to solve it. All the methods of any
precision have depended on the Copernican fact that the earth in June
was 184 million miles away from its position in December, and that
accordingly the grouping and aspect of the heavens should be somewhat
different when seen from so different a point of view. An apparent
change of this sort is called generally parallax; _the_ parallax of a
star being technically defined as the angle subtended at the star by the
radius of the earth's orbit: that is to say, the angle E[sigma]S;
where E is the earth, S the sun, and [sigma] a star (Fig. 91).

Plainly, the further off [sigma] is, the more nearly parallel will
the two lines to it become. And the difficulty of determining the
parallax was just this, that the more accurately the observations were
made, the more nearly parallel did those lines become. The angle was, in
fact, just as likely to turn out negative as positive--an absurd result,
of course, to be attributed to unavoidable very minute inaccuracies.

For a long time absolute methods of determining parallax were attempted;
for instance, by observing the position of the star with respect to the
zenith at different seasons of the year. And many of these
determinations appeared to result in success. Hooke fancied he had
measured a parallax for Vega in this way, amounting to 30" of arc.
Flamsteed obtained 40" for [gamma] Draconis. Roemer made a serious
attempt by comparing observations of Vega and Sirius, stars almost the
antipodes of each other in the celestial vault; hoping to detect some
effect due to the size of the earth's orbit, which should apparently
displace them with the season of the year. All these fancied results
however, were shown to be spurious, and their real cause assigned, by
the great discovery of the aberration of light by Bradley.

After this discovery it was possible to watch for still outstanding very
minute discrepancies; and so the problem of stellar parallax was
attacked with fresh vigour by Piazzi, by Brinkley, and by Struve. But
when results were obtained, they were traced after long discussion to
age and gradual wear of the instrument, or to some other minute
inaccuracy. The more carefully the observation was made, the more nearly
zero became the parallax--the more nearly infinite the distance of the
stars. The brightest stars were the ones commonly chosen for the
investigation, and Vega was a favourite, because, going near the zenith,
it was far removed f