What Is a Conjunction? Two Planets Lining Up From Where We Happen to Stand

A conjunction occurs when two bodies appear at the same or nearly the same position in the sky as seen from Earth, which is a statement about lines of sight and not about proximity. Two planets appearing a fraction of a degree apart may be separated by hundreds of millions of kilometres, with one far beyond the other along the same direction. The appearance arises because the planets all orbit in roughly the same plane, so from within that plane they trace paths across the sky that lie close to a single line, and any two of them will therefore pass one another periodically. The formal definition specifies matching in one particular coordinate, which is why published times of conjunction sometimes differ slightly from the moment of closest visual approach.

The interval between conjunctions of a given pair follows from their orbital periods:

• •The faster planet laps the slower one, and the lap time sets the interval
• •Jupiter and Saturn meet roughly every twenty years, which is the longest interval among the bright planets
• •Venus and Jupiter meet roughly every thirteen months
• •Mars and Jupiter meet roughly every two and a quarter years
• •The inner planets produce frequent conjunctions but many are lost in the sun's glare
• •A conjunction is only observable if the pair is far enough from the sun in the sky at the time

Some events are genuinely unusual and worth distinguishing from the routine. A very close conjunction, where the pair are separated by less than the width of the full moon, happens for any given pair rarely, since most meetings occur with the planets displaced above or below one another. The Jupiter and Saturn conjunction of December 2020 brought them about a tenth of a degree apart, closer than at any time since 1623 and closer than any observable pairing since the thirteenth century, which is the kind of interval that makes such an event genuinely once in a lifetime. An occultation, where one planet passes directly in front of the other, is rarer still and happens a handful of times per century for any pair. Groupings of several planets within a small area occur at intervals of decades and are the events usually described in the press as an alignment.

Observing one of these events needs less equipment than people expect and a little planning. The pair must be far enough from the sun to be visible, which usually means shortly after sunset in the west or before sunrise in the east, and a low unobstructed horizon matters more than anything else. Bright pairings involving Venus or Jupiter are obvious to the eye and need nothing. Binoculars show both planets in one field and reveal the moons of Jupiter alongside, which turns a pretty sight into something more interesting. A small telescope at low magnification can hold both in one view when they are very close, which is a genuinely rare opportunity since the two planets are almost never within the same field. The event builds and fades over several evenings, so a single clouded night does not lose it.

The historical significance of these events is a large subject in itself. Astrological traditions across Mesopotamia, Greece, India, China and medieval Europe treated conjunctions as the most significant events in the sky, with the meeting of the two slowest visible planets attracting particular attention because of its long cycle and its slow drift around the zodiac, which supplied a framework for periodising history. Predicting them accurately was a major driver of astronomical work, since the practical demand for horoscopes funded the observation and calculation that produced the tables. Proposals connecting a particular conjunction to the star described in the nativity accounts have been made repeatedly since the seventeenth century and remain popular without being settled. The prediction of these events is now purely a matter of celestial mechanics and is accurate for thousands of years in either direction.

Two planets appearing close together share a line of sight and may be hundreds of millions of kilometres apart, with the effect arising because all the orbits lie near one plane. The interval between meetings follows from how long the faster planet takes to lap the slower. The Jupiter and Saturn pairing of December 2020 was closer than any observable one since the thirteenth century.