lay on his back examining the stars. He had no telescope, and
his other instruments were such crude things as he put together
himself. The most important was what he calls the _Instrumentum
parallacticum_, a wooden isosceles triangle with legs eight feet long
divided into 1000 {619} divisions by ink marks, and a hypotenuse
divided into 1414 divisions. With this he determined the height of the
sun, moon and stars, and their deviation from the vernal point. To
this he added a square (quadrum) which told the height of the sun by
the shadow thrown by a peg in the middle of the square. A third
instrument, also to measure the height of a celestial body, was called
the Jacob's staff. His difficulties were increased by the lack of any
astronomical tables save those poor ones made by Greeks and Arabs. The
faults of these were so great that the fundamental star, _i.e._, the
one he took by which to measure the rest, Spica, was given a longitude
nearly 40 degrees out of the true one.

[Sidenote: Copernican hypothesis]

Nevertheless with these poor helps Copernicus arrived, and that very
early, at his momentous conclusion. His observations, depending as
they did on the weather, were not numerous. His time was spent largely
in reading the classic astronomers and in working out the mathematical
proofs of his hypothesis. He found hints in quotations from ancient
astronomers in Cicero and Plutarch that the earth moved, but he, for
the first time, placed the planets in their true position around the
sun, and the moon as a satellite of the earth. He retained the old
conception of the primum mobile or sphere of fixed stars though he
placed it at an infinitely greater distance than did the ancients, to
account for the absence of any observed alteration (parallax) in the
position of the stars during the year. He also retained the old
conception of circular orbits for the planets, though at one time he
considered the possibility of their being elliptical, as they are.
Unfortunately for his immediate followers the section on this subject
found in his own manuscript was cut out of his printed book.

The precise moment at which Copernicus {620} formulated his theory in
his own mind cannot be told with certainty, but it was certainly before
1516. He kept back his books for a long time, but his light was not
placed under a bushel nevertheless. [Sidenote: 1520] The first rays
of it shown forth in a tract by Celio Calcagnini of whic