h enough to enable it to be seen
distinctly. The observer and the observatory are then to be transferred
to the other side of the earth. How is this to be done? Say, rather, how
we could prevent it from being done. Is not the earth rotating on its
axis, so that in the course of a few hours the observatory on the
equator is carried bodily round for thousands of miles? As the morning
approaches the observations are to be repeated. The planet is found to
have changed its place very considerably with regard to the stars. This
is partly due to its own motion, but it is also largely due to the
parallactic displacement arising from the rotation of the earth, which
may amount to so much as twenty seconds. The measures on a single night
with the heliometer should not have a mean error greater than one-fifth
of a second, and we might reasonably expect that observations could be
secured on about twenty-five nights during the opposition. Four such
groups might be expected to give the sun's distance without any
uncertainty greater than the thousandth part of the total amount. The
chief difficulty of the process arises from the movement of the planet
during the interval which divides the evening from the morning
observations. This drawback can be avoided by diligent and repeated
measurements of the place of the planet with respect to the stars among
which it passes.

In the monumental piece of work which issued in 1897 from the Cape
Observatory, under the direction of Dr. Gill, the final results from the
observations of Iris, Victoria, and Sappho have been obtained. From this
it appears that the angle which the earth's equatorial radius subtends
at the centre of the sun when at its mean distance has the value
8".802. If we employ the best value of the earth's equatorial radius we
obtain 92,870,000 miles as the mean distance of the centre of the sun
from the centre of the earth. This is probably the most accurate
determination of the scale of the solar system which has yet been made.

CHAPTER XII.

JUPITER.

The Great Size of Jupiter--Comparison of his Diameter with that of
the Earth--Dimensions of the Planet and his Orbit--His
Rotation--Comparison of his Weight and Bulk with that of the
Earth--Relative Lightness of Jupiter--How Explained--Jupiter still
probably in a Heated Condition--The Belts on Jupiter--Spots on his
Surface--Time of Rotation of different Spots various--Storms on
Jupit