A planet is said in conjunction with a star when it passes in front of it in the immediate approach. Naturally it is a perspective effect, since the stars are much more distant than the planets of the solar system with respect to the Earth. However, the earth observer can see the two overlapping bodies or side by side.
Planets whose orbit is interior with respect to that of the Earth (Mercury and Venus), may be in lower conjunction when they are between the Sun and Earth, or in higher conjunction when they are on the other side of the Sun with respect to the Land. On the other hand, the planets whose orbits are external with respect to that of the Earth (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), can obviously be found only in higher conjunction.
When, in the case of lower conjunction, a planet like Mercury or Venus is perfectly aligned with the Sun and the Earth, we can see it pass as a small black dot on the Sun's disk. This fact is defined as transit.
When any planet in higher conjunction is exactly aligned with the Sun and the Earth, it means that it is hidden behind the Sun's disk. The term conjunction does not mean, however, that necessarily the Earth, the Sun and the planet in question they must be exactly aligned, but occupy approximately the type of configuration described.
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