Conjunction (Astronomy one of the aspects)
Collier's New Encyclopedia (1921)
in astronomy, one of the aspects of the planets. Two heavenly bodies are in conjunction when they curved; and (2) Cupressex, with the 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 moon. 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 Planets are invisible when in consun. 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, according as they are actually witnessed from the earth rth, 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 center; by this means the exact times of conjunction 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. Chinese history records one in the reign of the Emperor Tehuen-hiu (2514-2436 b. c.), which astronomers calculate to have actually taken place.
What Has Changed in the Last Century
The geometry Collier's set out in 1921 is exactly the geometry taught today: two bodies share the same celestial longitude, the sun and moon are in conjunction at new moon, and Mercury and Venus have both inferior and superior conjunctions. Two small shifts: modern almanacs usually reckon conjunction in right ascension rather than ecliptic longitude, so listed times can differ slightly from the older definition, and the "rare cases" of an inferior planet crossing the sun's disk have proved rarer than the 1921 reader might have hoped — after the transits of Venus in 2004 and 2012 the next is not until 2117. The 1921 remark that planets are invisible at conjunction with the sun now has a practical edge unimagined then: during Mars solar conjunction, roughly every 26 months, NASA suspends commands to its rovers and orbiters because the sun corrupts the radio link. The most publicised recent event was the Jupiter–Saturn great conjunction of December 2020, the closest such pairing since 1623.
Written for this edition, 2026.