Every 20 years or so Jupiter laps Saturn, appearing together in the same zodiacal region of the sky (technically with the same right ascension). It’s called a Great Conjunction and the next one occurs on 21 December 2020. That’s not too far away. Jupiter is chasing down Saturn and will catch it on that day, and the reflected light of the two gas giants will be some spectacle. This blog develops a useful formula for working out lapping times, the time required for a faster object to lap a slower object. The blog also follows the efforts of Johannes Kepler in his attempt to make sense of the heavenly phenomena.