TYCHO, THE GREAT OBSERVER
Clear as Copernicus had made the demonstration of the truth of his new
system, it nevertheless failed of immediate and universal acceptance.
The Ptolemaic system was too strongly intrenched, and the motions of all
the bodies in the sky were too well represented by it. Accurate
observations were greatly needed, and the Landgrave William IV. of Hesse
built the Cassel Observatory, which made a new catalogue of stars, and
introduced the use of clocks to carry on the time as measured by the
uniform motion of the celestial sphere. Three years after the death of
Copernicus, Tycho Brahe was born, and when he was 30 the King of Denmark
built for him the famous observatory of Uraniborg, where the great
astronomer passed nearly a quarter of a century in critically observing
the positions of the stars and planets. Tycho was celebrated as a
designer and constructor of new types of astronomical instruments, and
he printed a large volume of these designs, which form the basis of many
in use at the present day. Unfortunately for the genius of Tycho and the
significance of his work, the invention of the telescope had not yet
been made, so that his observations had not the modern degree of
accuracy. Nevertheless, they were destined to play a most important part
in the progress of astronomy.
Tycho was sadly in error in his rejection of the Copernican system,
although his reasons, in his day, seemed unanswerable. If the outer
planets were displaced among the stars by the annual motion of the earth
round the sun, he argued, then the fixed stars must be similarly
displaced--unless indeed they be at such vast distances that their
motions would be too slight to be visible. Of course we know now that
this is really true, and that no instruments that Tycho was able to
build could possibly have detected the motions, the effects of which we
now recognize in the case of the nearer fixed stars in their annual, or
parallactic, orbits.
The remarkably accurate instruments devised by Tycho Brahe and employed
by him in improving the observations of the positions of the heavenly
bodies were no doubt built after descriptions of astrolabes such as
Hipparchus used, as described by Ptolemy. In his "Astronomiae Instauratae
Mechanica" we find illustrations and descriptions of many of them.
One is a polar astrolabe, mounted somewhat as a modern equatorial
telescope is, and the meridian circle is adjustable so that it can be
used in any place, no matter what its latitude might be. There is a
graduated equatorial ring at right angles to the polar axis, so that the
astrolabe could be used for making observations outside the meridian as
well as on it. This equatorial circle slides through grooves, and is
furnished with movable sights, and a plumb line from the zenith or
highest point of the meridian circle makes it possible to give the
necessary adjustment in the vertical. Screws for adjustment at the
bottom are provided, just as in our modern instruments, and two
observers were necessary, taking their sights simultaneously; unless,
as in one type of the instrument, a clock, or some sort of measure of
time, was employed.
Another early type of instrument is called by Tycho the ecliptic
astrolabe (_Armillae Zodiacales_, or the Zodiacal Rings). It resembles
the equatorial astrolabe somewhat, but has a second ring inclined to the
equatorial one at an angle equal to the obliquity of the ecliptic. In
observing, the equatorial ring was revolved round till the ecliptic ring
came into coincidence with the plane of the ecliptic in the sky. Then
the observation of a star's longitude and latitude, as referred to the
ecliptic plane, could be made, quite as well as that of right ascension
and declination on the equatorial plane. But it was necessary to work
quickly, as the adjustment on the ecliptic would soon disappear and have
to be renewed.
Tycho is often called the father of the science of astronomical
observation, because of the improvements in design and construction of
the instruments he used. His largest instrument was a mural quadrant, a
quarter-circle of copper, turning parallel to the north-and-south face
of a wall, its axis turning on a bearing fixed in the wall. The radius
of this quadrant was nine feet, and it was graduated or divided so as to
read the very small angle of ten seconds of arc--an extraordinary degree
of precision for his day.
Tycho built also a very large alt-azimuth quadrant, of six feet radius.
Its operation was very much as if his mural quadrant could be swung
round in azimuth. At several of the great observatories of the present
day, as Greenwich and Washington, there are instruments of a similar
type, but much more accurate, because the mechanical work in brass and
steel is executed by tools that are essentially perfect, and besides
this the power of the telescope is superadded to give absolute
direction, or pointing on the object under observation.
Excellent clocks are necessary for precise observation with such an
instrument; but neither Tycho Brahe, nor Hevelius was provided with such
accessories. Hevelius did not avail himself of the telescope as an aid
to precision of observation, claiming that pinhole sights gave him more
accurate results. It was a dispute concerning this question that Halley
was sent over from London to Danzig to arbitrate.
There could be but one way to decide; the telescope with its added power
magnifies any displacement of the instrument, and thereby enables the
observer to point his instrument more exactly. So he can detect smaller
errors and differences of direction than he can without it. And what is
of great importance in more modern astronomy, the telescope makes it
possible to observe accurately the position of objects so faint that
they are wholly invisible to the naked eye.