This is when a planet has the same longitude as the Sun. A planet can only be observed at conjunction if there is a total eclipse or if it transits the Sun. Unlike the other planets, Mercury and Venus have two types of conjunction, inferior when the planet lies between the Sun and the Earth, superior when it lies on the opposite side of the Sun to the Earth. At inferior conjunction, if the inclination of the orbit of Mercury or Venus coincide with that of the Earth, a transit is observed. These are rare phenomena.

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