Oppositions, conjunctions, quadratures, elongations

access to the tables of dates of oppositions and elongations

These words correspond to specific configurations of celestial bodies, selected because they allow an easy observation of a planet or because they mean that no observation is possible.

Definitions :

Opposition:
Opposition of a planet with another celestial body: phenomenon for which the celestial geocentric longitudes of the planet and of the other body differ from 180° . An opposition may concern two planets but it is mainly the opposition of a planet and the Sun which is looked for because such configuration corresponds to favorable conditions for the observation of the planet.
Opposition of a planet with the Sun: this configuration concerns only the superior planets Mars, Jupiter, Saturn, Uranus, Neptune. It is at the time of the opposition that the distance between the planet and the Earth is minimum. At the time of the opposition, the planet rises about at the same time that the Sun sets.

Figure n°1: opposition, conjunction, quadrature

Conjunction:
Conjunction between a planet and one or several celestial bodies: phenomenon corresponding to the time when two or more celestial bodies have the same celestial geocentric longitudes, that means that these bodies are very close in the sky.
Conjunction of a superior planet with the Sun : the celestial geocentric longitudes (or right ascensions) of the planet and the Sun are equal. Conjunction of Mercury or Venus with the Sun : the celestial geocentric longitudes (or right ascensions) of the planet and the Sun are equal, the conjunction is superior or inferior depending that the Sun is between the Earth and the planet or that the planet is between the Earth and the Sun.
Conjunction of two planets, of a planet with the Moon or with a star: the right ascensions of the two bodies are considered to be equal. The conjunction between a planet and the Moon is often spectacular when the Moon occults the planet.

Quadrature:
Configuration concerning the superior planets only:
Quadrature of a planet superior with the Sun: configuration for which the celestial geocentric longitudes of the planet and the Sun differ from 90°.

Figure n°2: elongations

Elongation:
Configuration concerning the inferior planets only:
Maximum elongation: it is the configuration for which the angular apparent distance between the planet and the Sun is the largest. The inferior planets may never be in opposition with the Sun and the maximum elongation is the configuration allowing the best condition for observing the planet. When Mercury and Venus are close to their largest "East" elongation, they are visible after Sunset. Similarly, when they are close to their largest "West" elongation, they are visible before Sunrise.
For the superior planets of the Solar System, the maximum elongation corresponds to an opposition, i.e. to an elongation of 180°.

Duration between two oppositions (example of Mars):
The mean angular velocity (sidereal) of the Earth is n1 = 6.28307585 radians/year and the one of Mars n2 = 3.34061243 radians/year. If we start at a given opposition, the next one will occur when the Earth gains one revolution to Mars, i.e.
n1*t = n2*t + 2 pi
so that, after (in years)
t1 = 2 pi/(n1-n2)
i.e., in number of revolution for the Earth (or number of years)
nopp = t1/T1=n1/(n1-n2) = 2.135311456 revolutions or years
So it is necessary to wait 2 years and 49 days to have a new opposition.

Figure n°3: recent oppositions of Mars: oppositions occurring near the perihelion of Mars
are the most favorable. Note that the aphelion of the Earth is not far that make such opposition more favorable.

Specific case of Mars:
the planet Mars has its closest approach of the Earth at the time of the opposition but this distance varies for each opposition because of the ellipticity of the orbits of the Earth and Mars. This minimum distance at opposition may vary from one to two and, some oppositions are much more favorable to the observation of Mars.
The oppositions of Mars at the present time are as follows:

Dates	Distance to the Earth in AU	Apparent diameter in arcsec
13 June 2001	0.4556	21"
28 August 2003	0.3728	25"
7 November 2005	0.4700	21"
24 December 2007	0.5929	17"
29 January 2010	0.6644	14"
3 March 2012	0.6741	14"
8 April 2014	0.6209	15"
22 May 2016	0.5094	19"
27 July 2018	0.3862	24"
13 October 2020	0.4192	23"
8 December 2022	0.5496	18"
16 January 2025	0.6437	15"
19 February 2027	0.6779	14"
25 March 2029	0.6491	15"

The moire favorable oppositions of Mars:

   date              distance Earth-Mars      apparent diameter
                     (au)     (millions of km)   of Mars
25 August 1719       0.374010       55.951          25.05"
13 August 1766       0.373260       55.839          25.10"
18 August 1845       0.373021       55.803          25.11"
22 August 1924       0.372846       55.777          25.12"
27 August 2003       0.372719       55.758          25.13"
15 August 2050       0.374051       55.957          25.04"
30 August 2082       0.373564       55.884          25.08"
19 August 2129       0.373276       55.841          25.10"
24 August 2208       0.372794       55.769          25.13"
28 August 2287       0.372254       55.688          25.16"

All best oppositions occur in August: it is the time when the Earth is in the direction of the perihelion of Mars. If Mars is there, then, the distance Mars-Sun is minimum so thet the distance Mars-Earth is also minimum.

access to the tables of dates of oppositions and elongations

Credit : J.E. Arlot, IMCCE/observatoire de Paris