itudes and
latitudes.]

The circles of latitude are smaller in proportion as one approaches the
poles. The circumference of the world is 40,076,600 meters at the
equator. At the latitude of Paris (48 deg. 50') it is only 26,431,900
meters. A point situated at the equator has more ground to travel over
in order to accomplish its rotation in twenty-four hours than a point
nearer the pole.

We have already stated that this velocity of rotation is 465 meters per
second at the equator. At the latitude of Paris it is not more than 305
meters. At the poles it is _nil_.

The longitudes, or meridians, are great circles of equal length,
dividing the Earth into quarters, like the parts of an orange or a
melon. These circumvent the globe, and measure some 40,000,000
(40,008,032) meters. We may remember in passing that the length of the
meter has been determined as, by definition, the ten-millionth part of
the quarter of a celestial meridian.

Thus, while rotating upon itself, the Earth spins round the Sun, along a
vast orbit traced at 149,000,000 kilometers (93,000,000 miles) from the
central focus, a sensibly elliptical orbit, as we have already pointed
out. It is a little nearer the Sun on January 1st than on July 1st, at
its perihelion (_peri_, near, _helios_, Sun), than at its aphelion
(_apo_, far, _helios_, Sun). The difference = 6,000,000 kilometers
(3,720,000 miles), and its velocity is a little greater at perihelion
than at aphelion.

This second motion produces the _year_. It is accomplished in three
hundred and sixty-five days, six hours, nine minutes, nine seconds.
Such is the complete revolution of our planet round the orb of day. It
has received the name of sidereal year. But this is not how we calculate
the year in practical life. The civil year, known also as the tropical
year, is not equivalent to the Earth's revolution, because a very slow
gyratory motion, called "the precession of the equinoxes," the cycle of
which occupies 25,765 years, drags the spring equinox back some twenty
minutes in each year.

The civil year is, accordingly, three hundred and sixty-five days, five
hours, forty-eight minutes, forty-six seconds.

In order to simplify the calendar, this accumulating fraction of five
hours, forty-eight minutes, forty-six seconds (about a quarter day) is
added every four years to a bissextile year (leap-year), and thus we
have uneven years of three hundred and sixty-five, and three hundred and
sixty-s