Opposition (planets)

In positional astronomy, two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place (usually the Earth).

A planet (or asteroid or comet) is said to be "in opposition" when it is in opposition to the Sun. Since most orbits in the Solar System are nearly coplanar, this occurs when the Sun, Earth, and the body are approximately in a straight line, that is, the Earth and the body are in the same direction as seen from the Sun. The instant of opposition is defined as that when the apparent geocentric celestial longitude of the body differs by 180° from the apparent geocentric longitude of the Sun. At that time, a body is

• visible almost all night, rising around sunset, culminating around midnight and setting around sunrise

• at the point in its orbit where it is roughly closest to the Earth, making it appear bigger and brighter
• in apparent retrograde motion
• nearly completely illuminated; we see a "full planet", analogous to a full Moon
• at the place where the opposition effect increases the reflected light from bodies with unobscured rough surfaces

Opposition occurs only in superior planets (see the diagram).

The Moon, which orbits the Earth rather than the Sun, is in opposition to the Sun at full moon. When it is exact in opposition, a lunar eclipse occurs.

The astronomical symbol for opposition is ☍ (U+260D). Handwritten:

As seen from a planet that is superior, an inferior planet on the opposite side of the Sun is in superior conjunction with the Sun. An inferior conjunction occurs when the two planets lie in a line on the same side of the Sun. At inferior conjunction, the superior planet is "in opposition" to the Sun as seen from the inferior planet (see the diagram).