A clock model for planetary conjunctions

Abstract

This article is inspired by the great conjunction of Jupiter and Saturn on December 21st, 2020. We will address the following questions from a mathematical standpoint. What is a conjunction? How often do we have a conjunction of two planets? How are great conjunctions distributed in the sky? How long will it take for a cycle of great conjunctions to return to the same point in the sky and time of the year? Can there be a conjunction of 3 outer planets? How often can that happen? We will begin by setting up the geometric framework for formulating these questions. Then we will analyze these questions by first examining similar questions in a more mundane context: the three hands in a clock. Even though the clock model is a simple model that ignores the more subtle and complex elements of planets' cosmic dance, it captures the main ideas succinctly.