By the aid of telescopes, we discern irregularities in the surface of
the moon which can be no other than mountains and valleys,--for this
plain reason, that we see the shadows cast by the former in the exact
proportion as to length which they ought to have when we take into
account the inclinations of the sun’s rays to that part of the moon’s
surface on which they stand. From micrometrical measurements of the
lengths of the shadows of the more conspicuous mountains, Messrs. Baer
and Maedler have given a list of heights for no less than 1095 lunar
mountains, among which occur all degrees of elevation up to 22,823
British feet, or about 1400 feet higher than Chimborazo in the Andes.
If Chimborazo were as high in proportion to the earth’s diameter as
a mountain in the moon known by the name of Newton is to the moon’s
diameter, its peak would be more than sixteen miles high.
Arago calls to mind, that with a 6000-fold magnifying power, which
nevertheless could not be applied to the moon with proportionate
results, the mountains upon the moon would appear to us just as Mont
Blanc does to the naked eye when seen from the Lake of Geneva.
We sometimes observe more than half the surface of the moon, the
eastern and northern edges being more visible at one time, and the
western or southern at another. By means of this libration we are
enabled to see the annular mountain Malapert (which occasionally
conceals the moon’s south pole), the arctic landscape round the crater
of Gioja, and the large gray plane near Endymion, which conceals in
superficial extent the _mare vaporum_.
Three-sevenths of the moon are entirely concealed from our observation;
and must always remain so, unless some new and unexpected disturbing
causes come into play.--_Humboldt._
The first object to which Galileo directed his telescope was the
mountainous parts of the moon, when he showed how their summits
might be measured: he found in the moon some circular districts
surrounded on all sides by mountains similar to the form of
Bohemia. The measurements of the mountains were made by the method
of the tangents of the solar ray. Galileo, as Helvetius did still
later, measured the distance of the summit of the mountains from
the boundary of the illuminated portion at the moment when the
mountain summit was first struck by the solar ray. Humboldt found
no observations of the lengths of the shadows of the mountains:
the summits were “much higher than the mountains on our earth.”
The comparison is remarkable, since, according to Riccioli,
very exaggerated ideas of the height of our mountains were then
entertained. Galileo like all other observers up to the close of
the eighteenth century, believed in the existence of many seas and
of a lunar atmosphere.