We are very lucky this Christmas season to be able to enjoy a superb, bright, easily visible planetary conjunction. On the evening of December 21 (which, by coincidence, is the winter solstice), look to the southwest just after the sun has set. Low in the sky during twilight the planets Jupiter and Saturn will be very close to one another, and will form a bright “star.” I advise you to practice by looking southwest the night before to be certain your view won’t be blocked by a tree or building. Recall Psalm 19:1, “The heavens declare the glory of God, the skies proclaim the work of his hands.”
On that night the two planets will be within one-tenth of one degree of each other, still visible as two points of light, but this is closer than they have been in many years or will be for many years to come. Recall that the full moon has an average diameter of about 0.5 degree, so 0.1 degree represents just one-fifth the width of the full moon.
This unusual alignment of the planets can be illustrated by drawing three concentric circles using a quarter, a medium water glass, and a large mug almost twice the diameter of the water glass. Put the clock numbers 1 to 12 around the center circle, and do the same for the middle and outermost circle. This is a rough sketch of our solar system, with the sun near the center. The inner circle is the orbit of the Earth, the middle circle shows the orbit of Jupiter, and the outer circle shows the orbit of Saturn.
Put an E on the inner circle at 12 (that’s the Earth), a J on the middle circle at 2 for Jupiter, and an S on the outer circle also at 2 for Saturn. The planets are orbiting counterclockwise. Though the drawing is very approximate, you can see that once in a while (about every 20 earth-years), the Earth, Jupiter, and Saturn will temporarily fall in line. Standing in your backyard, you will be looking past Jupiter out to Saturn, which is much farther away. Enjoy this view on December 21, because this alignment won’t happen again until 2040.
One theory about the star of Bethlehem is that such a conjunction of Jupiter and Saturn occurred just before the birth of Jesus, and this was viewed by some as a portent of important things happening. In fact, mapping out mathematically the precise orbital motions proves that in the year we now call 7 BC there was a rare triple conjunction of Jupiter and Saturn over a few months. According to Joe Rao, a science writer and lecturer at New York’s Hayden Planetarium, the two planets appeared within one degree of each other on May 29, then separated a little and moved close again on September 30, and after separating they drew close again on December 5. But we’re really lucky because Jupiter and Saturn will appear 10 times closer this year. Merry Christmas!
James Magner, MD is an endocrinologist and scientist who spent years studying the biochemistry and physiology of the pituitary hormone, TSH, and providing medical supervision for several projects within the pharmaceutical industry. He is an avid chess player and expert poker player who placed 27th in the world in 2015. Dr. Magner is married and has two adult daughters. Seeking Hidden Treasures, his third book and debut collection of fiction, was published in 2019 by Archway Publishing. He is a member of the board of directors of Today’s American Catholic.