Oppositions of Mars
by Martin J. Powell
Diagram showing oppositions of Mars between 2012 and 2027 (click on thumbnail for full-size image, 156 KB). The relative brightness of the planet at each opposition is indicated by the colour of the date label, from orange (faintest) to yellow (brightest). The distances between Mars and the Earth, and between the Earth and the Sun, are also shown, in Astronomical Units (AU). The constellation in which Mars appears, as seen from the Earth, is shown in green. The apparent size of the planet at each opposition, together with other relevant data, is shown in the table below.
The heliocentric longitude of Mars at each opposition is marked in dark grey alongside an arrow. Heliocentric longitude (symbol ) is the longitude of a planet (from 0° to 360°) measured counter-clockwise from the First Point of Aries (Earth's Vernal Equinox direction in space) with the Sun at the centre. The First Point of Aries is the 'zero point' from which the longitudes of all Solar System bodies are measured.
The points of perihelion (the planet's closest point to the Sun) and aphelion (its most distant point from the Sun) are marked for both Mars and the Earth by the letters 'P' and 'A', respectively. The year 2012 saw Mars positioned at nearly the worst possible orbital position when seen from the Earth, the planet passing its aphelion point just two weeks before opposition. The opposition of 2027 - some 15 years later - is equally poor, being positioned about the same angular distance (of heliocentric longitude) from the planet's aphelion point.
Whenever a superior planet reaches opposition, it is positioned directly opposite the Sun in the Earth's sky (i.e. its solar elongation is 180º - the Sun, the Earth and the superior planet in question being lined up in space). The planet is then visible all night long, rising opposite the Sun around local sunset and setting opposite the Sun around local sunrise. Northern Hemisphere observers see the planet positioned due South around local midnight whilst Southern Hemisphere observers see the planet positioned due North around local midnight.
Opposition day is the best time of the year to observe a superior planet. For observers using telescopes in particular, it is an important time for several reasons. Firstly, the Earth is then at its closest point to the planet for the whole year. Secondly, the planet appears largest in the night sky for the whole year (i.e. it attains its greatest apparent size). Thirdly, the planet's fully illuminated side is then facing towards the Earth (i.e. its phase is 100%). The full illumination, combined with the planet's large apparent size, means that the planet then shines at its greatest apparent magnitude (brightness) for the year, an effect which is clearly visible to the naked-eye. In practice, a superior planet is observed most frequently during the period about one or two months to either side of the actual opposition date.
Mars seen through the Telescope by three different observers at the perihelic oppositions of 1971, 1988 and 2003 (click on thumbnail for full-size image, 16 KB). All images are orientated with South up and East to right (Left) Lowell Observatory, 24" Alvan Clark refractor (Centre) Tim Printy, 10" SCT reflector (Right) Damian Peach, 10" SCT reflector.
Because of the eccentricity of the planets' orbits, some oppositions are more favourable than others, the planets then shining brighter than at other oppositions. This is particularly true for Mars, whose orbit is quite eccentric, the result being that its distance from the Earth varies considerably from one opposition to the next.
The distance between Earth and Mars at opposition varies from around 0.3728 Astronomical Units (34.6 million miles or 55.7 million kms) at perihelion to 0.6780 AU (63 million miles or 101.4 million kms) at aphelion. This is easily the greatest variation in opposition distance of all the Solar System planets. The Red Planet's apparent magnitude at opposition varies from -2.9 (at perihelion) to -1.2 (at aphelion), which means that the planet is nearly five times brighter at its closest oppositions than at its most distant oppositions.
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Mars' brightest and best oppositions occur in groups of two or three which repeat at intervals of 15 years and 17 years, when it reaches the closest point in its orbit to the Earth. Whenever opposition occurs close to the planet's perihelion point (its closest point to the Sun) it is often referred to as a perihelic opposition. Likewise, when oppositions occur close to the planet's aphelion point (its furthest point from the Sun) it is referred to as an aphelic opposition. Oppositions taking place in-between best and worst (i.e. the most frequent) are sometimes referred to as intermediate oppositions. Of all the Solar System planets, Mars' perihelic and aphelic oppositions are particularly significant because its apparent size is considerably different in each case.
The time between successive oppositions (known as the synodic period) varies between 765 and 800 days, averaging around 780 days or a little over 2 years, the opposition date drifting about a month or two later through the calendar. Between each opposition, Mars completes a full orbit, moving an average angular distance of 408° before arriving at the next opposition, some 48° or so counter-clockwise along the planet's orbit. Viewed from the Earth, Mars then appears one or two constellations further Eastwards along the zodiac. After a period of (15 + 17) = 32 years, the planet returns to a position in its orbit which is within ±12° of its starting point.
Perihelic oppositions are seen to take place in the constellations of Capricornus or Aquarius, whilst aphelic oppositions take place in Leo. Perihelic oppositions are therefore rather better observed from Southern hemisphere latitudes, since the planet is positioned South of the celestial equator at such times. Northern hemisphere observers are better able to observe Mars at the following opposition, when the planet has moved further Eastwards and, therefore, is positioned at a higher declination and appears higher in the sky (the changing transit altitude of Mars from one opposition to the next, which is largely determined by the latitude of the observer, is explained in more detail on the current Mars page). Hence Northern hemisphere observers will obtain an improved view of the planet at its 2020 opposition rather than the 'ideal' 2018 opposition, despite the fact that the Martian disk will appear a little smaller through their telescopes.
Perihelic oppositions typically occur in August or September; in the last 50 years they took place in August 1971, September 1988 and August 2003. Aphelic oppositions take place on the opposite side of Earth's orbit, i.e. in February or March. Over the last half-century, these were in February 1963, February 1980, February 1995 and March 2012.
Opposition Data for Mars from 2012 to 2027 (click on thumbnail for full-size table, 87 KB). The Declination is the angle of the planet to the North (+) or South (-) of the celestial equator at the time of the planet's opposition. Mars' apparent disk size at the 2012 aphelic opposition was the smallest since the opposition of February 1995; it begins to increase once more from the 2014 opposition. The Martian disk images were derived from NASA's Solar System Simulator and are shown at the same scale as those in the current Mars apparition data table. A graphic showing opposition data for the years 2001-2010 is shown here. The changing aspect of the planet as seen from the Earth at each opposition is shown in an animation on the Martian Seasons page.
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Note that the date of Mars' opposition does not normally fall upon the same day as its closest approach to the Earth. The eccentricity of Mars' orbit is such that the dates of opposition and closest approach would only coincide if the planet were positioned precisely at its perihelion or aphelion point at the moment it reached opposition. Essentially, oppositions taking place between heliocentric longitudes of 336° ( = 336°, at perihelion) measured counter-clockwise through to = 156° (aphelion) will take place after closest approach to Earth, whilst oppositions taking place between = 156° and = 336° will take place before closest approach.
The maximum difference between opposition date and closest approach date, which amounts to around 10 days, takes place at Martian orbital positions about 90° from the perihelion and aphelion points (i.e. at around = 66° and = 246°). Hence at the opposition of May 22nd 2016, when = 241°, the date of closest approach to Earth is May 31st, i.e. 9 days after opposition day. Conversely, when Mars reaches opposition on December 8th 2022, at an orbital position nearly opposite its 2016 point, closest approach takes place on December 1st, i.e. 7 days before opposition day. At the perihelic opposition of July 27th 2018 ( = 304°), closest approach to Earth takes place on July 31st, just 4 days later.
The maximum theoretical apparent size of Mars when seen from the Earth is a little under 26" (26 arcseconds, where 1 arcsecond = 1/3600th of a degree), though this occurs very rarely (Jeff Beish of ALPO reports that an apparent diameter of 26".04 will be attained in the year 25,695 AD!). At the opposition of August 2003, when Mars was in Aquarius ( = 334°), the planet came closer to the Earth than it had done for almost 60,000 years. This was due to the fact that it reached the perihelion point in its orbit just two days after its opposition date. As seen from the Earth, the apparent equatorial diameter of the Martian disk then reached 25".1. The next perihelic opposition of Mars will take place in July 2018, the planet positioned in South-western Capricornus, when it will attain an apparent diameter of 24".2 and shine at magnitude -2.8.
In contrast, Mars' aphelic opposition of 2012 saw the planet's apparent diameter attaining just 13".9, a little over half of its perihelic opposition value. This is by far the greatest apparent size variation of any of the superior planets at opposition.
[Terms in yellow italics are explained in greater detail in an associated article describing planetary movements in the night sky.]
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Copyright Martin J Powell October 2013
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