The December 2020 Great Conjunction
The year 2020 will end with a special astronomical event: the closest great conjunction of Jupiter and Saturn in 397 years. On December 21, the two planets will almost touch in the sky.
What Is a Conjunction?
Generally speaking, a conjunction is when two objects appear close to each other in the sky. A conjunction of Jupiter and Saturn—which only happens about once every 20 years—is called a great conjunction.
In the technical language of astronomers, there are a number of ways to define a conjunction. One way is to say it is the moment of minimum separation between two objects as viewed from the Earth. By this definition, the 2020 great conjunction of Jupiter and Saturn will occur at about 18:20 UTC on December 21. Convert to your time zone
What Can We See, and When Can We See It?
Since September 2020, Jupiter has been moving closer and closer to Saturn in the early evening sky. Wherever you are in the world—even in light-polluted urban environments—the two planets are an impressive sight, and easy to find after sunset. If you're in the Northern Hemisphere, look toward the southwest. If you're in the Southern Hemisphere, look toward the west. Jupiter is the brighter of the pair.
As November begins, Jupiter and Saturn are five degrees apart, which is the width of your three middle fingers held at arm's length. (Another way to think about angular distances in the sky is to remember that the Moon's size is about half a degree.) At the start of December, the planets will be two degrees apart, and still moving closer together.
A few days before December 21, a thin Crescent Moon will pass close to Jupiter and Saturn in the sky. In the Western Hemisphere, the Moon will be closest to the two planets on December 16. In the Eastern Hemisphere, it will be closest on December 17.
Just 0.1 Degrees Apart on December 21
On December 21, the day of the conjunction, Jupiter and Saturn will be separated by a mere 0.1 degrees, and may appear as a single bright “star.” The two planets are completely merged together on our Night Sky Map for this date, although a careful observer should be able to separate them in the sky with the naked eye. It will be the closest great conjunction since July 16, 1623.
The pair of planets will become visible at twilight, close to the southwestern horizon in the Northern Hemisphere, or the western horizon in the Southern Hemisphere. They will set within a couple of hours or so, so it is important to have a clear view toward the horizon.
Jupiter and Saturn will continue to be an impressive sight in the early evenings following December 21. During January 2021, however, the two planets will become lost in the glare of the Sun.
Why Do Conjunctions Happen?
The solar system is the shape of a thin disk: the Earth, the Moon, and the planets orbit the Sun in roughly the same plane. Astronomers call this plane the ecliptic. Because of this alignment, the Sun, the Moon, and the five planets visible to the naked eye—Mercury, Venus, Mars, Jupiter, and Saturn—follow the same general path as they travel across the Earth's sky.
This is why the Sun, the Moon, and the planets sometimes meet in the sky. These meetings are conjunctions.
How Often Do Conjunctions Occur?
Conjunctions involving the Moon happen frequently. As it circles the Earth once a month, the Moon passes each of the planets in the sky. When the Moon passes the Sun, the result can be an eclipse (which is where the term ecliptic comes from). The reason an eclipse doesn't happen every month is that the Moon's orbit is slightly tilted in relation to the ecliptic, and it normally passes above or below the Sun.
A conjunction of Jupiter and Saturn, on the other hand, is relatively rare: it only happens about once every 20 years. For this reason, it is known as the great conjunction.
Why Are Jupiter-Saturn Conjunctions So Rare?
Jupiter and Saturn are much farther away than the other naked-eye planets. This means they move more slowly, because an object's orbital speed decreases with distance. The Earth takes one year to complete one orbit of the Sun, but Jupiter takes 12 years, and Saturn takes 30 years.
As a result of their long orbits, Jupiter and Saturn meet in the sky only once every 20 years. In this period of time, Saturn completes two-thirds of its 30-year orbit (since 20 is two-thirds of 30). In the same period, Jupiter completes one 12-year orbit, plus, in the remaining 8 years, two-thirds of its next orbit (since 8 is two-thirds of 12). In other words, 20 years is the time it takes Jupiter to catch up and pass Saturn again as they circle the Sun.
The above figures are round numbers. More precise figures are 11.86 years for Jupiter's orbital period, 29.46 years for Saturn's orbital period, and 19.86 years for the average frequency of a great conjunction.
Occasionally, Jupiter appears to pass Saturn three times, in a zigzag fashion. This phenomenon, which is called a triple conjunction, is an illusion caused by the Earth's own movement around the Sun. The most recent triple conjunction was in 1980-81, when Jupiter passed Saturn on December 31, 1980—and then again on March 4 and July 24, 1981.
When Will the Next Great Conjunctions Take Place?
After 2020, the next great conjunctions will occur on November 2, 2040 and April 7, 2060. On both these occasions, the minimum separation of Jupiter and Saturn will be 1.1 degrees—which means they will be eleven times farther apart than on December 21, 2020.
In fact, the 2020 great conjunction of Jupiter and Saturn is exceptionally close. Over a period of one thousand years, from 1600 to 2599, there are only six great conjunctions where the minimum separation between Jupiter and Saturn is less than 0.2 degrees: 1623, 1683, 2020, 2080, 2417, and 2477.
Does Jupiter Ever Pass Directly In Front of Saturn?
On extraordinarily rare occasions, a great conjunction is so close that Jupiter passes directly in front of Saturn. If Jupiter partially obscures Saturn, the event is known as a transit. If Jupiter completely covers Saturn, it is called an occultation.
Over the next 10,000 years, Jupiter will transit or occult Saturn three times: February 16, 7541 (a transit); June 17, 7541 (an occultation); and February 25, 8674 (a transit).