ow much was learned from the transits of Venus, and how much
remained to be settled. They left the result in doubt over a range of
about two million miles.

SUN'S DISTANCE, IN MILLIONS OF MILES, AS FOUND BY DIFFERENT OBSERVERS

=Before the Transits of Venus= estimates varied between =96= million
miles (Gilliss and Gould, 1856) and =91= million (Winneche, 1863), a
range of 5 million miles.

=The Transits of 1874 and 1882= gave results lying between =93-1/4=
million (Airy, from British observations of 1874), =92-1/2= million
(Stone, from British observations of 1882), and =91-1/2= million
(Puiseux, from French observations), a range of 1-3/4 millions.

=Gill's Heliometer results= all lie very near =93= millions. The
observations of Mars in 1877 give about 100,000 miles over this
figure: but the observations of Victoria, Iris, and Sappho, which are
more trustworthy, all agree in giving about 100,000 miles _less_ than
the 93 millions.

It became necessary, therefore, to look to other methods; and before the
second transit of 1882 was observed, an energetic astronomer, Dr. David
Gill, had already put into operation the method which may be now regarded
as the standard one.

[Sidenote: Modern method for sun's distance.]

[Sidenote: Photography.]

[Sidenote: Dr. Gill's expedition to Ascension.]

We have said that the _relative_ distance of Venus from the sun is
accurately known from observations of the exact time of revolution. It is
easy to see that these times of revolution can be measured accurately by
mere accumulation. We may make an error of a few seconds in noting the
time of return; but if the whole interval comprises 10 revolutions, this
error is divided by 10, if 100 revolutions by 100, and so on; and by this
time a great number of revolutions of all the planets (except those just
discovered) have been recorded. Hence we know their relative distances
with great precision; and if we can find the distance in miles of any one
of them, we can find that of the sun itself, or of any other planet, by a
simple rule-of-three sum. By making use of this principle many of the
difficulties attending the direct determination of the sun's distance can
be avoided; for instance, since the sun's light overpowers that of the
stars, it is not easy to directly observe the place of the sun among the
stars; but this is not so for the planets. We can photograph a plane