Planetary aspects Conjunction
Conjunction
Side by side to the eye, hundreds of millions of kilometres apart
Two bodies draw almost together on the sky while remaining vastly far apart.
- Geometry
- Equal ecliptic longitude
- Common kinds
- Planet with Moon, planet with planet, inferior and superior conjunction
- Jupiter–Saturn great conjunction
- About 20 years
01 / Overview
Meet Conjunction
Two bodies are in conjunction when they share the same ecliptic longitude. They lie along nearly the same line of sight and may appear less than a full Moon’s width apart, but that is only projection: in a conjunction of a planet with the Moon, the Moon is 380,000 km away and the planet usually hundreds of millions.
Some conjunctions you will meet
The commonest is a planet with the Moon, which passes close to one every month. Conjunctions between planets are far rarer: the great conjunction of Jupiter and Saturn comes around every 20 years, and in 2020 the pair closed to about 0.1° — near enough to merge for the unaided eye. The inner planets add superior and inferior conjunctions, behind and in front of the Sun.
Why a conjunction is worth watching
A conjunction puts two bodies of different brightness and colour in one field of view, and doubles as a lesson in scale: two equally bright points of light, one still inside the Earth–Moon system, the other far beyond the orbit of Jupiter.
02 / Key data
Core parameters
- Geometry
- Equal ecliptic longitude
- Common kinds
- Planet with Moon, planet with planet, inferior and superior conjunction
- Jupiter–Saturn great conjunction
- About 20 years
- 2020 great conjunction separation
- About 0.1°
- Recurrence
- Irregularly, all year round
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- The days around a conjunction are worth watching too: the separation changes daily.
- A conjunction close to the Sun can only be watched in twilight; keep clear of the Sun itself.
- Binoculars or a low-power telescope will often hold both in one field.
04 / Sources