superiority of the heliometer had been proved, the results would be
still better with the points of light shown by minor planets rather
than with the disc of Mars.

In 1888-9, at the Cape, he observed the minor planets Iris, Victoria,
and Sappho, and secured the co-operation of four other heliometers.
His final result was 92,870,000 miles, the parallax being 8",802
(_Cape Obs_., Vol. VI.).

So delicate were these measures that Gill detected a minute periodic
error of theory of twenty-seven days, owing to a periodically
erroneous position of the centre of gravity of the earth and moon to
which the position of the observer was referred. This led him to
correct the mass of the moon, and to fix its ratio to the earth's mass
= 0.012240.

Another method of getting the distance from the sun is to measure the
velocity of the earth's orbital motion, giving the circumference
traversed in a year, and so the radius of the orbit. This has been
done by comparing observation and experiment. The aberration of light
is an angle 20" 48, giving the ratio of the earth's velocity to the
velocity of light. The velocity of light is 186,000 miles a second;
whence the distance to the sun is 92,780,000 miles. There seems,
however, to be some uncertainty about the true value of the
aberration, any determination of which is subject to irregularities
due to the "seasonal errors." The velocity of light was experimentally
found, in 1862, by Fizeau and Foucault, each using an independent
method. These methods have been developed, and new values found, by
Cornu, Michaelson, Newcomb, and the present writer.