Elongation is the angular distance between the sun, and another object such a moon or a planet as seen from earth. There are several special names for these angular distances. The different names of these angles depend on the status, inferior or superior, of the planet. The planets closer to the sun than the earth are called inferior planets. The planets farther away from the sun than earth are called superior planets. Elongation is measured from earth as the angle between the sun and the planet. Sometimes the apparent relative position of a planet in relation to the sun is called the aspect, or configuration, of a planet.
Conjunction is the alignment of two solar system objects so that, seen from earth, they have the same longitude. For planets, this alignment is with the sun and a planet. A superior planet is said to be in conjunction when it is on the other side of the sun as seen from earth. An inferior planet can be in superior or inferior conjunction. Superior conjunction occurs when the planet is on the other side of the sun as seen from earth. Inferior conjunction occurs when planet is between the sun and the earth. The moon is at conjunction when it is between the earth and the sun.
Opposition can only occur for the moon or superior planets. This occurs when the earth is between the sun and the moon or planet. This is the most favorable time to observe superior planets because the planet is visible all through the night and it crosses the celestial meridian at about midnight (solar time).
Inferior planets are never very far from the sun as seen from earth. When the planet seems to follow the sun, as seen from earth, appearing east of the sun in the evening, the planet is in eastern elongation. When the planet seems to precede the sun, being west of the sun in the morning, it is in western elongation. The two most favorable times to observe an inferior planet are in its eastern or western greatest elongation, the two times when the planet appears farthest away from the sun as seen from earth.
Quadrature is when the angle from the sun to an object is a right angle. This can only occur for superior planets or the moon. A planet can be in either eastern or western quadrature. When a planet is in western quadrature, it is overhead at sunrise. When a planet is in eastern quadrature, it is overhead at sunset.
The types of elongation are shown in the following image.
|Figure 1: The
different configurations of the planets. The green planet
is earth, the red planet is any superior planet, the
light blue planet is any inferior planet. A list of the
abbreviations used in the image follows:
The time between two successive returns of a planet to the same elongation is known is an objects synodic period.
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