As the form of the Earth exerts a powerful influence on the motion
of other cosmical bodies, and especially on that of its neighbouring
satellite, a more perfect knowledge of the motion of the latter will
enable us reciprocally to draw an inference regarding the figure of
the earth. Thus, as Laplace ably remarks: “an astronomer, without
leaving his observatory, may, by a comparison of lunar theory with true
observations, not only be enabled to determine the form and size of
the earth, but also its distance from the sun and moon; results that
otherwise could only be arrived at by long and arduous expeditions
to the most remote parts of both hemispheres.” The compression which
may be inferred from lunar inequalities affords an advantage not
yielded by individual measurements of degrees or experiments with the
pendulum, since it gives a mean amount which is referable to the whole
planet.--_Humboldt’s Cosmos_, vol. i.
The distance of the moon from the earth is about 240,000 miles; and
if a railway-carriage were to travel at the rate of 1000 miles a-day,
it would be eight months in reaching the moon. But that is nothing
compared with the length of time it would occupy a locomotive to reach
the sun from the earth: if travelling at the rate of 1000 miles a-day,
it would require 260 years to reach it.