The “Great Conjunction” of Jupiter and Saturn Will Happen on December 21st. Here is What That Means.

Stefan 2499 - via Wikimedia Commons.

Four of the seven (excluding earth) planets in our solar system were discovered by the same man: Galileo Galilei. In 1609, upon learning of Hans Lippershey’s “Dutch Perspective Glasses” – designed a year earlier –  Galileo sought to design his own.

Only a few days later, Galileo had designed and built one of the world’s telescopes. Improving on Lippershey’s refractor design, Galileo’s prototype was orders of magnitude better than Lippershey’s. Pointing it at the sky, Galileo soon discovered Venus, Mars, Jupiter, and Saturn. 

As technology progressed, so did the telescopes. In five hundred years, we progressed from a telescope that could barely resolve individual stars to a space-based observatory that could see the heat of a bumblebee from the moon and a penny from twenty four miles away. 

The telescope is among our most important inventions; with it, Galileo discovered four planets and four moons, William Herschel discovered Uranus and created the Catalogue of Nebulae and Clusters of Stars, Charles Messier cataloged 110 large deep sky objects in his Messier Catalog, Edwin Hubble discovered the Hubble Constant (~70 km/s/Mpc*), Gerard Kuiper discovered the CO2 content on Mars and the icy rings of Saturn, alongside innumerable advances in physics – the cosmic microwave background, general relativity, even communication, to name a few. Although the telescope has served humanity more than perhaps any other tool, it is not useful for everything; some events are best seen with the naked eye.

December 21st is one of those times, for Jupiter and Saturn will come within .1 degrees of each other in the night sky in an event known as the Great Conjunction, which will form a superstar that has not been seen since 1226 (794 years ago!).

The conjunction

Every 19.6 years, Jupiter and Saturn align as Jupiter’s orbit “overtakes” Saturn in the night sky, as a result of Jupiter and Saturn’s revolutionary periods (11.9 years and 29.5 years, respectively).

A conjunction occurs when two astronomical/artificial objects, often spacecraft, background stars, deep-sky objects, the sun, or the planets, reach a similar or the same ecliptic longitude and right ascension, as observed from Earth; indeed, as this is an apparent conjunction, not two astronomical objects or spacecraft running into each other, the alignment on earth may not appear the same on, for example, Mars. 

Imagine you are looking at two isolated buildings in the middle of a field. You are diagonal to the two buildings at the moment, and you can easily see both of them, but if you were to move one-thousand feet to the west, you would see only one. If the conjunctions of the planets were so specific – and if Earth’s position in the sky relative to the planets were so perfect – the more distant planet, assuming its apparent size is smaller than the foreground object, would be completely invisible, eclipsed by the nearer object, for a short time.

The Great Conjunction of December 21st, 2020 is not a “perfect” conjunction, but Saturn and Jupiter will, nevertheless, be very close to one another. The two planets will be six arcminutes – or only a tenth of a degree – from one another. For context, their distance from each other will be about a fifth of the diameter of the full moon. The two planets will be so close together in the night sky that they will be a “superstar.” The two planets will be so close together that their light will be essentially combined, resulting in a much brighter “star” that combines the brightness of a -2.2 apparent magnitude planet and a .46 magnitude planet.

How to see it

If you wish to see Jupiter and Saturn in conjunction, you must know where to look. Regardless of where you are on earth (excluding far into the southern hemisphere), the conjunction should be to your southwest. As long as you live south of around sixty degrees latitude, you should be able to see it in the early evening, assuming a good view to your southwest. 

Historical implications of Great Conjunctions

The Great Conjunction, as it is visible every twenty years, has led to several interesting historical events and dogmas since its discovery.

In 4 B.C., around the time of the biblical birth of Jesus Christ, a series of three conjunctions of Jupiter and Saturn occurred. Johannes Kepler, who discovered the Laws of Planetary Motion and was a proponent of heliocentrism, first hypothesized that the Star of Bethlehem was the Great Conjunction of Jupiter and Saturn. He proposed that a series of three conjunctions between the two planets occurred in the year 4 B.C. such that the planets were close enough to resemble a star – which requires that the two planets be extremely close to one another (probably at most 10 arcminutes). After further analysis, it was discovered that the two planets were actually around a degree from each other in the night sky, around twice the size of the full moon.

As a result, the two planets were likely too far from one another to be considered a star. Another astronomer, Karlis Kaufmanis, noticed that the conjunction Kepler had referred to was actually a triple conjunction in the constellation Pisces; he proposed that this conjunction could have been the Star of Bethlehem in the New Testament. Nevertheless, the Star of Bethlehem could have been one of two supernovae that occurred in 4 BCE in the constellations Andromeda and Aquarius, both in the eastern skies at the time of the supernova; the conjunction of Jupiter, Venus, and the first-magnitude star Regulus in 3-2 BCE; the passage of a nearby comet; or the double occultation of the Moon and Jupiter in 6 BCE.

When we did not understand the Great Conjunction, some of us considered it, like every other peculiar event, an omen. According to a Wikipedia article on the Great Conjunction, “As successive great conjunctions occur nearly 120° apart, their appearances form a triangular pattern. In a series every third conjunction returns after some 60 years to the vicinity of the first. These returns are observed to be shifted by some 7–8°, so no more than four of them occur in the same zodiacal sign. To each triangular pattern astrologers have ascribed one from the series of four elements and thus four triplicities or trigons are formed. Particular importance has been accorded to the occurrence of a great conjunction in a new trigon, which is bound to happen after some 200 years at most. Even greater importance was attributed to the beginning of a new cycle after all four trigons had been visited, something which happens in about 800 years. Since each 'element' (trigon) consists of 3 signs it takes 800 × 3 = 2400 years for the whole process to start anew (relation with the cycle of precession).”

The astrologers appealed to the public view – for they were “scientists” – and many around the world awaited the end of the first cycle after it had been discovered in 1583. Nothing significant happened, unsurprisingly, leading to decreased public interest. Astrologers continued to warn of the omens until 1603, at the advent of a new trigon; and after nothing significant happened again, the public interest fell as well.

Wrapping it up

Overall, the Great Conjunction is a naturally occurring, visual phenomenon in which the planets Jupiter and Saturn have the same right ascension. The Great Conjunction is historically significant for its involvement in astrology and its potential place as the Star of Bethlehem in the New Testament.

If you are able to see the event from your house or nearby, I urge you to do so – astronomical events come and go, but they are always long apart. An event like this will never happen again in your lifetime, in my lifetime, or in any of our lifetimes. Take advantage of this incredible opportunity to experience the peculiarities of the night sky and our solar system. As always, take care and stay curious, everyone.

If you have any questions, comments, or corrections, please comment on this post or email learningbywilliam@gmail.com with your concerns. Thank you.

Note: As a result of temporary changes to my school account, which I use to create my citations, there will be no APA references until my password is, once again, reset and I am able to access school resources. Thank you for your patience.