, and Argelander, at the Bonn Observatory, brought
out a catalogue of 324,000 stars north of South declination 2°, a
work which has been completed by Schönfeld, who carried the census
down to South declination 23°, and by the two great astronomers of
Cordoba, South America, Dr. Gould and Dr. Thome, by whom it was
extended to the South Pole.
These last three catalogues embrace stars of all magnitudes down to
the 9th or 10th; but certain astronomers had endeavoured to go much
lower, and to make charts of limited portions of the sky down to
even the 14th magnitude.
From the very earliest days that men observed the stars, they
could not help noticing that 'one star differeth from another star
in glory,' and consequently they divided them into six classes,
according to their brightness--classes which are commonly spoken
of now as magnitudes. The ordinary 6th magnitude star is one which
can be clearly seen by average sight on a good night, and it gives
us about one-hundredth the light of an average 1st magnitude star.
Sirius, the brightest of all the fixed stars, is called a 1st
magnitude star, but is really some six or seven times as bright as
the average. It would take, therefore, more than two and a half
million stars of the 14th magnitude to give as much light as Sirius.
It is evident that so searching a census as to embrace stars of the
14th magnitude would involve a most gigantic chart. But the work
went on in more than one Observatory for a considerable time, until
at last the observers entered on to the region of the Milky Way.
Here the numbers of the stars presented to them were so great as to
baffle all ordinary means of observation. What could be done?
Just at this time immense interest was caused in the astronomical
world by the appearance of the great comet of 1882. It was watched
and observed and sketched by countless admirers, but more important
still, it was photographed, and some of its photographs, taken at
the Royal Observatory, Cape of Good Hope, showed not only the comet
with marvellous beauty of detail, but also thousands of stars,
and the success of these photographs suggested to her Majesty's
Astronomer at the Cape, Dr. Gill, that in photography we possessed
the means for making a complete sky census even to the 14th
magnitude.
The project was thought over in all its bearings, and in 1887
a great conference of astronomers at Paris resolved upon an
international scheme for photographing the entire heavens. The
work was to be divided between eighteen Observatories of different
nationalities. It was to result in a photographic chart extending to
the 14th magnitude, and probably embracing some forty million stars,
and a catalogue made from measures of the photographs down to the
11th magnitude, which would probably include between two and three
million stars.
[Illustration: THE CONTROL PENDULUM AND THE BASE OF THE THOMPSON
TELESCOPE.]
The eighteen Observatories all undertook to use instruments of the
same capacity. This was to be a photographic refractor, with an
object-glass of 13 inches aperture and 11 feet focus. At Greenwich
this telescope is mounted equatorially--that is, so as to follow the
stars in their courses--and is mounted on the top of the pier that
once supported Halley's quadrant. The telescope is driven by a most
efficient clock, whose motive power is a heavy weight. The rate of
the weight in falling is regulated by an ingenious governor, which
brings its speed very nearly indeed to that of the star, and any
little irregularities in its motion are corrected by the following
device. A seconds pendulum is mounted in a glass case on the wall
of the Observatory, and a needle at the lower end of the pendulum
passes at each swing through a globule of mercury. On one of the
wheels of the clock are arranged a number of little brass points,
at such intervals apart that the wheel, when going at the proper
rate, takes exactly one second to move through the distance between
any pair. A little spring is arranged above the wheel, so that
these points touch it as they pass. If this occurs exactly as the
pendulum point passes through the mercury nothing happens, but if
the clock is ever so little late or early, the electric current from
the pendulum brings into action a second wheel, which accelerates
or retards the driving of the clock, as the case may be. The total
motion, therefore, is most beautifully even.
[Illustration: THE ASTROGRAPHIC TELESCOPE.
(_Reproduced from 'Engineering' by permission._)]
But even this is not quite sufficient, especially as the plates
for the great chart have to be exposed for at least forty minutes.
Rigidly united with the 13-inch refractor, so that the two look
like the two barrels of a huge double-barrelled gun, is a second
telescope for the use of the observer. In its eyepiece are fixed
two pairs of cross spider lines, commonly called wires, and a
bright star, as near as possible to the centre of the field to be
photographed, is brought to the junction of two wires. Should the
star appear to move away from the wire, the observer has but to
press one of two buttons on a little plate which he carries in his
hand, and which is connected by an electric wire with the driving
clock, to bring it back to its position.
The photographs taken with this instrument are of two kinds. Those
for the great chart have but a single exposure, but this lasts for
forty minutes. Those for the great catalogue have three exposures on
them, the three images of a star being some 20 seconds of arc apart.
These exposures are of six minutes', three minutes', and twenty
seconds' duration, and the last exposure is given as a test, since,
if stars of the 9th magnitude are visible with an exposure of twenty
seconds, stars of the 11th magnitude should be visible with three
minutes' exposure.
Thus it will be seen that in three minutes an impression is got of
many scores of stars, whose places it would require many hours to
determine at the transit instrument. But the positions of these
stars on the plate still remain to be measured. For this purpose a
net-work of lines, at right angles to each other, is printed on the
photograph before its development, and, after it has been developed,
washed and dried, the distances of the stars from their nearest
cross-lines are measured in the measuring machine.
[Illustration: THE DRIVING CLOCK OF THE ASTROGRAPHIC TELESCOPE.
(_Reproduced from 'Engineering' by permission._)]
The measuring machine is constructed to hold two plates, one half
its breadth higher than the other. In fact, in each of the two
series of photographs the whole sky is taken twice, but the two
photographs of any region are not simply duplicates of each other.
The centre of each plate is at a corner of four other plates, and
in the micrometer the stars on the quarter common to two plates are
measured simultaneously.
In this way will be carried out a great census of the sky that will
exceed Flamsteed's ten thousand fold. And just as Flamsteed's was
but the first of many similar catalogues, so, no doubt, will this be
followed by others--not superseded, for its value will increase with
its age and the number of those that follow it, by comparison with
which it will prove an inexhaustible mine of information concerning
the motions of the stars and the structure of the universe.
There is a great difference between the work of the observer with
the 'Astrographic Telescope,' as this great twin photographic
instrument is called, and the work of the transit observer. The
latter sees the star gliding past him, and telegraphs the instant
that the star threads itself on each of the ten vertical wires in
succession. The astrographic observer, on the other hand, sees his
star shining almost immovably in the centre of his field, threaded
on the two cross wires placed there, for the driving-clock moves
the telescope so as to almost exactly compensate for the rotation
movement of the earth. The observer's duty in this case is to
telegraph to his driving-clock, when it has in the least come short
of or exceeded its duty, and so to bring back the 'guiding star' to
its exact proper place on the cross wires.
So far, the work of the Astrographic Department has been, as
mentioned above, a development on an extraordinary scale, but a
development still, of the original programme of the Observatory.
But the munificent gift of Sir Henry Thompson has put it within
the power of the Astronomer Royal to push this work of sidereal
photography a stage further. Sir Henry Thompson gave to the
Observatory, not merely the photographic refractor of 9 inches'
aperture, now used for solar photography, and known as the 'Thompson
photo-heliograph,' but also one of 26 inches' aperture and 22-1/2
feet focal length. This instrument was specially designed of exactly
double the dimensions of the standard astrographic telescope used
for the International Photographic Survey, the idea being that,
in the case of a field of special interest and importance, a
photograph could be obtained with the larger instrument on exactly
double the scale given by the smaller. It has rather, however,
found its usefulness in a slightly different field. The observation
of the satellites of Jupiter was suggested by Galileo as a means
of determining the longitude at sea. As already pointed out, the
suggestion did not prove to be a practical one for that purpose, but
observations of the satellites have been made none the less with
a view simply to improving our knowledge of their movements, and
of the mass of Jupiter. The utilitarian motive for the work having
fallen through, it has been carried on as a matter of pure science.
And the work has not stopped with the satellites of Jupiter; eight
satellites were in due time discovered to Saturn, four to Uranus,
and two to Mars; and though these could give not the remotest
assistance to navigation, they too have been made the subjects of
observation for precisely the same reason as those of Jupiter have
been.
[Illustration: THE THOMPSON TELESCOPE IN THE NEW DOME.]
In just the same way, when the discovery of Neptune was followed by
that of a solitary companion to it, this also had to be followed.
The difficulties in the way of observing the fainter of all these
satellites were considerable, and the work has been mostly confined
to two or three observatories possessing very large telescopes. As
the largest telescope at Greenwich was only 7 inches in aperture up
to 1859, and only 12-3/4 inches up to 1893, it is only very recently
that it has been able to take any very substantial part in satellite
measures. But since the Thompson photographic telescope was set up,
it has been found that a photograph of Neptune and its satellite can
be taken in considerably less time than a complete set of direct
measures can be made, whilst the photograph, which can be measured
at leisure during the day, gives distinctly the more accurate
results.
So, too, the places of the minor planets can be got more accurately
and quickly by means of photographs with this great telescope than
by direct observation, and photographs of the most interesting
of them all, the little planet Eros, have been very successfully
obtained. So that, though doing nothing directly to improve the
art of navigation, or to find the longitude at sea, the great
photographic refractor takes its share in the work of 'Rectifying
the Tables of the Planets.'
[Illustration: THE NEBULÆ OF THE PLEIADES.
(_From a photograph taken at the Royal Observatory, Greenwich,
December 3, 1899, with an exposure of three hours._)]
The reflector of 30 inches' aperture, which acts as a counterpoise
to the sheaf of telescopes of the Thompson, is intended for use with
the spectroscope, the quality which mirrors possess of bringing
all rays, whatever their colour, to the same focus being of great
importance for spectroscopic work. But the experiments which have
been made with it in celestial photography have proved so extremely
successful as to cause the postponement of the recommencement of
the spectroscopic researches. Chief amongst these photographs are
some good ones of the moon, and more recently some exceedingly fine
photographs of the principal nebulæ.
In no department of astronomy has photography brought us such
striking results as in regard to the nebulæ. Dr. Roberts' photograph
of the great nebula in Andromeda converted the two or three
meaningless rifts--which some of the best drawings had shown--into
the divisions between concentric rings; and what had appeared a
mere shapeless cloud was seen to be a vast symmetrical structure, a
great sidereal system in the making. The great nebula in Orion has
grown in successive photographs in detail and extent, until we have
a large part of the constellation bound together in the convolutions
of a single nebula of the most exquisite detail and most amazing
complexity. The group of the Pleiades has had a more wonderful
record still. Manifestly a single system even to the naked eye,
and showing some faint indications of nebulosity in the telescope,
the photographs have revealed its principal stars shining out from
nebulous masses, in appearance like carded wool, and have shown
smaller stars threaded on nebulous lines like pearls upon a string.
Such photographs are, of course, of no utilitarian value, and at
present they lead us to no definite scientific conclusions. They
lie, therefore, doubly outside the limits of the purely practical,
but they attract us by their extreme beauty, and by the amazing
difficulty of the problems they suggest. How are these weird masses
of gas retained in such complex form over distances which must
be reckoned by millions of millions of miles? By what agency are
they made to glow so as to be visible to us here? What conceivable
condition threads together suns on a line of nebula? What universes
are here in the making, or perhaps it may be falling into ruin and
decay?