Great Conjunction: A Guide to Understanding the Rare Celestial Event is a comprehensive guide to understanding the rare astronomical event known as a great conjunction. This guide will explain the science behind a great conjunction, the history of great conjunctions, and the significance
From Wikipedia
A great conjunction is a conjunction of the planets Jupiter and Saturn, when the two planets appear closest together in the sky. Great conjunctions occur approximately every 20 years when Jupiter "overtakes" Saturn in its orbit. They are named "great" for being by far the rarest of the conjunctions between naked-eye planets.