Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth the mass of the Sun. Jupiter is the third-brightest natural object in the Earth's night sky after the Moon and Venus. People have been observing it since prehistoric times; it was named after the Roman god Jupiter, the king of the gods, because of its observed size.
The Olympic spirit Bethor rules over Jupiter.
Observation
Observation of Jupiter dates back to at least the Babylonian astronomers of the 7th or 8th century BC. Trapezoidal rule was used by Babylonians before 50 BC for integrating the velocity of Jupiter along the ecliptic.
The ancient Chinese knew Jupiter as the "Suì Star" and established their cycle of 12 earthly branches based on the approximate number of years it takes Jupiter to rotate around the Sun; the Chinese language still uses its name (simplified as 歲) when referring to years of age. By the 4th century BC, these observations had developed into the Chinese zodiac, and each year became associated with a Tai Sui star and god controlling the region of the heavens opposite Jupiter's position in the night sky.
These beliefs survive in some Taoist religious practices and in the East Asian zodiac's twelve animals. The Chinese historian Xi Zezong has claimed that Gan De, an ancient Chinese astronomer, reported a small star "in alliance" with the planet, which may indicate a sighting of one of Jupiter's moons with the unaided eye. If true, this would predate Galileo's discovery by nearly two millennia.
In his 2nd century work the Almagest, the Hellenistic astronomer Claudius Ptolemaeus constructed a geocentric planetary model based on deferents and epicycles to explain Jupiter's motion relative to Earth, giving its orbital period around Earth as 4332.38 days, or 11.86 years.
In 1610, Italian polymath Galileo Galilei discovered the four largest moons of Jupiter (now known as the Galilean moons) using a telescope. This is thought to be the first telescopic observation of moons other than Earth's.
Moons
There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024. This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes.
All together, Jupiter's moons form a satellite system called the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto.
The Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century.
Exploration
Nine spacecraft have visited Jupiter. The first were Pioneer 10 in 1973, and Pioneer 11 a year later, taking low-resolution images of the four Galilean moons and returning data on their atmospheres and radiation belts. The Voyager 1 and Voyager 2 probes visited Jupiter in 1979, discovering the volcanic activity on Io and the presence of water ice on the surface of Europa. Ulysses further studied Jupiter's magnetosphere in 1992 and then again in 2000.
The Galileo spacecraft was the first to enter orbit around Jupiter, arriving in 1995 and studying it until 2003. During this period, Galileo gathered a large amount of information about the Jovian system, making close approaches to all of the Galilean moons and finding evidence for thin atmospheres on three of them, as well as the possibility of liquid water beneath the surfaces of Europa, Ganymede, and Callisto. It also discovered a magnetic field around Ganymede.
Then the Cassini probe to Saturn flew by Jupiter in 2000 and collected data on interactions of the Galilean moons with Jupiter's extended atmosphere. The New Horizons spacecraft flew by Jupiter in 2007 and made improved measurements of its satellites' orbital parameters.
In 2016, the Juno spacecraft imaged the Galilean moons from above their orbital plane as it approached Jupiter orbit insertion, creating a time-lapse movie of their motion. With a mission extension, Juno has since begun close flybys of the Galileans, flying by Ganymede in 2021 followed by Europa and Io in 2022. It flew by Io again in late 2023 and once more in early 2024.
Astrology
It takes Jupiter about 12 years to circle the zodiac (the planet visits an average of one sign a year). It is masculine energy and rules both Sagittarius and Pisces, and the Ninth and Twelfth Houses.
Jupiter is the thinking-person’s planet. As the guardian of the abstract mind, this planet rules higher learning, and bestows upon us a yen for exploring ideas, both intellectually and spiritually. Intellectually speaking, Jupiter assists us in formulating our ideology. In the more spiritual realm, Jupiter lords over religions and philosophy. A search for the answers is what Jupiter proposes, and if it means spanning the globe to find them, well, that’s probably why Jupiter also rules long-distance travel. In keeping with this theme, Jupiter compels us to assess our ethical and moral values; it also addresses our sense of optimism.
Astrologically, luck and good fortune are associated with Jupiter. This is a kind and benevolent planet, one that wants us to grow and flourish in a positive way. Jupiter may be judge and jury, but it’s mostly an honorable helpmate, seeing to it that we’re on the right path. While our success, accomplishments and prosperity are all within Jupiter’s realm, this largesse can, at times, deteriorate into laziness and sloth (Jupiter, at its worst, is associated with weight gain!). More often than not, however, Jupiter will guide us down the primrose path.
Leisure time is also strongly associated with Jupiter. Sports of all kinds, games of chance and a stroll in the park with the family pet (Jupiter loves animals) –- these are all ruled by this planet. Finally, Jupiter often presages great wealth, material and otherwise. This is a good friend in the heavens.
In Kabbalah, Jupiter rules over the sefirot of chesed and its opposing qlippoth Gha’ag Sheklah.