In astronomy, a conjunction occurs when two astronomical objects or spacecraft appear to be close to each other in the sky. This means they have either the same right ascension or the same ecliptic longitude, usually as observed from Earth.
When two objects always appear close to the ecliptic—such as two planets, the Moon and a planet, or the Sun and a planet—this fact implies an apparent close approach between the objects as seen in the sky. A related word, appulse, is the minimum apparent separation in the sky of two astronomical objects.
Conjunctions involve either two objects in the Solar System or one object in the Solar System and a more distant object, such as a star. A conjunction is an apparent phenomenon caused by the observer's perspective: the two objects involved are not actually close to one another in space. Conjunctions between two bright objects close to the ecliptic, such as two bright planets, can be seen with the naked eye.
The astronomical symbol for conjunction is (Unicode U+260C ☌). The conjunction symbol is not used in modern astronomy. It continues to be used in astrology.
Passing close
More generally, in the particular case of two planets, it means that they merely have the same right ascension (and hence the same hour angle). This is called conjunction in right ascension. However, there is also the term conjunction in ecliptic longitude. At such conjunction both objects have the same ecliptic longitude. Conjunction in right ascension and conjunction in ecliptic longitude do not normally take place at the same time, but in most cases nearly at the same time. However, at triple conjunctions, it is possible that a conjunction only in right ascension (or ecliptic length) occurs. At the time of conjunction – it does not matter if in right ascension or in ecliptic longitude – the involved planets are close together upon the celestial sphere. In the vast majority of such cases, one of the planets will appear to pass north or south of the other.
Passing closer
However, if two celestial bodies attain the same declination (or ecliptic latitude) at the time of a conjunction then the one that is closer to the Earth will pass in front of the other. If one object moves into the shadow of another, the event is an eclipse. For example, the Moon passing through the shadow of Earth is called a lunar eclipse. If the visible disk of the nearer object is considerably smaller than that of the farther object, the event is called a transit, such as a transit of Mercury or a transit of Venus across the sun. When the nearer object appears larger than the farther one, it will completely obscure its smaller companion; this is called an occultation. An example of an occultation is when the Moon is relatively near and therefore large and passes between Earth and the Sun, causing the Sun to disappear entirely (a total solar eclipse). Occultations in which the larger body is neither the Sun nor the Moon are very rare. More frequent, however, is an occultation of a planet by the Moon. Several such events are visible every year from various places on Earth.
Position of the observer A conjunction, as a phenomenon of perspective, is an event that involves two astronomical bodies seen by an observer on the Earth. Times and details depend only very slightly on the observer's location on the Earth's surface, with the differences being greatest for conjunctions involving the Moon because of its relative closeness, but even for the Moon the time of a conjunction never differs by more than a few hours.
Superior and inferior conjunctions with the Sun A planet is said to be superior to another if it is farther from the sun. As seen from a superior planet, if an inferior planet is on the opposite side of the Sun, it is in superior conjunction, and it is in inferior conjunction if on the same side of the Sun. In an inferior conjunction, the superior planet is "in opposition" to the Sun as seen from the inferior planet.
The terms "inferior conjunction" and "superior conjunction" are used in particular for the planets Mercury and Venus, which are inferior planets as seen from Earth. However, this definition can be applied to any pair of planets, as seen from the one farther from the Sun.
A planet (or asteroid or comet) is simply said to be in conjunction, when it is in conjunction with the Sun, as seen from Earth. The Moon is in conjunction with the Sun at New Moon.
Multiple conjunctions and quasiconjunctions Conjunctions between two planets can be single, triple, or even quintuple. Quintuple conjunctions involve Mercury, because it moves rapidly east and west of the sun, in a synodic cycle just 116 days in length. An example will occur in 2048, when Venus, moving eastward behind the Sun, encounters Mercury five times (February 16, March 16, May 27, August 13, and September 5).Compare the longitudes of Mercury and the longitudes of Venus in the period January through September 2048, calculated by the JPL Horizons On-Line Ephemeris System.
There is also a so-called quasiconjunction, when a planet in retrograde motion — always either Mercury or Venus, from the point of view of the Earth — will "drop back" in right ascension until it almost allows another planet to overtake it, but then the former planet will resume its forward motion and thereafter appear to draw away from it again. This will occur in the morning sky, before dawn. The reverse may happen in the evening sky after dusk, with Mercury or Venus entering retrograde motion just as it is about to overtake another planet (often Mercury and Venus are both of the planets involved, and when this situation arises they may remain in very close visual proximity for several days or even longer). The quasiconjunction is reckoned as occurring at the time the distance in right ascension between the two planets is smallest, even though, when declination is taken into account, they may appear closer together shortly before or after this.
Average interval between conjunctions
The interval between two conjunctions involving the same two planets is not constant, but the average interval between two similar conjunctions can be calculated from the periods of the planets. The "speed" at which a planet goes around the Sun, in terms of revolutions per time, is given by the inverse of its period, and the speed difference between two planets is the difference between these. For conjunctions of two planets beyond the orbit of Earth, the average time interval between two conjunctions is the time it takes for 360° to be covered by that speed difference, so the average interval is:
This does not apply of course to the intervals between the individual conjunctions of a triple conjunction.
Conjunctions between a planet inside the orbit of Earth (Venus or Mercury) and a planet outside are a bit more complicated. As the outer planet swings around from being in opposition to the Sun to being east of the Sun, then in superior conjunction with the Sun, then west of the Sun, and back to opposition, it will be in conjunction with Venus or Mercury an odd number of times. So the average interval between, say, the first conjunction of one set and the first of the next set will be equal to the average interval between its oppositions with the Sun. Conjunctions between Mercury and Mars are usually triple, and those between Mercury and planets beyond Mars may also be. Conjunctions between Venus and the planets beyond Earth may be single or triple.
As for conjunctions between Mercury and Venus, each time Venus goes from maximum elongation to the east of the Sun to maximum elongation west of the Sun and then back to east of the Sun (a so-called synodic cycle of Venus), an even number of conjunctions with Mercury take place. There are usually four, but sometimes just two,Compare the longitudes of Mercury and the longitudes of Venus in the period November 4, 2005, to June 9, 2007, calculated by the JPL Horizons On-Line Ephemeris System. There is a quasiconjunction on August 9, 2006. and sometimes six, as in the cycle mentioned above with a quintuple conjunction as Venus moves eastward, preceded by a singlet on August 6, 2047, as Venus moves westward. The average interval between corresponding conjunctions (for example the first of one set and the first of the next) is 1.599 years (583.9 days), based on the orbital speeds of Venus and Earth, but arbitrary conjunctions occur at least twice this often. The synodic cycle of Venus (1.599 years) is close to five times as long as that of Mercury (0.317 years). When they are in phase and move between the Sun and the Earth at the same time they remain close together in the sky for weeks.
The following table gives these average intervals, between corresponding conjunctions, in Julian years of 365.25 days, for combinations of the nine traditional planets.The periods are calculated from the "mean mean motions" given by Conjunctions with the Sun are also included. Since Pluto is in resonance with Neptune the period used is 1.5 times that of Neptune, slightly different from the current value. The interval is then exactly thrice the period of Neptune.
+ Average interval between similar conjunctions Mercury Venus Sun Mars Jupiter Saturn Uranus Neptune Pluto Period 0.241 0.615 1.000 1.881 11.862 29.457 84.020 164.770 247.155Mercury0.241 1.599 0.317 2.135 1.092 1.035 1.012 1.006 1.004Venus 0.615 1.599 1.599 2.135 1.092 1.035 1.012 1.006 1.004Sun 1.000 0.317 1.599 2.135 1.092 1.035 1.012 1.006 1.004Mars 1.881 2.135 2.135 2.135 2.235 2.009 1.924 1.903 1.895Jupiter11.862 1.092 1.092 1.092 2.235 19.859 13.812 12.782 12.460Saturn 29.457 1.035 1.035 1.035 2.009 19.859 45.360 35.870 33.443Uranus 84.020 1.012 1.012 1.012 1.924 13.812 45.360 171.444 127.294Neptune164.770 1.006 1.006 1.006 1.903 12.782 35.870 171.444 494.310Pluto 247.155 1.004 1.004 1.004 1.895 12.460 33.443 127.294 494.310
Approximate conjunctions of more than two planets A conjunction in which three or more planets simultaneously have the same longitude will almost surely never happen, but because the ratios of the synodic cycles are not rational numbers, this situation can be approached arbitrarily closely. In 1953 BC Mercury, Venus, Mars, Jupiter, and Saturn were all in a longitude range of 4.3° (see below). The graph below shows the standard deviation of the differences between the ecliptic longitudes of the five naked-eye planets (not including Uranus) and that of the sun, showing times when these five planets were fairly close together. In 1961 and again in 1997 Jean Meeus found several such groupings over the millennia.
Since three planets having the same longitude and the same latitude involves four equations (equating longitude and latitude of the second and third planets to those of the first) and four variables (the positions of the earth and the three other planets in their orbits), it is in principle possible to have three planets perfectly lined up. In reality this will almost surely never happen, but is approached arbitrarily closely given enough time. But by the same token, a perfect alignment of four planets as seen from Earth is not possible (there are six equations to be solved but only five variables) unless the Solar System is very special.