Opposition (astronomy)

Opposition is the aspect or configuration of two bodies that are, in relation to the Earth, at two diametrically opposite points in the sky. Two stars with geocentric celestial longitude that differs by 180º.

Only the outer planets and the Moon can be found in opposition to the Sun. Earth lies between the Sun and the planet. When it happens, the planet passes through the meridian of the place at midnight. The planet is visible throughout the night and occupies its closest position to Earth, so its diameter is the largest possible and the conditions for telescopic observation are ideal.

From the point of view of telescopic observation, if the opposition takes place near the perihelion of the planet (perihelic oppositions) the distance from the planet to the Earth is minimal and the observation is very favorable. Conversely, if the opposition occurs close to aphelion, it is very unfavorable.

Lunar oppositions occur on a full moon. If the Moon is close to the nodes of its orbit, a lunar eclipse will occur.

The oppositions are repeated every synodic period of the planet. By visual observation, the synodic periods of Mars, Jupiter and Saturn have been known since the most remote antiquity. Those of Uranus and Neptune, planets discovered more recently through the use of the telescope, are of more modern knowledge:

Planet	Mars	Jupiter	Saturn	Uranus	Neptune
Synodic period	780 days	399 days	378 days	370 days	367 days

As the translation movement of the Earth and the planets around the Sun is not uniform but rather follows Kepler's Law of areas, the time elapsed between two oppositions varies. If the mean value is calculated, the previous value is found.

The table of oppositions of the planet Jupiter during one turn of the planet in its orbit:

Datejuliana	Date	TU (h)	Length Heliocentric		Synodic period (N)
2449076,99	30 March 1993	12	189°	47'	395.47
2449472,86	30 April 1994	9	219°	47	395.87
2449869,97	1 June 1995	11	250°	31	397,11
2450268,98	4 July 1996	11	282°	46	399.01
2450670,06	9 August 1997	13	317°	00	401,08
2451072,62	16 September 1998	3	353°	03	402,56
2451475,29	13 October 1999	19	29°	56	402,67
2451876,59	28 November 2000	2	66°	09	401,3
2452275,74	1 January 2002	6	100°	38	399.15
2452672,88	2 February 2003	9	133°	06	397,14
2453068,70	4 March 2004	5	163°	58	395.82
2453464,14	3 April 2005	15	193°	58	395.44
Average:					398.55
Oscillation:					7.23

The table of oppositions of the planet Mars in its orbit:

Date Juliana	Date	TU		Minimum distance(U.A.)	Length Heliocentric		Period Synodic (N)
2449760.60	12 February 1995	2h	25m	0.67570	142°	56'	765.16
2450524,83	17 March 1997	7	48	0.65939	176°	49	764,23
2451293,23	14 April 1999	17	31	0.557846	214°	10	768,4
2452074,24	13 June 2001	17	39	0.45016	262°	51	781.01
2452880.25	28 August 2003	17	53	0.37271	335°	08	806,01
2453681.83	7 November 2005	7	52	0.46406	45°	08	801,58
2454459,32	24 December 2007	19	41	0.58935	92°	43	777,49
2455226,32	29 January 2010	19	37	0.66398	129°	52	767
2455990,34	3 March 2012	20	04	0.67368	163°	46	764,02
2456756,37	8 April 2014	20	57	0.61756	199°	05	766.03
Average:							777,31
Oscillation:							41,99

Due to the high eccentricity of Mars in its orbit, this effect is especially important at the oppositions of Mars. These occur every 2 years and 50 days. Thus, if an opposition occurs at one longitude, the next one will occur at a longitude 48.8° higher (the angle the Earth advances in 50 days). Thus, in 7 or 8 oppositions, the lengths make a complete turn. When the longitude of the opposition is close to 335° (longitude of the perihelion of the orbit of Mars) the opposition is very favorable perihelic. This means that every 15 years or 17 years there will be a perihelic opposition.