Page:Astronomy for Everybody.djvu/177

A little consideration will show that a superior planet can never be in inferior conjuction, but an inferior planet has both kinds of conjunction.

A planet is said to be in opposition when it is in the opposite direction from the sun. It then rises at sunset, and vice versa. Of course, an inferior planet can never be in opposition.

The perihelion of an orbit is that point of it which is nearest the sun; the aphelion its most distant point from the sun.

As the inferior planets, Mercury and Venus, perform their revolutions they seem to us to swing from one side of the sun to the other. Their apparent distance from the sun at any time is called their elongation.

The greatest elongation of Mercury is generally about twenty-five degrees, being sometimes more and sometimes less, owing to the great eccentricity of the orbit of this planet. The greatest elongation of Venus is almost forty-five degrees.

When the elongation of one of these planets is east from the sun we may see it in the west after sunset; when west we may see it in the east in the morning sky. As neither of them ever wanders from the sun farther than the distances we have stated, it follows that a planet seen in the east in the evening, or in the west in the morning, cannot be either Mercury or Venus.

No two orbits of the planets lie exactly in the same plane. That is, if we regard any one orbit as horizontal, all the others will be tipped by small amounts toward one side or the other. Astronomers find it convenient to take