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). In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°.

The astronomical symbol for opposition is ☍ (Unicode #x260d). Handwritten:

A planet (or asteroid or comet) is said to be "in opposition" when it is in opposition to the Sun, as seen from the Earth. When a planet is like this,

• it is visible almost all night, rising around sunset, culminating around midnight and setting around sunrise;
• at this point of its orbit it is roughly closest to the Earth, making it appear bigger and brighter.
• the half of the planet visible from Earth is then completely illuminated ("full planet")
• the opposition effect increases the reflected light from bodies with unobscured rough surfaces.

Opposition occurs only in superior planets.

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.

Superior and inferior

As seen from a planet that is superior, if an inferior planet is on the opposite side of the Sun, it 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. In an inferior conjunction, the superior planet is "in opposition" to the Sun as seen from the inferior planet.

See also

• Astronomical conjunction
• Astrological aspects
• Positional astronomy
• Syzygy