Even though its orbit was dominated by an influx of earthly struggles, 2020 was an incredible year for astronomy. Whether it was SpaceX making headlines by launching Starlink satellites to provide WiFi for the world or sending astronauts to the International Space Station (twice!) or the unexpected delight of watching Comet NEOWISE journey across the sky, there were some inspiring reasons to keep gazing up throughout the year.
The roller coaster of a year wrapped up with a massive astronomical event that was centuries in the making: the Great Planetary Conjunction of Jupiter and Saturn on Dec. 21, 2020. This celestial event offered a once-in-a-lifetime opportunity to see the gas giants of our solar system appear so close together Earth's night sky that they looked to be touching. (They were actually more than 400 million miles apart — it's all a matter of perspective!)
A planetary conjunction occurs when two or more planets appear close together in the sky from an Earthly perspective. This alignment is a result of their orbits, creating a striking visual phenomenon. While the planets maintain their individual positions in space, they seem to converge due to their orbital motion and our viewing angle.
Conjunctions can be visible to the naked eye and are captivating astronomical events. They provide insights into celestial mechanics and offer opportunities for stargazers to witness rare and beautiful displays of cosmic proximity.
The Great Conjunction
A great planetary conjunction, often referred to as a "Great Conjunction," specifically involves the two largest gas giants in our solar system, Jupiter and Saturn. This rare event occurs when the planets appear exceptionally close to each other in the sky, as a result of their slower orbits.
Based on their orbits, Jupiter (which orbits the sun every 11.9 Earth years) and Saturn (every 29.5 Earth years) appear close together roughly every 19.6 Earth years.
The most famous of these occurred on Dec. 21, 2020, when the two planets appeared as a "Christmas Star" from Earth. But what does this event have to do with Christmas?
The conjunction coincided with the winter solstice, leading some to draw parallels with the biblical Star of Bethlehem, which is said to have guided the Three Wise Men to the birthplace of Jesus. The term "Christmas Star" captures the symbolic and visual significance of this rare astronomical event during the holiday season.
A "Close" Call 800 Years in the Making
The 2020 Great Conjunction was the closest these two planets had appeared in the sky since the 13th century. "This conjunction is exceptionally rare because of how close the planets will appear to one another," said Patrick Hartigan, an astronomer at Rice University in Texas in a press release. "You had to go all the way back to just before dawn on March 4, 1226, to see a closer alignment between these objects visible in the night sky."
Basically, it had been nearly 800 years since Jupiter and Saturn had appeared this close together, due to the two planets' orbits — as well as our earthly one. (Although there was a close Jupiter-Saturn conjunction in 1623, it was too near to the sun to be seen without a telescope and likely wasn't observed by many — the telescope being a new object at the time.)
The planets appeared very close: less than a fifth of the diameter of a full moon, or roughly 0.1 degrees apart in the sky. (Astronomers use degrees as the largest unit of distance between objects in the sky; most times when two planets appear close together, they're within 2–4 degrees of one another.)
With Saturn and Jupiter being so close together, they might have looked like a double planet.
"For most telescope viewers, each planet and several of their largest moons were visible in the same field of view that evening," Hartigan said at the time.
Viewers who didn't have a telescope were still able to see the event with a pair of binoculars, a clear horizon and some patience.
The Great Conjunction was visible across Earth, though the timing depended on your location and latitude. The best viewing prospects were near the equator, but those in the Northern Hemisphere had a shorter viewing window before the planets set beyond the horizon.
For U.S. viewers, the best time to observe was around twilight. People could have started looking from Dec. 17 through Christmas, though the closest approach was on Dec. 21.
And if you missed out on the very special 2020 Great Conjunction, you'll have to wait till March 15, 2080 (just 50-ish years from now), to see the gas giants get super up close and personal again.
Planetary Alignment in the Sky
The frequency of planets aligning in the evening sky varies depending on their orbits. Some planets line up more frequently due to their shorter orbits and relatively close proximity to each other in the solar system.
For example, Mercury and Venus, which orbit closer to the sun than Earth, often appear near the sun in the sky. They can sometimes be seen in the evening, shortly after sunset, or in the morning before sunrise.
On the other hand, outer planets like Mars, Jupiter and Saturn have longer orbits and appear farther from the sun. Consequently, they align less often in the evening sky. Jupiter and Saturn's conjunctions, like the Great Conjunction, are relatively rare and occur once every few decades.
Overall, the frequency of planets aligning in the evening sky can range from a few times a year for inner planets like Mercury and Venus, to more sporadic occurrences for outer planets like Mars, Jupiter and Saturn.
'Naked Eye' Planets
Several planets can be seen with the naked eye from Earth. Keep in mind that their visibility depends on factors such as their current positions in their orbits, the time of year and local light pollution.
Venus: Often referred to as the "Evening Star" or "Morning Star," Venus is the brightest planet in the sky and can be seen shortly after sunset or before sunrise.
Mars: Known for its reddish hue, Mars is visible as a bright point of light and can vary in brightness over time.
Jupiter: A giant gas planet, Jupiter is one of the brightest objects in the night sky. Its four largest moons, known as the Galilean moons, are visible through binoculars or a small telescope.
Saturn: Recognizable by its distinct ring system, Saturn is another prominent planet visible to the naked eye. You can observe its rings by using a telescope.
Mercury: The closest planet to the sun can appear briefly in the evening sky after sunset or in the morning sky before sunrise. It is often more challenging to spot due to its proximity to the sun.
Uranus and Neptune: These two planets are also visible to the naked eye under very dark and clear conditions, but they appear as faint points of light and are more easily observed with binoculars or telescopes.
Other Notable Planetary Conjunctions
In March 2023, Jupiter stepped out on Saturn to share a "celestial kiss" with bright Venus. The two planets converged to within approximately half a degree of each other — roughly the span of a full moon. Over preceding weeks, these two entities had been gradually drawing nearer, culminating in a conjunction.
This visual phenomenon, naturally, stemmed from an optical illusion. In actuality, these two luminous planets were separated by millions of miles; only from our terrestrial vantage point did they seem poised for contact. The cyclical paths traced by Earth, Jupiter and Venus lead to these conjunctions occurring around once per year.
This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.
Now That's Interesting
To get a sense of how close Jupiter and Saturn were in the dark sky on Dec. 21, 2020, hold your arm straight out and make a fist. Stick up your pinky finger. At arm's length, the diameter of your pinky finger is roughly equivalent to 1 degree of distance in the sky. Jupiter and Saturn appeared 1/10th of the diameter of your pinky apart!
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