If, on the contrary, it is displaced,
it will in the year describe a minute ellipse, which is only the
reflection, the perspective in miniature, of the revolution of our
planet round the Sun.

The annual parallax of a star is the angle under which one would see the
radius, or half-diameter, of the terrestrial orbit from it. This radius
of 149,000,000 kilometers (93,000,000 miles) is indeed, as previously
observed, the unit, the meter of celestial measures. The angle is of
course smaller in proportion as the star is more distant, and the
apparent motion of the star diminishes in the same proportion. But the
stars are all so distant that their annual displacement of perspective
is almost imperceptible, and very exact instruments are required for its
detection.

[Illustration: FIG. 84.--Small apparent ellipses described by the stars
as a result of the annual displacement of the Earth.]

The researches of the astronomers have proved that there is not one star
for which the parallax is equal to that of another. The minuteness of
this angle, and the extraordinary difficulties experienced in measuring
the distance of the stars, will be appreciated from the fact that the
value of a second is so small that the displacement of any star
corresponding with it could be covered by a spider's thread.

A second of arc corresponds to the size of an object at a distance of
206,265 times its diameter; to a millimeter seen at 206 meters'
distance; to a hair, 1/10 of a millimeter in thickness, at 20 meters'
distance (more invisible to the naked eye). And yet this value is in
excess of those actually obtained. In fact:--the apparent displacement
of the nearest star is calculated at 75/100 of a second (0.75"), _i.e._,
from this star, [alpha] of Centaur, the half-diameter of the terrestrial
orbit is reduced to this infinitesimal dimension. Now in order that the
length of any straight line seen from the front be reduced until it
appear to subtend no more than an angle of 0.75", it must be removed to
a distance 275,000 times its length. As the radius of the terrestrial
orbit is 149,000,000 kilometers (93,000,000 miles), the distance which
separates [alpha] of Centaur from our world must therefore =
41,000,000,000,000 kilometers (25,000,000,000,000 miles). And that is
the nearest star. We saw in Chapter II that it shines in the southern
hemisphere. The next, and one that can be seen in our latitudes, is 61
of Cygnus, which floats in the Heave