Jupiter and Saturn have come closer than at any time in 400 years in the event dubbed the great conjunction, prompting people around the world to turn out and try for a glimpse. The orbital paths of the two huge planets ensure great conjunctions every 20 years, but many are impossible to see with the naked eye because they happen during the daytime. Great conjunctions happen when Jupiter, which laps the sun in a shade under 12 years, and Saturn, which orbits every 29.5 years, come into near alignment with the Earth. It will be 2080 before the planets align so closely again
source: theguardian.com