Page:Astronomy for Everybody.djvu/175

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Orbits and Aspects of the Planets

The orbits in which the planets revolve around their central luminary are in strictness ellipses, or slightly flattened circles. But the flattening is so slight that the eye would not notice it without measurement. The sun is not in the centre of the ellipse but in a focus, which in some cases is displaced from the centre by an amount that the eye can readily perceive. This displacement measures the eccentricity of the ellipse, which is much greater than the flattening. For example, in the case of Mercury, which moves in a very eccentric orbit, the flattening is only one fiftieth; that is, if we represent the greatest diameter of the orbit by fifty, the least diameter will be forty-nine. But the distance of the sun from the centre of the orbit is ten on the same scale.

To show this we give a diagram of the orbits of the inner group of planets showing quite nearly their forms and respective locations. A simple glance will show that the orbits are much nearer together at some points than at others.

In explaining the various aspects and motions, real and apparent, of the planets a number of technical expressions are used which we shall explain.

Inferior planets are those whose orbits lie within the

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