onds_. Written
short, the degree is indicated by a little zero (deg.) placed above the
figure; the minute by an apostrophe ('), and the second by two (").
These minutes and seconds of _arc_ have no relation with the same terms
as employed for the division of the duration of time. These latter ought
never to be written with the signs of abbreviation just indicated,
though journalists nowadays set a somewhat pedantic example, by writing,
_e.g._, for an automobile race, 4h. 18' 30", instead of 4h. 18m. 30s.

This makes clear the distinction between the relative measure of an
angle and the absolute measures, such, for instance, as the meter. Thus,
a degree may be measured on this page, while a second (the 3,600th part
of a degree) measured in the sky may correspond to millions of
kilometers.

Now the measure of the Moon's diameter gives us an angle of a little
more than half a degree. If it were exactly half a degree, we should
know by that that it was 114 times the breadth of its disk away from us.
But it is a little less, since we have more than half a degree (31'),
and the geometric ratio tells us that the distance of our satellite is
110 times its diameter.

Hence we have very simply obtained a first idea of the distance of the
Moon by the measure of its diameter. Nothing could be simpler than this
method. The first step is made. Let us continue.

This approximation tells us nothing as yet of the real distance of the
orb of night. In order to know this distance in miles, we need to know
the width in miles of the lunar disk.

[Illustration: FIG. 81.--Division of the Circumference into 360
degrees.]

This problem has been solved, as follows:

Two observers go as far as possible from each other, and observe the
Moon simultaneously, from two stations situated on the same meridian,
but having a wide difference of latitude. The distance that separates
the two points of observation forms the base of a triangle, of which the
two long sides come together on the Moon.

[Illustration: FIG. 82.--Measurement of the distance of the Moon.]

It is by this proceeding that the distance of our satellite was finally
established, in 1751 and 1752, by two French astronomers, Lalande and
Lacaille; the former observing at Berlin, the latter at the Cape of Good
Hope. The result of their combined observations showed that the angle
formed at the center of the lunar disk by the half-diameter of the Earth
is 57 minutes of arc (a little l