We must add that the axis of the Earth is a straight line that is supposed to pass through the center of the globe and come out at two diametrically opposite points called the poles. The diurnal rotation of the Earth is effected round this axis.
The name equator is given to a great circle situated between the two poles, at equal distance, which divides the globe into two hemispheres. The equator is divided into 360 parts or degrees, by other circles that go from one pole to the other. These are the longitudes or meridians (see Fig. 62). The distance between the equator and the pole is divided into larger or smaller circles, which have received the name of latitudes, 90 degrees are reckoned on the one side and the other of the equator, in the direction of the North and South poles, respectively. The longitudes are reckoned from some point either to East or West: the latitudes are reckoned North and South, from the equator. In going from East to West, or inversely, the longitude changes, but in passing from North to South of any spot, it is the latitude that alters.
Fig. 62.—The divisions of the globe. Longitudes 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° 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.