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,