|Semi-major axis (AU)||1.00|
|Orbital Period (Years)||1.00|
|Inclination to Sun's equator (°)||7.25|
|Rotation Period (Days)||1.00|
|Axial Tilt (°)||23.44|
|Atmosphere||N2, O2, Ar|
Our grand celestial journey lands us upon our own little planet, on this globe that gravitates between Mars and Venus (between War and Love), circulating like her brothers of the solar system, around the colossal Sun. The Earth! The name evokes in us the image of Life, and calls up the theater of our activities, our ambitions, our joys and sorrows. Does it not, in fact, to ignorant eyes, represent the whole of the universe? And yet, what is the Earth?
The Earth is a star in the Heavens. We learned this much in our first lesson. It is a globe of opaque material, similar to the planets Mercury, Venus, Mars, Jupiter, etc., as previously described. Isolated on all sides in space, it revolves round the Sun, along a vast orbit that it accomplishes in a year. And while it thus glides along the lines of solar attraction, the terrestrial ball rotates rapidly upon itself in twenty-four hours. These statements may appear dubious at first sight, and contradictory to the evidence of our senses.
Now that the surface of the Earth has been explored in all directions, there is no longer room to doubt that it is a globe, a sort of ball that we adhere to. A journey round the world is common enough to-day, and always yields the most complete evidence of the spherical nature of the Earth. On the other hand, the curvature of the seas is a no less certain proof. When a ship reaches the dark-blue line that appears to separate the sky from the ocean, it seems to be hanging on the horizon. Little by little, however, as it recedes, it drops below the horizon line; the tops of the masts being the last to disappear. The observer on board ship witnesses the same phenomenon. The low shores are first to disappear, while the high coasts and mountains are much longer visible.
The aspect of the Heavens gives another proof of the Earth's rotundity. As one travels North or South, new stars rise higher and higher above the horizon in the one direction or the other, and those which shine in the latitude one is leaving, gradually disappear. If the surface of the Earth were flat, the ships on the sea would be visible as long as our sight could pierce the distance, and all the stars of the Heavens would be equally visible from the different quarters of the world.
Lastly, during the eclipses of the Moon, the shadow projected by the Earth upon our satellite is always round. This is another proof of the spherical nature of the terrestrial globe.
We described the Earth as an orb in the Heavens, similar to all the other planets of the great solar family. We see these sister planets of our world circulating under the starry vault, like luminous points whose brilliancy is sometimes dazzling. For us they are marvelous celestial birds hovering in the ether, upheld by invisible wings. The Earth is just the same. It is supported by nothing. Like the soap-bubble that assumes a lovely iridescence in the rays of the Sun, or, better, like the balloon rapidly cleaving the air, it is isolated from every kind of support.