Mercury and Venus are called inferior planets as they are closer to the sun than the earth, and can never be seen opposite the sun in the sky. The phenomena associated with them are:

Inferior conjuction - The planet passes between the earth and the sun, so is invisible, except in very rare cases when it passes in transit across the sun.

Superior conjuction - The planet is behind the sun and is invisible.

Greatest elongation east - The planet is at its greatest angular separation east from the sun. At an eastern elongation the planet sets after the sun so is an evening star, and at greatest elongation it is generally at its best in the evening sky. The most favorable eastern elongations occur in spring.

Greatest elongation west - The planet is at its greatest angular separation west from the sun. At a western elongation the planet rises before the sun so is a morning star, and at greatest elongation it is generally at its best in the morning sky. The most favorable western elongations occur in the fall.

Mars and the other planets outside the earth's orbit are called superior planets. The phenomena associated with them are:

Conjuction - As with a superior conjunction of Mercury and Venus, the planet is behind the sun and is invisible. However, as inferior conjuctions are impossible, the word "superior" is dropped.

Opposition - The planet is directly opposite the sun in the sky, so can be seen all night. At or near opposition the planet is also closest and brightest, so is the best time to observe.

Quadrature - The planet is either 90° east of the sun (eastern quadrature) which occurs a few months after opposition and is an evening object, or 90° west of the sun (western quadrature) which occurs a few months before opposition and is a morning object. A superior planet always shows a more or less full phase, at quadrature the greatest departure from full phase occurs, and in the case of Mars a very appreciable gibbous phase can be seen. For the other superior planets this effect is much smaller.

All planets, including the earth, move in elliptical orbits around the sun, so their distances from the sun vary. In the case of the earth this effect is fairly small, and is swamped by the tilt of the axis which governs the seasons. We have the following two terms:

Perihelion - The point in the planet's orbit when it is closest to the sun, (from the Greek for near and sun).

Aphelion - The point in the planet's orbit when it is furthest from the sun, (from the Greek for far and sun).

In theory for all planets with an appreciable tilt of the rotation axis relative to the normal of its orbital plane, we have the following four terms, which, however, are normally used only for the earth and occasionally for Mars, and govern the seasons. In the northern hemisphere:

Summer solstice - The planet's axis is tilted at its most towards the sun, and marks the longest day and the beginning of summer, (in the soutern hemisphere this is the winter solstice).

Winter solstice - The planet's axis is tilted at its most away from the sun, and marks the shortest day and the beginning of winter, (in the southern hemisphere this is the summer solstice).

Vernal equinox - The sun is on the equator moving north, and marks the beginning of spring, (in the southern hemisphere this is the beginning of autumn).

Autumnal equinox - The sun is on the equator moving south, and marks the beginning of autumn, (in the sothern hemisphere this is the beginning of spring). At both equinoxes the day is the same length (12 hours on the earth) everywhere except at the poles.

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