Step outside after sunset this month and you’ll see a brilliant gold-white star high, far brighter than any other, high in the east. It’s not the legendary star in the east that the three magi supposedly followed to a manger outside Bethlehem. It’s not even a star at all. It’s the planet Jupiter.
Jupiter is the largest planet in our solar system, and descriptions of it can involve lots of big numbers. It has over 90 moons. It’s 11 times the size of Earth with a diameter of 88,000 miles. Thirteen hundred Earths could fit inside it. In fact, all the other planets could fit inside Jupiter.
Like Saturn, the other planet to watch in the winter night sky, Jupiter is what astronomers call a gas giant. This means it has a thick, dense atmosphere of hydrogen mixed with some helium and trace amounts of ammonia and methane. Our own atmosphere has a depth of about 60 miles; Jupiter’s is 1,900 miles.
The pressure and heat of such a deep atmosphere create strange chemistry. At the lowest altitudes, the pressure compresses the gaseous hydrogen so much that it transitions into an immense sea of liquid hydrogen. Farther in, as pressure and heat soar, electrons are stripped from nuclei, transforming the liquid hydrogen into a bizarre metallic superfluid where electricity flows freely. Theoretically, Jupiter has a core made of nickel and iron like our own and larger than the Earth.
If we zoom out to a comfortable distance of 10,000 miles or so, the planet presents a visual spectacle. The topmost layer of its atmosphere contains clouds of ammonia crystals. Jupiter’s fast rotational speed (it rotates once every 10 hours as opposed to our 24 hours) causes powerful jet streams and turbulence. Sheared by these forces, the clouds separate into many distinct bands. Other material, like hydrocarbons, sulphur, and phosphorus, rises from the depths and reacts with ultraviolet light from the Sun, taking on different colors. This colorful material then gets caught up with the motion of the cloud bands, creating Jupiter’s whirling, striped appearance.
The frenzied activity in Jupiter’s uppermost layer reaches its apex in the Great Red Spot — a most underwhelming name for such an astonishing phenomenon. This mighty storm, larger than two Earths, has been raging at least since 1831 and possibly since 1665, when the Italian astronomer Giovanni Cassini first recorded observations of a “permanent spot” in Jupiter’s south equatorial belt. The Great Red Spot is an anti-cyclone, a storm rotating counterclockwise, with wind speeds of over 250 miles per hour. You can see it with any backyard telescope. And I recommend seeing it while you can. The storm appears to be at last diminishing in size and intensity. It may go on for only another century or so.
While looking for the Great Red Spot in a telescope, you can’t miss the four largest of Jupiter’s moons. These four — Io, Europa, Ganymede, and Callisto — appear as pinprick stars in line on one or both sides of the planet.
In 1610, Galileo Galilei first pointed one of the earliest telescopes at Jupiter. Besides Jupiter’s cloud bands, he observed these four “stars.” He also observed their motion.
Stay at the telescope for an hour or two and you’ll witness the moons changing position as they orbit Jupiter. One might disappear behind the planet, while another appears on the other side. This clockwork procession around Jupiter provided the first empirical proof that Earth was not the center of the universe.
Galileo got into serious trouble with the Catholic Church for promoting his observations and deductions. In honor of his dedication to science and evidence-based thinking, these four satellites are known now as the Galilean Moons.
Jupiter’s name, like that of Saturn, also comes to us from the Romans. They called it the Star of Jupiter, a god of the Roman pantheon. Other cultures have associated this bright star with important mythological figures. Ancient Hindu astrologers called it Brihaspati, who was the teacher and counselor of their gods. The Babylonians connected the star with Marduk, who, like Jupiter (and the Greeks’ Zeus before him), was the king of their pantheon.
Jupiter will shine after sunset until early spring. You can also still see Saturn (details on how can be found in last month’s column), but it is already dipping toward the horizon at sunset and setting earlier every night. By the end of January, Saturn will vanish for a time as it moves behind the Sun from our point of view, leaving Jupiter alone to rule the early evening sky. For a time.
Come spring, Jupiter will vanish behind the Sun, too, giving another planet a chance to claim the title of brightest evening star. Venus perhaps? Clear skies!