THE ALTAZIMUTH DEPARTMENT
The determining of the places of the fixed stars which Flamsteed
carried out so efficiently in his _British Catalogue of Stars_--the
first 'Census of the Sky' made with the aid of a telescope--was but
half of the work imposed upon him. The other half, equally necessary
for the solution of the problem of the longitude at sea, was the
'Rectifying the Tables of the Motions of the Heavens.'
This second duty was not less necessary than the other, for, if we
may again use the old simile of the clock-face, the fixed stars
may be taken to represent the figures on the vast dial of the sky,
whilst the moon, as it moves amongst them, corresponds to the moving
hand of the timepiece. To know the places of the stars, then,
without being able to predict the place of the moon, would be much
like having a clock without its hands. But if not less necessary,
it was certainly more difficult. The secret of the movements of the
moon and planets had not then been grasped, and the only tables
which had been calculated were based upon observations made before
the days of telescopes.
It is one of the most fortunate and remarkable coincidences in the
whole history of science, that at the very time that Greenwich
Observatory was being called into existence, the greatest of
all astronomers was working out his demonstration of the great
fundamental law of the material universe--the law that every
particle of matter attracts every other particle with a force which
varies directly with the mass and inversely with the square of the
distance.
Several other of the great minds of that time, in particular
Dr. Hooke, the Gresham Professor of Astronomy, had seen that
it was possible that some such law might supply the secret of
planetary motion; but it is one thing to make a suggestion, and
a very different matter indeed to be able to demonstrate it; and
the latter was in Newton's power alone. He did much more than
demonstrate it--he brought out a whole series of most important and
far-reaching consequences. He showed that the ebb and flow of the
tides was due to the attraction of both sun and moon, especially
the latter, upon the waters of our oceans. He pointed out certain
irregularities which must take place in the motion of our moon, due
to the influence of the sun upon it. He showed, too, what was the
cause of that swinging of the axis of the earth which gives rise to
precession. He deduced the relative weights of the earth, the sun,
and of Jupiter and Saturn, the planets with satellites. He proved
also that comets, which had seemed hitherto to men as perfectly
lawless wanderers, obeyed in their orbits the self-same law which
governed the moon and planets. The whole vast system of celestial
movements, which had long seemed to men irregular and uncontrolled,
now fell, every one of them, into its place, as but the necessary
manifestations of one grand, simple order.
This great discovery gave a new and additional importance to the
regular observation of the moon and planets. They were needed now,
not only to assist in the practical work of navigation, but for
the development of questions of pure science. Halley, the second
Astronomer Royal, and Maskelyne, the fifth, devoted themselves
chiefly to this department of work, to the partial neglect of the
observation of the places of stars. Airy, the seventh, whilst making
catalogue-work a part of the regular routine of the Observatory,
developed the observation of the members of the solar system, and
especially of the moon, in a most marked degree, and collected and
completely reduced the vast mass of material which the industry of
his predecessors had gathered. It is pre-eminently of the work of
Airy that the memorable words quoted before of Professor Newcomb,
the great American mathematician and astronomer, are applicable:
'that if this branch of astronomy were entirely lost, it could be
reconstructed from the Greenwich observations alone.'
A most important step taken by Airy was the construction of an
altazimuth. An altazimuth is practically a theodolite on a large
scale. Its purpose is to determine, not the declination and right
ascension of some celestial body, as is the case with the transit
circle, but its altitude, _i.e._ its height above the horizon, and
its azimuth, _i.e._ the angle measured on the horizontal plane from
the north point. The altazimuth, then, like the transit circle,
consists of a telescope revolving on a horizontal axis, but, unlike
the transit circle, both the telescope and the piers which carry its
pivots can be rotated so as to point not merely due north and south,
but in any direction whatsoever.
[Illustration: AIRY'S ALTAZIMUTH.]
The observations with the altazimuth are rather more complicated
than those with the transit circle. Looking in the telescope, the
observer sees a double set of spider threads or 'wires'; and when a
star or other heavenly body enters the field, it will generally be
observed to move obliquely across both sets of wires. The observer
usually determines to make an observation either in altitude or
azimuth. In the former case he presses the little contact button,
which, as in the transit circle, is provided close to the eyepiece,
as the star reaches each of the horizontal wires in succession. If
in azimuth, it is the times of crossing the vertical wires that are
in like manner telegraphed to the chronograph. The transit over,
the appropriate circle is read; for the telescope itself is rigidly
attached to a vertical wheel having a carefully engraved circle on
its face and read by four microscopes, whilst the entire instrument
carries another set of microscopes, pointing to a fixed horizontal
circle, and upon which the azimuth can be read. A complete
observation involves four such transits and sets of circle readings,
two of altitude, and two of azimuth; for after one of altitude
and one of azimuth the telescope is turned round, and a second
observation is taken in each element.
The observation gives us the altitude and azimuth of the star. These
particulars are of no direct value to us. But it is a mere matter
of computation, though a long and laborious one, to convert these
elements into right ascension and declination.
The usefulness of the altazimuth will be seen at once. It will be
remembered that with the transit circle any particular object can
only be observed as it crosses the meridian. If the weather should
be cloudy, or the observer late, the chance of observation is
lost for four and twenty hours, and in the case of the moon, for
which the altazimuth is specially used, it is on the meridian only
in broad daylight during that part of the month which immediately
precedes and follows new moon. At such times it is practically
impossible to observe it with the transit circle; with the
altazimuth it may be caught in the twilight before sunrise or after
sunset; and at other times in the month, if lost on the meridian in
the transit circle, the altazimuth still gives the observer a chance
of catching it any time before it sets. But for this instrument, our
observations of the moon would have been practically impossible over
at least one-fourth of its orbit.
Airy's altazimuth was but a small instrument of three and
three-quarter inches aperture, mounted in a high tower built on
the site of Flamsteed's mural arc; and, after a life history of
about half a century, has been succeeded by a far more powerful
instrument. The 'New Altazimuth' has an aperture of eight inches,
and is housed in a very solidly constructed building of striking
appearance, the connection of the Observatory with navigation being
suggested by a row of circular lights which strongly recall a ship's
portholes. This building is at the southern end of the narrow
passage, 'the wasp's waist,' which connects the older Observatory
domain with the newer. It is the first building we come to in the
south ground. The computations of the department are carried on in
the south wing of the new Observatory.
It will be seen from the photograph that the instrument is much
larger, heavier, and less easy to move in azimuth than the old
altazimuth. It is, therefore, not often moved in azimuth, but is set
in some particular direction, not necessarily north and south, in
which it is used practically as a transit circle.
[Illustration: NEW ALTAZIMUTH BUILDING.]
There is quite another way of determining the place of the moon,
which is sometimes available, and which offers one of the prettiest
of observations to the astronomer. As the moon travels across the
sky, moving amongst the stars from west to east, it necessarily
passes in front of some of them, and hides them from us for a time.
Such a passage, or 'occultation,' offers two observations: the
'disappearance,' as the moon comes up to the star and covers it; the
'reappearance,' as it leaves it again, and so discloses it.
[Illustration: THE NEW ALTAZIMUTH.
(_From a photograph by Mr. Lacey._)]
Except at the exact time of full moon, we do not see the entire face
of our satellite; one edge or 'limb' is in darkness. As the moon
therefore passes over the star, either the limb at which the star
disappears, or that at which it reappears, is invisible to us. To
watch an occultation at the bright limb is pretty; the moon, with
its shining craters and black hollows, its mountain ranges in bright
relief, like a model in frosted silver, slowly, surely, inevitably
comes nearer and nearer to the little brilliant which it is going to
eclipse. The movement is most regular, most smooth, yet not rapid.
The observer glances at his clock, and marks the minute as the two
heavenly bodies come closer and closer to each other. Then he counts
the clock beats: 'five, six, seven,' it may be, as the star has
been all but reached by the advancing moon. 'Eight,' it is still
clear; ere the beat of the clock rings to the 'nine,' perhaps the
little diamond point has been touched by the wide arch of the moon's
limb, and has gone! Less easy to exactly time is a reappearance at
the bright limb. In this case the observer must ascertain from the
_Nautical Almanac_ precisely where the star will reappear; then
a little before the predicted time he takes his place at the
telescope, watches intently the moon's circumference at the point
indicated, and, listening for the clock-beats, counts the seconds as
they fly. Suddenly, without warning, a pin-point of light flashes
out at the edge of the moon, and at once draws away from it. The
star has 'reappeared.'
Far more striking is a disappearance or reappearance at the 'dark
limb.' In this case the limb of the moon is absolutely invisible,
and it may be that no part of the moon is visible in the field
of the telescope. In this case the observer sees a star shining
brightly and alone in the middle of the field of his telescope. He
takes the time from his faithful clock, counting beat after beat,
when suddenly the star is gone! So sudden is the disappearance that
the novice feels almost as astonished as if he had received a slap
in the face, and not unfrequently he loses all count or recollection
of the clock beats. The reappearance at the dark limb is quite as
startling; with a bright star it is almost as if a shell had burst
in his very face, and it would require no very great imagination to
make him think that he had heard the explosion. One moment nothing
was visible; now a great star is shining down serenely on the
watcher. A little practice soon enables the observer to accustom
himself to these effects, and an old hand finds no more difficulty
in observing an occultation of any kind than in taking a transit.
Such an observation is useful for more purposes than one. If the
position of the star occulted is known--and it can be determined at
leisure afterwards--we necessarily know where the limb of the moon
was at the time of the observation. Then the time which the moon
took to pass over the star enables us to calculate the diameter of
our satellite; the different positions of the moon relative to the
star, as seen from different observatories, enable us to calculate
its distance.
But if the disappearance takes place at the bright limb, the
reappearance usually takes place at the dark, and _vice versâ_;
and the two observations are not quite comparable. There is one
occasion, however, when both observations are made under similar
circumstances, namely, at the full. And if the moon happens also
to be totally eclipsed, the occultations of quite faint stars can
be successfully observed, much fainter than can ordinarily be
seen close up to the moon. Total eclipses of the moon, therefore,
have recently come to be looked upon as important events for the
astronomer, and observatories the world over usually co-operate in
watching them. October 4, 1884, was the first occasion when such an
organised observation was made; there have been several since, and
on these nights every available telescope and observer at Greenwich
is called into action.
It may be asked why these different modes of observing the moon
are still kept up, year in and year out. 'Do we not know the
moon's orbit sufficiently well, especially since the discovery of
gravitation?' No; we do not. This simple and beautiful law--simple
enough in itself, gives rise to the most amazing complexity of
calculation. If the earth and moon were the only two bodies in the
universe, the problem would be a simple one. But the earth, sun,
and moon are members of a triple system, each of which is always
acting on both of the others. More, the planets, too, have an
appreciable influence, and the net result is a problem so intricate
that our very greatest mathematicians have not thoroughly worked it
out. Our calculations of the moon's motions need, therefore, to be
continually compared with observation, need even to be continually
corrected by it.
There is a further reason for this continual observation, not only
in the case of the sun, which is our great standard star, since
from it we derive the right ascensions of the stars, and it is also
our great timekeeper; not only in that of the moon, but also in the
case of the planets. Their places as computed need continually to
be compared with their places as observed, and the discordances,
if any, inquired into. The great triumph which resulted to science
from following this course--to pure science, since Uranus is too
faint a planet to be any help to the sailor in navigation--is well
known. The observed movements of Uranus proved not to be in accord
with computation, and from the discordances between calculation and
observation Adams and Leverrier were able to predicate the existence
of a hitherto unseen planet beyond--
'To see it, as Columbus saw America from Spain. Its movements
were felt by them trembling along the far-reaching line of their
analysis, with a certainty hardly inferior to that of ocular
demonstration.'[5]
[5] From Sir John Herschel's address to the British Association,
September 10, 1846, thirteen days before Galle's first observation
of the planet.
The discovery of Neptune was not made at Greenwich, and Airy has
been often and bitterly attacked because he did not start on the
search for the predicted planet the moment Adams addressed his
first communication to him, and so allowed the French astronomer
to engross so much of the honour of the exploit. The controversy
has been argued over and over again, and we may be content to leave
it alone here. There is one point, however, which is hardly ever
mentioned, which must have had much effect in determining Airy's
conduct. In 1845, the year in which Adams sent his provisional
elements of the unseen disturbing planet to Airy, the largest
telescope available for the search at Greenwich was an equatorial of
only six and three-quarter inches aperture, provided with small and
insufficient circles for determining positions, and housed in a very
small and inconvenient dome; whilst at Cambridge, within a mile or
so of Adams' own college, was the 'Northumberland' equatorial, of
nearly twelve inches aperture, under the charge of the University
Professor of Astronomy, Professor Challis, and which was then much
the largest, best mounted and housed equatorial in the entire
country. The 'Northumberland' had been begun from Airy's designs and
under his own superintendence, when he was Professor of Astronomy at
Cambridge. Naturally, then, knowing how much superior the Cambridge
telescope was to any which he had under his care, he thought the
search should be made with it. He had no reason to believe that his
own instrument was competent for the work.
[Illustration: THE NEW OBSERVATORY AS SEEN FROM FLAMSTEED'S
OBSERVATORY.]
On the other hand, it is hard for the ordinary man to understand
how it was that Adams not only left unnoticed and unanswered for
three-quarters of a year, an inquiry of Airy's with respect to his
calculations, but also never took the trouble to visit Challis,
whom he knew well, and who was so near at hand, to stir him up
to the search. But, in truth, the whole interest of the matter
for Adams rested in the mathematical problem. The irregularities
in the motion of Uranus were interesting to him simply for the
splendid opportunity which they gave him for their analysis. A
purely imaginary case would have served his purpose nearly as well.
The actuality of the planet which he predicted was of very little
moment; the _éclat_ and popular reputation of the discovery was less
than nothing; the problem itself and the mental exercise in its
solution, were what he prized.
But it was not creditable to the nation that the Royal Observatory
should have been so ill-provided with powerful telescopes; and a
few years later Airy obtained the sanction of the Government for
the erection of an equatorial larger than the 'Northumberland,'
but on the same general plan and in a much more ample dome. This
was for thirty-four years the 'Great' or 'South-East' equatorial,
and the mounting still remains and bears the old name, though the
original telescope has been removed elsewhere. The object-glass had
an aperture of twelve and three-quarter inches and a focal length of
eighteen feet, and was made by Merz of Munich, the engineering work
by Ransomes and Sims of Ipswich, and the graduations and general
optical work by Simms, now of Charlton, Kent. The mounting was so
massive and stable that the present Astronomer Royal has found it
quite practicable and safe to place upon it a telescope (with its
counterpoises) of many times the weight, one made by Sir Howard
Grubb, of Dublin, of twenty-eight inches aperture and twenty-eight
feet focal length, the largest refractor in the British Empire,
though surpassed by several American and Continental instruments.
The stability of the mounting was intended to render the telescope
suitable for a special work. This was the observation of 'minor
planets.' On the first day of the present century the first of these
little bodies was discovered by Piazzi at Palermo. Three more were
discovered at no great interval afterwards, and then there was an
interval of thirty-eight years without any addition to their number.
But from December 8, 1845, up to the present time, the work of
picking up fresh individuals of these 'pocket planets' has gone on
without interruption, until now more than 400 are known. Most of
these are of no interest to us, but a few come sufficiently near to
the earth for their distance to be very accurately determined; and
when the distance of one member of the solar system is determined,
those of all the others can be calculated from the relations which
the law of gravitation reveals to us. It is a matter of importance,
therefore, to continue the work of discovery, since we may at any
time come across an interesting or useful member of the family; and
that we may be able to distinguish between minor planets already
discovered and new ones, their orbits must be determined as they
are discovered, and some sort of watch kept on their movements.
A striking example of the scientific prizes which we may light upon
in the process of the rather dreary and most laborious work which
the minor planets cause, has been recently supplied by the discovery
of Eros. On August 13, 1898, Herr Witt, of the Urania Observatory,
Berlin, discovered a very small planet that was moving much faster
in the sky than is common with these small bodies. The great
majority are very much farther from the sun than the planet Mars,
many of them twice as far, and hence, since the time of a planet's
revolution round the sun increases, in accordance with Kepler's law,
more rapidly than does its distance, it follows that they move much
more slowly than Mars. But this new object was moving at almost the
same speed as Mars; it must, therefore, be most unusually near to
us. Further observations soon proved that this was the case, and
Eros, as the little stranger has been called, comes nearer to us
than any other body of which we are aware except the moon. Venus
when in transit is 24-1/2 millions of miles from us, Mars at its
nearest is 34-1/2 millions, Eros at its nearest approach is but
little over 13 millions.
The use of such a body to us is, of course, quite apart from any
purpose of navigation, except very indirectly. But it promises to
be of the greatest value in the solution of a question in which
astronomers must always feel an interest, the determination of
the distance of the earth from the sun. We know the _relative_
distances of the different planets, and, consequently if we could
determine the absolute distance of any one, we should know the
distances of all. As it is practically impossible to measure our
distance from the sun directly, several attempts have been made
to determine the distances of Venus, Mars, or such of the minor
planets as come the nearest to us. Three of these in particular, the
little planets Iris, Victoria, and Sappho, have given us the most
accurate determinations of the sun's distance (92,874,000 miles)
which we have yet obtained; but Eros at its nearest approach will
be six times as near to us as either of the three mentioned above,
and therefore should give us a value with only one-sixth of the
uncertainty attaching to that just mentioned.
The discovery of minor planets has lain outside the scope of
Greenwich work, but their observation has formed an integral part of
it. The general public is apt to lay stress rather on the first than
on the second, and to think it rather a reproach to Greenwich that
it has taken no part in such explorations. Experience has, however,
shown that they may be safely left to amateur activity, whilst the
monotonous drudgery of the observation of minor planets can only be
properly carried out in a permanent institution.
The observation of these minute bodies with the transit circle
and altazimuth is attended with some difficulties; but precise
observations of various objects may be made with an equatorial;
indeed, comets are usually observed by its means.
The most ordinary way of observing a comet with an equatorial is as
follows: Two bars are placed in the eye-piece of the telescope, at
right angles to each other, and at an angle of forty-five degrees
to the direction of the apparent daily motion of the stars. The
telescope is turned to the neighbourhood of the comet, and moved
about until it is detected. The telescope is then put a little in
front of the comet, and very firmly fixed. The observer soon sees
the comet entering his field, and by pressing the contact button he
telegraphs to the chronograph the time when the comet is exactly
bisected by each of the bars successively. He then waits until a
bright star, or it may be two or three, have entered the telescope
and been observed in like manner. The telescope is then unclamped,
and moved forward until it is again ahead of the comet, and the
observations are repeated; and this is done as often as is thought
desirable. The places of the stars have, of course, to be found out
from catalogues, or have to be observed with the transit circle, but
when they are known the position of the comet or minor planet can
easily be inferred.
Next to the glory of having been the means of bringing about the
publication of Newton's _Principia_, the greatest achievement of
Halley, the second Astronomer Royal, was that he was the first to
predict the return of a comet. Newton had shown that comets were
no lawless wanderers, but were as obedient to gravitation as were
the planets themselves, and he also showed how the orbit of a comet
could be determined from observations on three different dates.
Following these principles, Halley computed the orbits of no fewer
than twenty-four comets, and found that three of them, visible at
intervals of about seventy-five years, pursued practically the same
path. He concluded, therefore, that these were really different
appearances of the same object, and, searching old records, he
found reason to believe that it had been observed frequently
earlier still. It seems, in fact, to have been the comet which is
recorded to have been seen in 1066 in England at the time of the
Norman invasion; in A.D. 66, shortly before the commencement of
that war which ended in the destruction of Jerusalem by Titus;
and earlier still, so far back as B.C. 12. Halley, however,
experienced a difficulty in his investigation. The period of the
comet's revolution was not always the same. This, he concluded,
must be due to the attraction of the planets near which the comet
might chance to travel. In the summer of 1681 it had passed very
close to Jupiter, for instance, and in consequence he expected
that instead of returning in August 1757, seventy-five years after
its last appearance, it would not return until the end of 1758 or
the beginning of 1759. It has returned twice since Halley's day,
a triumphant verification of the law of gravitation; and we are
looking for it now for a third return some ten years hence, in 1910.
Halley's comet, therefore, is an integral member of our solar
system, as much so as the earth or Neptune, though it is utterly
unlike them in appearance and constitution, and though its path is
so utterly unlike theirs that it approaches the sun nearer than our
earth, and recedes farther than Neptune. But there are other comets,
which are not permanent members of our system, but only passing
visitors. From the unfathomed depths of space they come, to those
depths they go. They obey the law of gravitation so far as our sight
can follow them, but what happens to them beyond? Do they come under
some other law, or, perchance, in outermost space is there still a
region reserved to primeval Chaos, the 'Anarch old,' where no law
at all prevails? Gravitation is the bond of the solar system; is it
also the bond of the Universe?