What exactly is an astronomical conjunction? To define it is simple: as seen from some place (usually the Earth), two celestial bodies appear near one another in the sky. It can not get any simpler than that. Having defined the term, you have to delve deeper into the possibilities and idiosyncrasies of the actual happening. So, let’s go ahead and do that.
Most generally, conjunction relates to the case of two planets, meaning that they merely have the same right ascension and hour angle. This is called conjunction in right ascension. There is also the term conjunction in ecliptical longitude. In this type of 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. During triple conjunctions it is possible that a conjunction only in right ascension 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.
Now, to make things a little more murky. Should 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 syszygy (a kind of unity, especially through coordination or alignment) takes place. One object can move into the shadow of another and become an 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 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, but is truly an occultation. Occultations in which the larger body is neither the Sun nor the Moon are very rare.
The terms “inferior conjunction” and “superior conjunction” are used in particular for the planets Mercury and Venus, which are inferior planets(referring to a planet’s orbit size compared to Earth’s) as seen from the Earth. However, this definition can be applied to any pair of planets, as seen from the one further from the Sun. “Quasi-conjunctions” are also possible; in this scenario, a planet in retrograde motion 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; 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 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. Like I said, a simple definition until you get into the ins and outs of the thing.
Here is a link that explains conjunction further as well as defines some of the terms used. Here on Universe Today we offer a great article about a recent conjunction of Venus. Astronomy Cast offers a good episode about an interesting phenomenon: the proof that the Christmas star was actually a planetary conjunction.
Source: Wikipedia
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