CONJUNCTION, in astronomy, one of the aspects of the planets. Two heav enly bodies are in conjunction when they have the same longitude—that is, when the same perpendicular to the ecliptic passes through both. If they have, at the same time, the same latitude—that is, if they are both equally far north or south of the ecliptic—they appear from the earth to be in the same spot of the heavens, and to cover one another. The sun and moon are in conjunction at the period of new moan. In the case of the inferior planets Mercury and Venus, there is an inferior conjunction when the planet is between the earth and the sun, and a superior when the sun is between the earth and the planet. In general, a heavenly body is in conjunction with the sun when it is on the same side of the earth, and in a line with him; and it is in opposition to the sun when it is on the opposite side of the earth, the earth being in a line between it and the sun. Planets are invisible when in con
junction with the sun, except in rare cases when an inferior planet passes over the sun's disk, and may be seen as a speck on his surface. Conjunctions are either geocentric or heliocentric, accord ing as they are actually witnessed from the earth, or as they would be witnessed if observed from the sun. In observing a conjunction from the earth's surface it is usual to reduce the observation to what it would be if made from the earth's cen ter; by this means the exact times of con junction are more accurately fixed, and the observations of an astronomer made available to every other, wherever he may be on the earth's surface. Grand conjunctions are those where several stars or planets are found together. Chi nese history records one in the reign of the Emperor Tehuen-hiu (2514-2436 B. C.) , which astronomers calculate to have actually taken place.