The conjunction of Jupiter and Saturn on December 21, 2020, marks the closest they have been seen in the sky since 1623. Known as a Great Conjunction, this phenomenon was once regarded as an omen. Nowadays, it is a good reminder of how these two giant planets have appealed to the human imagination and propelled exploration of worlds in the solar system and beyond.
In Roman mythology, Jupiter (on the right) is the king of gods while Saturn (left) is the god of time. With their slow march across the sky (a cycle of roughly 12 years for Jupiter and 29 years for Saturn), and with Jupiter being one the brightest celestial objects visible to the naked eye, the two planets caused awe and wonder since ancient times and across cultures. In Western astrology, the conjunction of the two planets, or Great Conjunction, was regarded as an omen signaling both good and bad. Shown here are two pages from the first printed edition of an influential treatise on this topic titled The Great Conjunctions, originally written in the 9th century by the Persian astrologer Albumasar.
Until the 17th century, Saturn, Jupiter, and the other planets visible to the unaided eye (Mercury, Venus, and Mars) were just dots of light that moved against the so-called “fixed stars.” This would change dramatically with the development of the telescope from the early 1600s onwards. In this image from 1651, angelic creatures hold the Moon (bottom), Jupiter (just above), and Saturn (top right), which sport features revealed by the telescope in the preceding years. Note how Jupiter is depicted with two bands across its disc and surrounded by the four satellites discovered by Galileo, while Saturn is surrounded by a ring.
A Planetary Mystery
This illustration from Christiaan Huygens’s "Systema Saturnium", published in 1659, presents different telescopic depictions of Saturn recorded by several observers over the preceding decades. Various hypotheses were put forward to explain Saturn’s intriguing “anses” or “handles”—the terms given to the features around the planet’s disc seen through the telescope. Huygens came up with an explanation that is essentially correct: Saturn is surrounded by a ring that produces—combined with the tilt of its axis and its orbital motion—varying appearances as seen from Earth.
The invention of the telescope in the 17th century also enabled astronomers to learn more about the sizes of the planets. The central illustration in this 19th-century educational chart shows the relative sizes of the planets of the solar system. Though fancifully colored and not totally accurate by today’s standards, it conveys a sense of proportion that is essentially correct. Jupiter, first on the left, evidently outsizes all the other planets, with Saturn (a little larger than we would depict it today) coming second. Both Jupiter and Saturn are significantly larger than the other two gas giants, Neptune and Uranus, third and fourth respectively (Neptune is actually slightly smaller than Uranus). Our Earth, as well as the other rocky planets and the Moon (shown on the right end of the diagram), are way smaller in comparison.
The Most Famous Spot in Astronomy
The Great Red Spot is one of Jupiter’s most distinctive and iconic features. It corresponds to a high-pressure region in the planet’s atmosphere that produces an enduring anticyclonic storm. It shows prominently in this watercolor painting, which is based on observations of Jupiter carried out at Chicago’s first Dearborn Observatory in 1880. Though spots on Jupiter have been reported over the last 350 years or so, it was only in the late 1870s that systematic observations of the Great Red Spot began, showing that this feature has persisted at least since that time to this day. Also noticeable in the image (which has the south at the top to show the planet as seen through a refracting telescope) are Jupiter’s typical pattern of alternating dark and light bands (respectively called “belts” and “zones”), plus the shadows of two of its satellites, one of which falls within the Great Red Spot.
In 1944, Chesley Bonestell painted a series of images for Life magazine in which he imagined how Saturn would look as seen from its satellites. This particular painting, often cited as the most iconic work of space art, shows Saturn from the perspective of an observer standing on the surface of its largest satellite, Titan. Bonestell combined his experience as an architectural draftsman and Hollywood matte painter with the astronomical knowledge available at the time to produce this striking rendition. Now we know that Titan is surrounded by a dense atmosphere that would render such a sight impossible. But we discovered this fact in large part thanks to Bonestell's inspiring imagination. His compelling and realistic portraits of otherworldly landscapes got scientists, engineers, and the public equally wired for the Space Age, showing the worth of sending spacecraft to those distant places so that we could learn more about them.
Jupiter and Beyond
This scene showing a spacecraft approaching Jupiter was painted by Chesley Bonestell to illustrate Arthur C. Clarke’s book Beyond Jupiter: Worlds of Tomorrow, published in 1972. In this book Clarke addresses what was known about the outer planets (Jupiter, Saturn, Uranus, Neptune) at the time, and explores what might be found by space probes sent to those distant worlds. In the same year that Beyond Jupiter was published, the Pioneer 10 probe was launched. It became the first spacecraft to cross the asteroid belt and to obtain close-up images of Jupiter.
Jupiter: The Biggest Boi
Jupiter is the largest planet in our solar system by quite a bit - it contains more than two times the mass of all of the other planets in the solar system combined! Even small telescopes reveal some interesting features of this gas giant: Jupiter has horizontal stripes called cloud bands, formed by strong jet streams that run side to side and separate the planet’s clouds into darker and brighter regions. A small or mid-size telescope can also reveal the Great Red Spot, a storm twice as wide as the earth with winds up to 400mph.
Saturn: The Solar System Put a Ring on It
Saturn, the second largest planet in our solar system, is most well known for its spectacular rings. While it isn’t the only planet with rings - Jupiter, Uranus, and Neptune also have rings - Saturn’s are by far the easiest to see. While Saturn’s rings look like a single solid disc from a distance, they’re formed of thousands of separate rings and made of particles that range from the size of a grain of sand to the size of a mountain. They extend 175,000 miles from the planet, but are relatively thin by comparison: they’re between just 30 to 300 feet in height.
Dr. Shawn Brooks - Wild Flights of Fancy
Dr. Shawn Brooks currently works at NASA's Jet Propulsion Laboratory as one of the Investigation Scientists on NASA's future Europa Clipper mission: investigating the suitability of Europa, the smallest of Jupiter's four Galilean satellites, as a potential abode for extraterrestrial life within our solar system. Dr. Brooks has been interested in astronomy in general, and specifically planetary science, since his days as a schoolchild in Queens, New York. “I recall poring over outdated science books about our solar system that must have predated our first spacecraft missions to the planets of our solar system. The wild flights of fancy about what might exist underneath the clouds of Venus and Jupiter and how Saturn's rings came to be stoked my imagination and set me on the path to my current career.”
Posing New Mysteries - Dr. Brooks
Having studied the surfaces of Jupiter's moons and the nature of Jupiter rings as a graduate student, Dr. Brooks came to JPL to work on the Cassini mission to Saturn. His work on Cassini afforded him the opportunity to help plan, design and analyze hundreds observations of Saturn's rings and moons. “Looking forward, I am very excited to be a part of NASA's return to Jupiter's moon Europa. The data that will be obtained will be the first close-up look at Europa that we have had since the Galileo mission to Jupiter. It will answer questions raised while I was a graduate student and certainly pose many new mysteries.” In addition, Dr. Brooks has been serving the planetary science community as a member of the Planetary Culture and Climate subcommittee of the Division for Planetary Sciences, a committee that advises the Division for Planetary Science committee and the broader community on issues related to equity, diversity and inclusion.
Dr. Emily Martin - The Search for Habitable Environments
Dr. Emily Martin is a planetary geologist in the Center for Earth and Planetary Studies in the Smithsonian Institution’s National Air and Space Museum. Dr. Martin had no idea she would grow up to be a scientists when she was a child: but she discovered the field of planetary science in her senior year of college when she was struck by the wonderfully weird moons of Jupiter and Saturn. These moons are mostly made up of water ice and have cold, salty oceans in their interiors. ‘Ocean’ worlds are exciting targets in the search for environments that might be habitable.
Mapping a Moon - Dr. Martin
Dr. Martin is most interested in how the surfaces of these icy moons fracture and crack which can tell us a lot about the oceans on the inside. Places like Saturn’s moon Enceladus are extra exciting because some of that ocean is erupting out of Enceladus’s south pole draping the surface with a snow-like material and feeding Saturn’s E-ring. Dr. Martin also explores the surfaces of icy moons by creating geological maps. She is currently leading a project to create a map of the surface of Triton, Neptune’s largest moon.
Dr. Lynnae C. Quick - Modeling the Physics of Eruptions
Dr. Lynnae C. Quick is a planetary geophysicist and Ocean Worlds Planetary Scientist at NASA Goddard Space Flight Center. She models geological processes, specifically cryovolcanic eruptions, on the moons of Jupiter and Saturn and on dwarf planets such as Ceres. Dr. Quick also studies volcanism on Venus and Earth's Moon and uses knowledge of our solar system's planets and moons to characterize low-mass extrasolar planets. She is a member of several teams that will send spacecraft to the Jupiter and Saturn systems, including NASA’s Europa Clipper Mission and the Dragonfly Mission to Titan.
The Coolest Application of Physics - Dr. Quick
“On Earth, we’re used to volcanic eruptions, during which molten rock and ash erupt. The icy moons of Jupiter and Saturn have cryovolcanism, during which briny mixtures of water and ice erupt instead. We can think of cryovolcanism as an eruption of icy slush. Studying the process of cryovolcanism, how it works, where it happens, and how it differs from classical volcanism really excites me. It’s the coolest way I can think of (no pun intended!) to apply my physics and math background to learn about Jupiter and Saturn’s moons and icy worlds around other stars.” - Dr. Lynnae Quick
Jupiter and Saturn Through the Looking Glass
Amateur observing tools have advanced far beyond the days in which Galileo looked through his telescope and thought Saturn’s rings looked like ears. Small telescopes and binoculars can allow us to observe these two gas giants in more detail. We can see rings, cloud bands, and even moons! Binoculars alone can reveal the four brightest moons of Jupiter that we call the Galilean moons: Io, Ganymede, Callisto, and Europa. Small telescopes can also see Saturn’s brightest moon, Titan. Jupiter and Saturn continue to be some of the best objects for amateur observers to view and photograph.
When can you see Jupiter and Saturn?
Jupiter and Saturn are the farthest planets from Earth than can be easily observed with just the naked eye. While the conjunction is an exciting time to observe them, they’re easy to find in the sky for many months a year. Because they move around the sun at different speeds than the Earth does, their visibility in our sky changes from year to year. In 2020, these planets are still visible in the early evening through the end of December. In 2021, they’ll be visible in the evening from August through to the end of January of 2022. In later 2022, expect to see them even later in the year, from September 2022 to January 2023 with Jupiter staying up a bit later until February 2023.
Viewing the Great Conjunction in Your Area
You can easily find Jupiter and Saturn in the sky using Stellarium. Stellarium is a web-based program for learning what’s in your night sky and where. You just need to enter your location, the date, and the time, and Stellarium will show you what is up in the sky at that exact moment. Visit https://stellarium-web.org
Viewing the Great Conjunction in Chicago
The Great Conjunction of Jupiter and Saturn will be visible right after sunset on December 21st, 2021. If you’re in or near Chicago, take a look close to the horizon in your southwest sky. You’ll see two bright objects that look like stars, close enough to each other that you could cover them both with your thumb outstretched at arm’s length. If you are able to observe them with a telescope or binoculars, you can capture them both in the same telescope or binocular view at moderate magnification.
Celebrate the Great Conjunction with Adler
The Adler Planetarium will host several events and continue to create so you can learn more about this once in a lifetime celestial event. Sky Observers Hangout, a live YouTube show about observing the night sky, will host a show all about the conjunction on December 21st, 2020, 5pm CT. Skywatch Weekly will produce a short YouTube video about the conjunction as well. Visit the Adler Planetarium’s YouTube channel for this and more out-of-this-world space content!
Thank you to the staff of the Adler Planetarium for their assistance in creating this exhibition, and to NASA, the Jet Propulsion Lab, and the Smithsonian for the use of their images.
The Adler also thanks Dr. Emily Martin, Dr. Lynnae C. Quick, and Dr. Shawn Brooks for their permission and assistance in creating this exhibition.