conjunction, in astronomy, alignment of two celestial bodies as seen from the earth. Conjunction of the moon and the planets is often determined by reference to the sun. When a body is in conjunction with the sun, it rises with the sun, and thus cannot be seen; its elongation is 0°. The moon is in conjunction with the sun when it is new; if the conjunction is perfect, an eclipse of the sun will occur. Mercury and Venus, the two inferior planets, have two positions of conjunction. When either lies directly between the earth and the sun, it is in inferior conjunction; when either lies on the far side of the sun from the earth, it is in superior conjunction.