es. No doubt
this seems an enormous distance, when estimated by any standard adapted
for terrestrial measurements; it is, however, hardly greater than the
distance of Venus when nearest, and it is much less than the distance
from the earth to the sun.

We have explained how the _form_ of the solar system is known from
Kepler's laws, and how the absolute size of the system and of its
various parts can be known when the direct measurement of any one part
has been accomplished. A close approach of Mars affords a favourable
opportunity for measuring his distance, and thus, in a different way,
solving the same problem as that investigated by the transit of Venus.
We are thus led a second time to a knowledge of the distance of the sun
and the distances of the planets generally, and to many other numerical
facts about the solar system.

On the occasion of the opposition of Mars in 1877 a successful attempt
was made to apply this refined process to the solution of the problem of
celestial measurement. It cannot be said to have been the first occasion
on which this method was suggested, or even practically attempted. The
observations of 1877 were, however, conducted with such skill and with
such minute attention to the necessary precautions as to render them an
important contribution to astronomy. Dr. David Gill, now her Majesty's
Astronomer at the Cape of Good Hope, undertook a journey to the Island
of Ascension for the purpose of observing the parallax of Mars in 1877.
On this occasion Mars approached to the earth so closely as to afford an
admirable opportunity for the application of the method. Dr. Gill
succeeded in obtaining a valuable series of measurements, and from them
he concluded the distance of the sun with an accuracy somewhat superior
to that attainable by the transit of Venus.

There is yet another method by which Mars can be made to give us
information as to the distance of the sun. This method is one of some
delicacy, and is interesting from its connection with the loftiest
enquiries in mathematical astronomy. It was foreshadowed in the
Dynamical theory of Newton, and was wrought to perfection by Le Verrier.
It is based upon the great law of gravitation, and is intimately
associated with the splendid discoveries in planetary perturbation which
form so striking a chapter in modern astronomical discovery.

There is a certain relation between two quantities which at first sight
seems quite independent. These qu