in the Discovery--The Distinction of Mercury
from a Star--Mercury in the East and in the West--The
Prediction--How to Observe Mercury--Its Telescopic
Appearance--Difficulty of Observing its Appearance--Orbit of
Mercury--Velocity of the Planet--Can there be Life on the
Planet?--Changes in its Temperature--Transit of Mercury over the
Sun--Gassendi's Observations--Rotation of Mercury--The Weight of
Mercury.

Long and glorious is the record of astronomical discovery. The
discoveries of modern days have succeeded each other with such rapidity,
they have so often dazzled our imaginations with their brilliancy, that
we are sometimes apt to think that astronomical discovery is a purely
modern product. But no idea could be more fundamentally wrong. While we
appreciate to the utmost the achievements of modern times, let us
endeavour to do justice to the labours of the astronomers of antiquity.

And when we speak of the astronomers of antiquity, let us understand
clearly what is meant. The science is now growing so rapidly that each
century witnesses a surprising advance; each generation, each decade,
each year, has its own rewards for those diligent astronomers by whom
the heavens are so carefully scanned. We must, however, project our
glance to a remote epoch in time past, if we would view the memorable
discovery of Mercury. Compared with it, the discoveries of Newton are to
be regarded as very modern achievements; even the announcement of the
Copernican system of the heavens is itself a recent event in comparison
with the detection of this planet now to be discussed.

By whom was this great discovery made? Let us see if the question can
be answered by the examination of astronomical records. At the close of
his memorable life Copernicus was heard to express his sincere regret
that he never enjoyed an opportunity of beholding the planet Mercury. He
had specially longed to see this body, the movements of which were to
such a marked extent illustrative of the theory of the celestial motions
which it was his immortal glory to have established, but he had never
been successful. Mercury is not generally to be seen so easily as are
some of the other planets, and it may well have been that the vapours
from the immense lagoon at the mouth of the Vistula obscured the horizon
at Frauenburg, where Copernicus dwelt, and thus his opportunities of
viewing Mercury were probably even rarer than they are at