Planetary aspects Opposition
Opposition
An outer planet’s best night of the year
Planet and Sun stand on opposite sides of Earth: visible all night, and at its brightest.
- Geometry
- Sun, Earth and planet nearly in line
- Visible for
- All night
- Jupiter oppositions
- About 13 months
01 / Overview
Meet Opposition
An outer planet is at opposition when its ecliptic longitude differs from the Sun’s by 180°, with Earth between the two. It then rises at sunset and sets at sunrise, stays up all night, and sits near its closest to Earth — so it looks brightest and shows its largest disc in a telescope.
The interval follows the synodic period
Jupiter comes to opposition about every 13 months, Mars about every 26. The difference is the gap in angular speed between Earth and the planet: the further out the planet, the closer to one year Earth needs to lap it.
Not every opposition is equally close
Mars has a markedly eccentric orbit: at a perihelic opposition it comes within about 0.37 AU, at an ordinary one more than 0.6 AU, nearly doubling the apparent diameter. Saturn’s brightness at opposition also depends on the tilt of its rings, so no two oppositions of the same planet are alike.
02 / Key data
Core parameters
- Geometry
- Sun, Earth and planet nearly in line
- Visible for
- All night
- Jupiter oppositions
- About 13 months
- Mars oppositions
- About 26 months
- Recurrence
- Once every one or two years for an outer planet
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- The planet rides highest around midnight, where the seeing is usually steadiest.
- Conditions stay close to best for weeks either side of opposition.
- Mercury and Venus orbit inside Earth’s, so they never come to opposition.
04 / Sources