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A conjunction occurs when two astronomical objects have either the same right ascension or the same ecliptical longitude, normally when observed from the Earth. In the case of two objects that always appear close to the ecliptic – such as two planets, or the Moon and a planet, or the Sun and a planet – this implies an apparent close approach between the objects as seen on the sky. In contrast, the term appulse is defined as the minimum apparent separation on the sky of two astronomical bodies.
Conjunctions therefore involve two Solar System bodies, or one Solar System body and one more distant object such as a star. A conjunction is an apparent phenomenon caused by perspective only: there is no close physical approach in space between the two objects involved. Conjunctions between two bright objects close to the ecliptic, such as two bright planets, can be easily seen with the naked eye and can attract some public interest.
The astronomical symbol of conjunction is ☌ (in Unicode x260c) and handwritten: . However, this symbol is never used in modern astronomy and is of historical interest only.
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 ecliptical longitude. At such conjunction both objects have the same ecliptical longitude. Conjunction in right ascension and conjunction in ecliptical 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 ecliptical length) occur. At the time of conjunction – it does not matter if in right ascension or in ecliptical 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.
However, if two celestial bodies attain the same declination at the time of a conjunction in right ascension (or the same ecliptical latitude at a conjunction in ecliptical longitude), the one that is closer to the Earth will pass in front of the other. In such a case, a syzygy takes place. If one object moves into the shadow of another, the event is an eclipse. For example, if the Moon passes into the shadow of Earth and disappears from view, this event 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. When Mercury passes in front of the Sun, it is a transit of Mercury, and when Venus passes in front of the Sun, it is a transit of Venus. 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 passes between Earth and the Sun, causing the Sun to disappear either entirely or partially. This phenomenon is commonly known as a 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.
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.
As seen from a planet that is superior, if an inferior planet is on the opposite side of the Sun, it is in superior conjunction with the Sun. An inferior conjunction occurs when the two planets lie in a line 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 the Earth. However, this definition can be applied to any pair of planets, as seen from the one farther from the Sun.
"Quasiconjunctions" are also possible. In these situations, 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. Else 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.
|This section may be confusing or unclear to readers. (May 2007)|
On 1 December 2008, a conjunction of Venus and Jupiter occurred, and several hours later both planets separately reached conjunction with the crescent Moon. An occultation of Venus by the Moon was visible from some locations. The three objects appeared close together in the sky from any location on the Earth.
A planetary/galactic configuration occurred on 23–24 December 2007. The configuration on December 23 – Mars, Earth, Sun, Mercury, Jupiter, Galactic Centre – is shown in the graphic simulation linked below. It was accompanied by the Full Moon (conjunct Mars) at about 2 a.m. on December 24. It is even more remarkable in that the Sun-Pluto conjunction appears exactly on the December Solstice, just past conjunction with the center of the Galaxy .
Link below is the view from Mars toward the Jupiter, Mars, Earth, Mercury, Pluto alignment toward the center of the Galaxy on 23 December 2007 which occurs just after the Pluto-Jupiter (Heliocentric) conjunction on 23 November 2007. NASA Solar System Simulator for 23 December 2007
Solar System on 22 December 2007. What is not shown is Pluto (which would be to right of Jupiter). Saturn, which follows down from Venus, and Uranus and Neptune. Saturn and Neptune form the T-member of the cross. There will be a full Moon on 23 December 2007.
In late April 2002, a rare great conjunction occurred. In this one, Saturn, Jupiter, Mars, Venus, and Mercury were all visible concurrently in the northwestern sky, shortly after sundown. This will happen again in early July 2060, except that on that occasion the quintet will be bunched in the northeastern sky shortly before dawn.
In May 2000, the five brightest planets were aligned within 20 degrees of the Sun, as seen from the Earth. This could not be observed since they were too close to the Sun.
On August 24, 1987, the five major objects closest to the Earth – the Sun, the Moon, Mercury, Venus, and Mars – were within approximately five degrees of one another, the Sun setting first, followed by Mars, Venus, Mercury, and the Moon, in that order, within 20 minutes. As in the 2000 conjunction above, this event was unobservable due to the Sun being part of the line-up.
During the new moon and solar eclipse of February 4–5, 1962, an extremely rare great conjunction of the classical planets occurred (it included all five of the naked-eye planets plus the Sun and the Moon), all of them within 16 degrees of one another on the ecliptic. At the precise moment of the new moon and a solar eclipse, five celestial bodies (the Sun, the Moon, Mercury, Venus, and Jupiter) were clustered within three degrees of each other, with the Earth in close conjunction with them. Taken in totality though, this grand conjunction included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn, with the Earth also in alignment with the Sun and the Moon at the exact moment of the new moon and a solar eclipse (eight celestial bodies in total).
During the new moon on December 2–3, 1899, a near-grand conjunction of the classical planets and several other binocular bodies occurred. The Sun, the Moon, Mercury, Mars, and Saturn were all within 15 degrees of each other, with Venus five degrees ahead of this conjunction and Jupiter 15 degrees behind. Accompanying the classical planets in this grand conjunction were Uranus (technically visible unaided in pollution-free skies), Ceres, and Pallas.