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Early Instruments of Astronomy

Posted on July 23rd, 2007 by Admin.
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Having now traced the progress of physical astronomy up to the time
when very striking proofs of the universality of the law of
gravitation convinced the most sceptical, it must still be borne in
mind that, while gravitation is certainly the principal force
governing the motions of the heavenly bodies, there may yet be a
resisting medium in space, and there may be electric and magnetic
forces to deal with. There may, further, be cases where the effects of
luminous radiative repulsion become apparent, and also Crookes’
vacuum-effects described as “radiant matter.” Nor is it quite certain
that Laplace’s proofs of the instantaneous propagation of gravity are
final.

And in the future, as in the past, Tycho Brahe’s dictum must be
maintained, that all theory shall be preceded by accurate
observations. It is the pride of astronomers that their science stands
above all others in the accuracy of the facts observed, as well as in
the rigid logic of the mathematics used for interpreting these facts.

It is interesting to trace historically the invention of those
instruments of precision which have led to this result, and, without
entering on the details required in a practical handbook, to note the
guiding principles of construction in different ages.

It is very probable that the Chaldeans may have made spheres, like the
armillary sphere, for representing the poles of the heavens; and with
rings to show the ecliptic and zodiac, as well as the equinoctial and
solstitial colures; but we have no record. We only know that the tower
of Belus, on an eminence, was their observatory. We have, however,
distinct records of two such spheres used by the Chinese about 2500
B.C. Gnomons, or some kind of sundial, were used by the Egyptians and
others; and many of the ancient nations measured the obliquity of the
ecliptic by the shadows of a vertical column in summer and winter. The
natural horizon was the only instrument of precision used by those who
determined star positions by the directions of their risings and
settings; while in those days the clepsydra, or waterclock, was the
best instrument for comparing their times of rising and setting.

About 300 B.C. an observatory fitted with circular instruments for
star positions was set up at Alexandria, the then centre of
civilisation. We know almost nothing about the instruments used by
Hipparchus in preparing his star catalogues and his lunar and solar
tables; but the invention of the astrolabe is attributed to him.[1]

In more modern times Nuremberg became a centre of astronomical
culture. Waltherus, of that town, made really accurate observations of
star altitudes, and of the distances between stars; and in 1484
A.D. he used a kind of clock. Tycho Brahe tried these, but discarded
them as being inaccurate.

Tycho Brahe (1546-1601 A.D.) made great improvements in armillary
spheres, quadrants, sextants, and large celestial globes. With these
he measured the positions of stars, or the distance of a comet from
several known stars. He has left us full descriptions of them,
illustrated by excellent engravings. Previous to his time such
instruments were made of wood. Tycho always used metal. He paid the
greatest attention to the stability of mounting, to the orientation of
his instruments, to the graduation of the arcs by the then new method
of transversals, and to the aperture sight used upon his
pointer. There were no telescopes in his day, and no pendulum
clocks. He recognised the fact that there must be instrumental
errors. He made these as small as was possible, measured their amount,
and corrected his observations. His table of refractions enabled him
to abolish the error due to our atmosphere so far as it could affect
naked-eye observations. The azimuth circle of Tycho’s largest quadrant
had a diameter of nine feet, and the quadrant a radius of six feet. He
introduced the mural quadrant for meridian observations.[2]

[Illustration: ANCIENT CHINESE INSTRUMENTS, Including quadrant, celestial
globe, and two armillae, in the Observatory at Peking. Photographed in
Peking by the author in 1875, and stolen by the Germans when the
Embassies were relieved by the allies in 1900.]

The French Jesuits at Peking, in the seventeenth century, helped the
Chinese in their astronomy. In 1875 the writer saw and photographed,
on that part of the wall of Peking used by the Mandarins as an
observatory, the six instruments handsomely designed by Father
Verbiest, copied from the instruments of Tycho Brahe, and embellished
with Chinese dragons and emblems cast on the supports. He also saw
there two old instruments (which he was told were Arabic) of date
1279, by Ko Show-King, astronomer to Koblai Khan, the grandson of
Chenghis Khan. One of these last is nearly identical with the armillae
of Tycho; and the other with his “armillae aequatoriae maximae,” with
which he observed the comet of 1585, besides fixed stars and
planets.[3]

The discovery by Galileo of the isochronism of the pendulum, followed
by Huyghens’s adaptation of that principle to clocks, has been one of
the greatest aids to accurate observation. About the same time an
equally beneficial step was the employment of the telescope as a
pointer; not the Galilean with concave eye-piece, but with a
magnifying glass to examine the focal image, at which also a fixed
mark could be placed. Kepler was the first to suggest this. Gascoigne
was the first to use it. Huyghens used a metal strip of variable width
in the focus, as a micrometer to cover a planetary disc, and so to
measure the width covered by the planet. The Marquis Malvasia, in
1662, described the network of fine silver threads at right angles,
which he used in the focus, much as we do now.

In the hands of such a skilful man as Tycho Brahe, the old open
sights, even without clocks, served their purpose sufficiently well to
enable Kepler to discover the true theory of the solar system. But
telescopic sights and clocks were required for proving some of
Newton’s theories of planetary perturbations. Picard’s observations at
Paris from 1667 onwards seem to embody the first use of the telescope
as a pointer. He was also the first to introduce the use of Huyghens’s
clocks for observing the right ascension of stars. Olaus Romer was
born at Copenhagen in 1644. In 1675, by careful study of the times of
eclipses of Jupiter’s satellites, he discovered that light took time
to traverse space. Its velocity is 186,000 miles per second. In 1681
he took up his duties as astronomer at Copenhagen, and built the first
transit circle on a window-sill of his house. The iron axis was five
feet long and one and a-half inches thick, and the telescope was fixed
near one end with a counterpoise. The telescope-tube was a double
cone, to prevent flexure. Three horizontal and three vertical wires
were used in the focus. These were illuminated by a speculum, near the
object-glass, reflecting the light from a lantern placed over the
axis, the upper part of the telescope-tube being partly cut away to
admit the light. A divided circle, with pointer and reading
microscope, was provided for reading the declination. He realised the
superiority of a circle with graduations over a much larger
quadrant. The collimation error was found by reversing the instrument
and using a terrestrial mark, the azimuth error by star observations.
The time was expressed in fractions of a second. He also constructed a
telescope with equatoreal mounting, to follow a star by one axial
motion. In 1728 his instruments and observation records were destroyed
by fire.

Hevelius had introduced the vernier and tangent screw in his
measurement of arc graduations. His observatory and records were burnt
to the ground in 1679. Though an old man, he started afresh, and left
behind him a catalogue of 1,500 stars.

Flamsteed began his duties at Greenwich Observatory, as first
Astronomer Royal, in 1676, with very poor instruments. In 1683 he put
up a mural arc of 140 degrees, and in 1689 a better one, seventy-nine
inches radius. He conducted his measurements with great skill, and
introduced new methods to attain accuracy, using certain stars for
determining the errors of his instruments; and he always reduced his
observations to a form in which they could be readily used. He
introduced new methods for determining the position of the equinox and
the right ascension of a fundamental star. He produced a catalogue of
2,935 stars. He supplied Sir Isaac Newton with results of observation
required in his theoretical calculations. He died in 1719.

Halley succeeded Flamsteed to find that the whole place had been
gutted by the latter’s executors. In 1721 he got a transit instrument,
and in 1726 a mural quadrant by Graham. His successor in 1742,
Bradley, replaced this by a fine brass quadrant, eight feet radius, by
Bird; and Bradley’s zenith sector was purchased for the observatory.
An instrument like this, specially designed for zenith stars, is
capable of greater rigidity than a more universal instrument; and
there is no trouble with refraction in the zenith. For these reasons
Bradley had set up this instrument at Kew, to attempt the proof of the
earth’s motion by observing the annual parallax of stars. He certainly
found an annual variation of zenith distance, but not at the times of
year required by the parallax. This led him to the discovery of the
“aberration” of light and of nutation. Bradley has been described as
the founder of the modern system of accurate observation. He died in
1762, leaving behind him thirteen folio volumes of valuable but
unreduced observations. Those relating to the stars were reduced by
Bessel and published in 1818, at Konigsberg, in his well-known
standard work, _Fundamenta Astronomiae_. In it are results showing the
laws of refraction, with tables of its amount, the maximum value of
aberration, and other constants.

Bradley was succeeded by Bliss, and he by Maskelyne (1765), who
carried on excellent work, and laid the foundations of the Nautical
Almanac (1767). Just before his death he induced the Government to
replace Bird’s quadrant by a fine new mural _circle_, six feet in
diameter, by Troughton, the divisions being read off by microscopes
fixed on piers opposite to the divided circle. In this instrument the
micrometer screw, with a divided circle for turning it, was applied
for bringing the micrometer wire actually in line with a division on
the circle–a plan which is still always adopted.

Pond succeeded Maskelyne in 1811, and was the first to use this
instrument. From now onwards the places of stars were referred to the
pole, not to the zenith; the zero being obtained from measures on
circumpolar stars. Standard stars were used for giving the clock
error. In 1816 a new transit instrument, by Troughton, was added, and
from this date the Greenwich star places have maintained the very
highest accuracy.

George Biddell Airy, Seventh Astronomer Royal,[4] commenced his
Greenwich labours in 1835. His first and greatest reformation in the
work of the observatory was one he had already established at
Cambridge, and is now universally adopted. He held that an observation
is not completed until it has been reduced to a useful form; and in
the case of the sun, moon, and planets these results were, in every
case, compared with the tables, and the tabular error printed.

Airy was firmly impressed with the object for which Charles II. had
wisely founded the observatory in connection with navigation, and for
observations of the moon. Whenever a meridian transit of the moon
could be observed this was done. But, even so, there are periods in
the month when the moon is too near the sun for a transit to be well
observed. Also weather interferes with many meridian observations. To
render the lunar observations more continuous, Airy employed
Troughton’s successor, James Simms, in conjunction with the engineers,
Ransome and May, to construct an altazimuth with three-foot circles,
and a five-foot telescope, in 1847. The result was that the number of
lunar observations was immediately increased threefold, many of them
being in a part of the moon’s orbit which had previously been bare of
observations. From that date the Greenwich lunar observations have
been a model and a standard for the whole world.

Airy also undertook to superintend the reduction of all Greenwich
lunar observations from 1750 to 1830. The value of this laborious
work, which was completed in 1848, cannot be over-estimated.

The demands of astronomy, especially in regard to small minor planets,
required a transit instrument and mural circle with a more powerful
telescope. Airy combined the functions of both, and employed the same
constructors as before to make a _transit-circle_ with a telescope of
eleven and a-half feet focus and a circle of six-feet diameter, the
object-glass being eight inches in diameter.

Airy, like Bradley, was impressed with the advantage of employing
stars in the zenith for determining the fundamental constants of
astronomy. He devised a _reflex zenith tube_, in which the zenith
point was determined by reflection from a surface of mercury. The
design was so simple, and seemed so perfect, that great expectations
were entertained. But unaccountable variations comparable with those
of the transit circle appeared, and the instrument was put out of use
until 1903, when the present Astronomer Royal noticed that the
irregularities could be allowed for, being due to that remarkable
variation in the position of the earth’s axis included in circles of
about six yards diameter at the north and south poles, discovered at
the end of the nineteenth century. The instrument is now being used
for investigating these variations; and in the year 1907 as many as
1,545 observations of stars were made with the reflex zenith tube.

In connection with zenith telescopes it must be stated that Respighi,
at the Capitol Observatory at Rome, made use of a deep well with a
level mercury surface at the bottom and a telescope at the top
pointing downwards, which the writer saw in 1871. The reflection of
the micrometer wires and of a star very near the zenith (but not quite
in the zenith) can be observed together. His mercury trough was a
circular plane surface with a shallow edge to retain the mercury. The
surface quickly came to rest after disturbance by street traffic.

Sir W. M. H. Christie, Eighth Astronomer Royal, took up his duties in
that capacity in 1881. Besides a larger altazimuth that he erected in
1898, he has widened the field of operations at Greenwich by the
extensive use of photography and the establishment of large
equatoreals. From the point of view of instruments of precision, one
of the most important new features is the astrographic equatoreal, set
up in 1892 and used for the Greenwich section of the great
astrographic chart just completed. Photography has come to be of use,
not only for depicting the sun and moon, comets and nebulae, but also
to obtain accurate relative positions of neighbouring stars; to pick
up objects that are invisible in any telescope; and, most of all
perhaps, in fixing the positions of faint satellites. Thus Saturn’s
distant satellite, Phoebe, and the sixth and seventh satellites of
Jupiter, have been followed regularly in their courses at Greenwich
ever since their discovery with the thirty-inch reflector (erected in
1897); and while doing so Mr. Melotte made, in 1908, the splendid
discovery on some of the photographic plates of an eighth satellite of
Jupiter, at an enormous distance from the planet. From observations in
the early part of 1908, over a limited arc of its orbit, before
Jupiter approached the sun, Mr. Cowell computed a retrograde orbit and
calculated the future positions of this satellite, which enabled
Mr. Melotte to find it again in the autumn–a great triumph both of
calculation and of photographic observation. This satellite has never
been seen, and has been photographed only at Greenwich, Heidelberg,
and the Lick Observatory.

Greenwich Observatory has been here selected for tracing the progress
of accurate measurement. But there is one instrument of great value,
the heliometer, which is not used at Greenwich. This serves the
purpose of a double image micrometer, and is made by dividing the
object-glass of a telescope along a diameter. Each half is mounted so
as to slide a distance of several inches each way on an arc whose
centre is the focus. The amount of the movement can be accurately
read. Thus two fields of view overlap, and the adjustment is made to
bring an image of one star over that of another star, and then to do
the same by a displacement in the opposite direction. The total
movement of the half-object glass is double the distance between the
star images in the focal plane. Such an instrument has long been
established at Oxford, and German astronomers have made great use of
it. But in the hands of Sir David Gill (late His Majesty’s Astronomer
at the Cape of Good Hope), and especially in his great researches on
Solar and on Stellar parallax, it has been recognised as an instrument
of the very highest accuracy, measuring the distance between stars
correctly to less than a tenth of a second of arc.

The superiority of the heliometer over all other devices (except
photography) for measuring small angles has been specially brought
into prominence by Sir David Gill’s researches on the distance of the
sun–_i.e.,_ the scale of the solar system. A measurement of the
distance of any planet fixes the scale, and, as Venus approaches the
earth most nearly of all the planets, it used to be supposed that a
Transit of Venus offered the best opportunity for such measurement,
especially as it was thought that, as Venus entered on the solar disc,
the sweep of light round the dark disc of Venus would enable a very
precise observation to be made. The Transit of Venus in 1874, in
which the present writer assisted, overthrew this delusion.

In 1877 Sir David Gill used Lord Crawford’s heliometer at the Island
of Ascension to measure the parallax of Mars in opposition, and found
the sun’s distance 93,080,000 miles. He considered that, while the
superiority of the heliometer had been proved, the results would be
still better with the points of light shown by minor planets rather
than with the disc of Mars.

In 1888-9, at the Cape, he observed the minor planets Iris, Victoria,
and Sappho, and secured the co-operation of four other heliometers.
His final result was 92,870,000 miles, the parallax being 8″,802
(_Cape Obs_., Vol. VI.).

So delicate were these measures that Gill detected a minute periodic
error of theory of twenty-seven days, owing to a periodically
erroneous position of the centre of gravity of the earth and moon to
which the position of the observer was referred. This led him to
correct the mass of the moon, and to fix its ratio to the earth’s mass
= 0.012240.

Another method of getting the distance from the sun is to measure the
velocity of the earth’s orbital motion, giving the circumference
traversed in a year, and so the radius of the orbit. This has been
done by comparing observation and experiment. The aberration of light
is an angle 20″ 48, giving the ratio of the earth’s velocity to the
velocity of light. The velocity of light is 186,000 miles a second;
whence the distance to the sun is 92,780,000 miles. There seems,
however, to be some uncertainty about the true value of the
aberration, any determination of which is subject to irregularities
due to the “seasonal errors.” The velocity of light was experimentally
found, in 1862, by Fizeau and Foucault, each using an independent
method. These methods have been developed, and new values found, by
Cornu, Michaelson, Newcomb, and the present writer.

Quite lately Halm, at the Cape of Good Hope, measured
spectroscopically the velocity of the earth to and from a star by
observations taken six months apart. Thence he obtained an accurate
value of the sun’s distance.[5]

But the remarkably erratic minor planet, Eros, discovered by Witte in
1898, approaches the earth within 15,000,000 miles at rare intervals,
and, with the aid of photography, will certainly give us the best
result. A large number of observatories combined to observe the
opposition of 1900. Their results are not yet completely reduced, but
the best value deduced so far for the parallax[6] is 8″.807 +/-
0″.0028.[7]

FOOTNOTES:

[1] In 1480 Martin Behaim, of Nuremberg, produced his _astrolabe_ for
measuring the latitude, by observation of the sun, at sea. It
consisted of a graduated metal circle, suspended by a ring which was
passed over the thumb, and hung vertically. A pointer was fixed to a
pin at the centre. This arm, called the _alhidada_, worked round the
graduated circle, and was pointed to the sun. The altitude of the sun
was thus determined, and, by help of solar tables, the latitude could
be found from observations made at apparent noon.

[2] See illustration on p. 76.

[3] See Dreyer’s article on these instruments in _Copernicus_,
Vol. I. They were stolen by the Germans after the relief of the
Embassies, in 1900. The best description of these instruments is
probably that contained in an interesting volume, which may be seen in
the library of the R. A. S., entitled _Chinese Researches_, by
Alexander Wyllie (Shanghai, 1897).

[4] Sir George Airy was very jealous of this honourable title. He
rightly held that there is only one Astronomer Royal at a time, as
there is only one Mikado, one Dalai Lama. He said that His Majesty’s
Astronomer at the Cape of Good Hope, His Majesty’s Astronomer for
Scotland, and His Majesty’s Astronomer for Ireland are not called
Astronomers Royal.

[5] _Annals of the Cape Observatory_, vol. x., part 3.

[6] The parallax of the sun is the angle subtended by the earth’s
radius at the sun’s distance.

[7] A. R. Hinks, R.A.S.; _Monthly Notices_, June, 1909.