t and
the stars surrounding it on the same plate, and then by careful
measurement determine its exact position among the stars; and since this
position differs slightly according to the situation of the observer on
the earth's surface, by comparing two photographs taken at stations a
known distance apart we can find the distance of the planet from the
earth; and hence, as above remarked, the distance of the sun and all the
other members of the solar system. Or, instead of taking photographs from
two different stations, we can take from the same station two photographs
at times separated by a known interval. For in that interval the station
will have been carried by the earth's rotation some thousands of miles
away from its former position, and becomes virtually a second station
separated from the first by a distance which is known accurately when we
know the elapsed time. Again, instead of taking photographs, and from them
measuring the position of the planet among the stars, we may make the
measurements on the planet and stars in the sky itself; and since in 1878,
when Dr. Gill set out on his enterprise of determining the sun's distance,
photography was in its infancy as applied to astronomy, he naturally made
his observations on the sky with an instrument known as a heliometer. He
made them in the little island of Ascension, which is suitably situated
for the purpose; because, being near the earth's equator, it is carried by
the earth's rotation a longer distance in a given time than places nearer
the poles, and in these observations for "parallax," as they are called,
it is important to have the displacement of the station as large as
possible. For a similar reason the object selected among the planets must
be as near the earth as possible; and hence the planet Mars, which at
favourable times comes nearer to us than any other superior planet[1] then
known, was selected for observation with the heliometer.

And now it will be seen why the discovery of the little planet Eros was
important, for Mars was no longer the known planet capable of coming
nearest to us; it had been replaced by this new arrival.

[Sidenote: Victoria, Iris, and Sappho.]

[Sidenote: Eros.]

Further, a small planet which is in appearance just like an ordinary star
has, irrespective of this great proximity, some distinct advantages over a
planet like Mars, which appears as a round disc, and is, moreover, of a
somewhat reddish colour. When the dis