An astronomical body is said to be at opposition when it makes its closest approach to the point directly opposite to the Sun in the night sky.
This means that the object will appear highest in the sky at around midnight, local time, and will be above the horizon for much of the night.
For objects which orbit further out in the Solar System than the Earth – almost all bodies other than Mercury and Venus – this configuration happens when the Solar System is aligned such that the object lies in a straight line with the Earth and the Sun, the Earth being in the middle.
Consequently, this is also the time when the object makes its closest approach to the Earth, making it appear at its largest and brightest in the night sky.
List of oppositions
The table below lists the dates when objects are at opposition in 2018, computed from NASA's DE405 planetary ephemeris. To show dates in other years, use the dropdown control.