This difference is called the parallax of the
stars; and the problem of measuring it is one of the most delicate
and difficult in the whole field of practical astronomy.
The nineteenth century was well on its way before the instruments
of the astronomer were brought to such perfection as to admit of
the measurement. From the time of Copernicus to that of Bessel
many attempts had been made to measure the parallax of the stars,
and more than once had some eager astronomer thought himself
successful. But subsequent investigation always showed that he had
been mistaken, and that what he thought was the effect of parallax
was due to some other cause, perhaps the imperfections of his
instrument, perhaps the effect of heat and cold upon it or upon
the atmosphere through which he was obliged to observe the star,
or upon the going of his clock. Thus things went on until 1837,
when Bessel announced that measures with a heliometer--the most
refined instrument that has ever been used in measurement--showed
that a certain star in the constellation Cygnus had a parallax of
one-third of a second. It may be interesting to give an idea of
this quantity. Suppose one's self in a house on top of a mountain
looking out of a window one foot square, at a house on another
mountain one hundred miles away.
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