Definition
A planet is at opposition when it lies on the opposite side of the Earth from the Sun. Meeus uses the precise definition: the instant when the planet's geocentric ecliptic longitude differs from the Sun's by exactly 180°. Seen from above the solar system, the Sun, the Earth and the planet are then nearly in a line, with the Earth in the middle.
Only the superior planets, those farther from the Sun than the Earth (Mars, Jupiter, Saturn, Uranus and Neptune), can come to opposition, together with asteroids and dwarf planets. Mercury and Venus always stay on the Sun's side of the sky; they have inferior and superior conjunctions instead. The full Moon is, in the same sense, the Moon at opposition.
Why opposition is the best time to observe
- The planet rises at sunset, culminates around local midnight and sets at sunrise: it is visible all night.
- It is at or near its smallest distance from the Earth, so its disc looks largest in a telescope.
- It is at its brightest. Saturn gets an extra boost from the opposition effect: its rings brighten sharply when the Sun is directly behind the observer.
- Its retrograde loop is centred on opposition: for a few weeks before and after, the planet moves westward against the stars, because the Earth is overtaking it on the inside track.
How often: the synodic period
Oppositions of a planet repeat after its synodic period S, the time the Earth needs to gain one full lap on it. With the sidereal periods of the Earth, E, and of the planet, P:
1/S = 1/E − 1/P| Planet | Sidereal period | Synodic period | Next opposition |
|---|---|---|---|
| Mars | 687.0 days | 779.9 days | 2 years 50 days later |
| Jupiter | 11.86 years | 398.9 days | 1 year 34 days later |
| Saturn | 29.46 years | 378.1 days | 1 year 13 days later |
| Uranus | 84.0 years | 369.7 days | 1 year 4 days later |
| Neptune | 164.8 years | 367.5 days | 1 year 2 days later |
The farther the planet, the more slowly it moves, and the closer its synodic period comes to one year. Mars is the exception among the bright planets: it moves fast enough that the Earth needs more than two years to catch up with it.
Mars: perihelic and aphelic oppositions
Because the orbit of Mars is eccentric (e = 0.093), not all its oppositions are equal. When opposition happens near the perihelion of Mars, in late August or September, Mars comes within about 56 million km and shows a disc of about 25″ at magnitude −2.9. On 28 August 2003 it came within 55.8 million km, the closest in almost 60,000 years. When opposition falls near aphelion, in February or March, the distance is about 100 million km and the disc only about 14″. The cycle of good and poor oppositions repeats roughly every 15 to 17 years.
For the same reason the closest approach and the opposition are not always on the same day: in 2003 Mars was closest on 27 August, a day before opposition. The orbit's eccentricity and its 1.85° inclination to the ecliptic shift the minimum distance by up to about a week.
Next oppositions
| Planet | Date | Note |
|---|---|---|
| Jupiter | 11 February 2027 | |
| Mars | 19 February 2027 | aphelic, about 101 million km |
| Saturn | 18 October 2027 | |
| Jupiter | 12 March 2028 | |
| Mars | 25 March 2029 | about 97 million km |
| Mars | 4 May 2031 | about 83 million km |
| Mars | 27 June 2033 | about 63 million km |
| Mars | 15 September 2035 | perihelic, about 57 million km |
Astronomical Algorithms gives a dedicated method for these instants: a mean date from the planet's mean motions, corrected by periodic terms. In the heliocentric view on this site, set one of these dates and the Sun, the Earth and the planet line up.
Opposition and the trip to Mars
Missions to Mars do not leave at opposition, when the planet is closest, but about three months earlier, while Mars is still some 44° ahead of the Earth. The spacecraft then coasts for about 260 days on a transfer ellipse and meets Mars after opposition, when the planet has caught up with the meeting point. The launch windows therefore repeat with the synodic period, every 26 months, just like the oppositions.
Explore it on AstroAlgos
Frequently asked questions
When is the next Mars opposition?
On 19 February 2027. It is an aphelic opposition, with Mars about 101 million km away. The next very favourable one, a perihelic opposition with Mars about 57 million km away, will be on 15 September 2035.
Can Venus or Mercury be at opposition?
No. Their orbits are inside the Earth's, so they can never appear opposite the Sun in the sky. Venus never strays more than about 47° from the Sun, and Mercury about 28°.
Is a planet closest to Earth exactly at opposition?
Nearly, but not always on the same day. Because orbits are eccentric and slightly tilted, the minimum distance can occur a few days before or after opposition. For Jupiter and Saturn the difference is small; for Mars it can reach about a week.
