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Conjunction is a term used in positional astronomy and astrology. It means that, as seen from some place (usually the Earth), two celestial bodies are at the same position in the sky.
More generally, in particular in the case of two planets, it means that they just have the same right ascension (and hence the same hour angle). When conjunctions occur, the involved planets are close to one another when viewed upon the celestial sphere. The vast majority of the cases, one of the planets will appear to pass north or south of the other.
Much more rarely, one planet may occult, or cross directly in front of another as seen from Earth, so that the two planets merge into a single object, i.e. they also have the same declination. Some historians believe that the Star of Bethlehem was caused by an occultation of Saturn by Jupiter. More frequent is an occultation of a planet by the Moon, which will generally occur every few years on average (and is often visible only from certain locations and not everywhere the two objects are above the horizon at the time).
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.
A planet (or asteroid or comet) is simply said to be in conjunction, when it is in conjunction with the Sun, as seen from the Earth. The Moon is in conjunction with the Sun at New Moon (or rather Dark Moon).
Strictly speaking one must distinguish between a conjunction in ecliptic longitude and a conjunction in right ascension. Both events need not take place at the same time. It is possible that two celestial bodies come together in conjunction in right ascension but do not reach a conjunction in length and vice versa.
"Quasi-conjunctions" are also possible; in this scenario, a planet in retrograde motion - always either Mercury or Venus - 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 move back ahead of it. This will occur in the morning sky, before dawn; or 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. The quasi-conjunction is reckoned as occurring at the time of the closest actual approach of the two planets.
Grand Conjunctions
Winter Solstice 2007 A very remarkable planetary/galactic configuration occurs on 23rd and 24th December 2007. The 23/12 configuration - Mars, Earth, Sun, Mercury, Jupiter, Galactic Centre, is shown in the graphic simulation (link) below; it becomes even more remarkable in that it will be joined/triggered by the full moon (conjunct mars) at about 2am on December the twenty-fourth when a simultaneous Venus square Neptune occurs. It's even more remarkable in that the Pluto/sun conjunction appears exactly on the Winter Solstice... just past conjunction with the Galactic Centre.
Link below is the view from Mars toward the Jupiter, Mars, Earth, Mercury, Pluto alignment toward the Galactic Centre on 23rd of Dec 2007 which occurs just after the Pluto/Jupiter (Heliocentric) conjunction on 23rd Nov 2007. NASA Solar System Simulator for 23rd Dec 2007 (http://space.jpl.nasa.gov/cgi-bin/wspace?tbody=5&vbody=4&month=12&day=23¢ury=20&decade=0&year=7&hour=00&minute=0&fovmul=1&rfov=30&bfov=30&porbs=1&brite=1)
Spring 2002 In late April of 2002, a rare grand conjunction occurred; in which Saturn, Jupiter, Mars, Venus and Mercury were all visible concomitantly in the west-northwest sky, shortly after sundown; this will happen again in early July of 2060, except that this time the quintet will be bunched in the east-northeast sky, shortly before dawn.
Spring 2000 In May 2000, the 5 brightest planets aligned within 20° of the Sun, as seen from the Earth. This could not be observed because they were too close to the Sun.
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