An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

CHAPTER Y.
OF ASTRONOMICAL INSTRUMENTS AND PROBLEMS-FIGURE AND
DENSITY OF THE EARTH.
117. THE most ancient astronomers employed no instruments for measuring angles, but acquired their knowledge of
the heavenly bodies by long-continued and most attentive inspection with the naked eye. In the Alexandrian school,
about 300 years before the Christian era, instruments began
to be freely used, and thenceforward trigonometry lent a powerful aid to the science of astronomy. Tycho Brahe, in the
16th century, formed a new era in practical astronomy, and
carried the measurement of angles to 10",-a degree of accuracy truly wonderful, considering that he had not the advantage of thle telescope. By the application of the telescope to
astronomic-al instruments, a far better defined view of objects
was acquired, and a far greater degree of refinement was attainable. The astronomers royal of Great Britain perfected
the art of observation, bringing the measurement of angles to
1", and the estimation of differences of time to J- of a second.
Beyond this degree of refinement it is supposed that we cannot advance, since unavoidable errors arising from the uncertainties of refraction, and the necessary imperfection of instruments, forbid us to hope for a more accurate determination
than this. But a little reflection will show us, that 1" on the
limb of an astronomical instrument, must be a space exceedingly small. Suppose the circle, on which the angle is lneas12x3. 14159
ured, be one foot in diameter. Then 3601- inch=
360 -
space occupied by 10. Hence 10x60-60 0=space of 1' and
3000=-space of 1". Such minute angles can be measured
only by large circles. If, for example, a circle is 20 feet in
diameter, a degree on its periphery would occupy a space 20