Most great conjunctions are not particularly notable. But occasionally, like this year, Jupiter and Saturn cross paths so close to each other that they can be barely distinguishable to the naked eye. Or sometimes the two planets cross paths when they are opposite the Sun, so their apparent retrograde motion results in a triple conjunction, as was the case in 7 BCE.
In 1604, while he was working in Prague, Kepler observed the tight arrangement of three planets — Mars, Saturn and Jupiter — and a bright new star, a supernova, that would slowly fade over the course of a year. This occurrence inspired him to consider a similar set of events that might have led the wise men to Bethlehem in time for Jesus Christ’s birth.
Knowing that Herod the Great had died in 4 BCE, he placed the birth of Christ before that date. And using his knowledge of planetary motion, he found that Jupiter and Saturn underwent a triple conjunction in 7 BCE, that conjunctions of Mars with each planet in 6 BCE were shortly followed by conjunctions of the planets with the Sun. Kepler suggested that these solar conjunctions aligned with the conception of Christ and that the wise men arrived the following year to witness Christ’s birth beneath the Star of Bethlehem.
Significance of the great conjunction
On Dec. 21 of this year, Jupiter and Saturn will be only one-tenth of a degree apart, well within the field of any telescope’s view. As we watch this year’s event, it is worth keeping in mind the historical significance previous conjunctions have had.
Kepler’s fascination with planetary motion led, only a handful of years later, to his discovery that planets follow elliptical paths around the Sun. And Kepler’s discovery would, before the end of that century, inspire Newton’s work on his most important contribution, the great Philosophiae Naturalis Principia Mathematica, where he laid down his ideas on the law of gravity, and which forever changed the world of science.
Without fear of exaggeration, it’s possible to link the wandering motion of the planets — never more clearly on display than when we can simultaneously see Saturn’s rings and the Galilean moons of Jupiter through a telescope — with the discovery that Earth is a planet within a solar system in which motions are dominated by a universal gravitation that acts between all massive bodies.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
This is a corrected version of a story originally published on Dec. 21. The earlier story said a conjunction aligned with Immaculate Conception instead of the conception of Christ.