to travel two days longer to recover its
position between the Sun and the Earth, so that the lunar month is
longer than the sidereal revolution of the Moon, and takes twenty-nine
days, twelve hours, forty-four minutes, three seconds. This is the
duration of the sequence of phases.

This revolution is accomplished at a distance of 384,000 kilometers
(238,000 miles). The velocity of the Moon in its orbit is more than 1
kilometer (0.6214 mile) per second. But our planet sweeps it through
space at a velocity almost thirty times greater.

The diameter of the Moon represents 273/1000 that of the Earth, _i.e._,
3,480 kilometers (2,157 miles).

Its surface = 38,000,000 square kilometers (15,000,000 square miles), a
little more than the thirteenth part of the terrestrial surface, which
= 510,000,000 (200,000,000 square miles).

In volume, the Moon is fifty times less than the Earth. Its mass or
weight is only 1/81 that of the terrestrial globe. Its density = 0.615,
relatively to that of the Earth, _i.e._, a little more than three times
that of water. Weight at its surface is very little: 0.174. A kilogram
transported thither would only weigh 174 grams.

* * * * *

At the meager distance of 384,000 kilometers (238,000 miles) that
separates us from it (about thirty times the diameter of the Earth), the
Moon is a suburb of our terrestrial habitation. What does this small
distance amount to? It is a mere step in the universe.

A telegraphic message would get there in one and a half second; a
projectile fired from a gun would arrive in eight days, five hours; an
express-train would be due in eight months, twenty-two days. It is only
the 1/388 part of the distance that separates us from the Sun, and only
the 100/1,000,000 part of the distance of the stars nearest to us. Many
men have tramped the distance that separates us from the Moon. A bridge
of thirty terrestrial globes would suffice to unite the two worlds.

Owing to this great proximity, the Moon is the best known of all the
celestial spheres. Its geographical (or more correctly,
selenographical, _Selene_, moon) map was drawn out more than two
centuries ago, at first in a vague sketch, and afterward with more
details, until to-day it is as precise and accurate as any of our
terrestrial maps of geography.

Before the invention of the telescope, from antiquity to the seventeenth
century, people lost themselves in conjectures as to the natu