A conjunction is when two celestial bodies within our solar system line up so that have the same right ascension (see equatorial coordinates), as viewed from Earth.

Conjunctions are a line-of-site effect, and the exact time of a conjunction will depend on the observer’s location on Earth, due to the effect of parallax.

Note that, for a conjunction to occur, it is not necessary for the declination coordinate to be aligned.

A celestial object that lies within the boundary of the Earth’s orbit (e.g. Mercury or Venus) can be said to be either in superior or inferior conjunction when it is aligned with the right ascension of the Sun, depending on whether it lies on the near or the far side of the Sun from the Earth, at the time of conjunction. Note that the Earth would appear to be in opposition to the Sun, as viewed from a planet that we consider to be positioned in inferior conjunction to the Sun.

Both a conjunction and an opposition can also be referred to as a syzygy.