A Great conjunction is a conjunction of the planets Jupiter and Saturn. Great conjunctions occur regularly (every 19.6 years, on average) due to the combined effect of Jupiter’s approximately 11.86-year orbital period and Saturn’s 29.5-year orbital period and due to the proximity of the orbits of the planets. The upcoming Great conjunction will occur on 21 December 2020.
Conjunctions occur in at least two coordinate systems: equatorial and ecliptic. The conjunctions in first system are measured in right ascension, along the celestial equator. The second system is based on the ecliptic, the plane of the Solar System. When measured along the ecliptic, the separations are usually smaller. Conjunctions are characterized by angular distance between planets and elongation (angular distance from Sun). The visibility of the exact moment of a conjunction depends on the observer’s location.
The upcoming Great conjunction will occur on 21 December at 13:30 UTC (in right ascension). At this time Jupiter will be 0.1 degree (6 arcmins, the one fifth of Moon diameter) south of Saturn and 30.3 degrees east (on the left) of the Sun. The closest approach of the planets will be at 18:25 UTC, elongation at this moment will be 30.1 degree. This Great conjunction will be the closest since 1623. This means that in telescopic field of view, both planets will be visible simultaneously. And also they will be distinguishable from each other without optical aid.
The Great conjunction will occur in the constellation of Capricornus. After sunset, the two planets will be visible at the southwestern part of the horizon, low above it. From mid-northern latitudes, the planets will be less than 15 degrees in altitude, one hour after sunset.
Check if the Great conjunction 2020 can be seen in your location using WinStars 3.
Source: Wikipedia (redacted version by Sergey Telukhin)