Callisto (moon) in the context of "Impact crater"

Gan De (Chinese: 甘德; fl. 4th century BC), also known as the Lord Gan (Gan Gong), was an ancient Chinese astronomer and astrologer born in the State of Qi. Along with Shi Shen, he is believed to be the first in history known by name to compile a star catalogue, preceded by the anonymous authors of the early Babylonian star catalogues and followed by the Greek Hipparchus who is the first known in the Western tradition of Hellenistic astronomy to have compiled a star catalogue. He also made observations of the planets, particularly Jupiter. His writings are lost, but some of his works' titles and fragments quoted from them are known from later texts.

Gan De may have been the first to describe one of the Galilean moons of Jupiter, usually invisible without the aid of telescopes. In the 20th century, a fragment of Gan's work, in a later compilation of astronomical texts, was identified by Xi Zezong as describing a naked-eye observation of either of the two largest and brightest moons, Ganymede or Callisto in summer 365 BC.

The Galilean moons (/ˌɡælɪˈleɪ.ən/), or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede, Callisto, Io, and Europa. They are the most readily visible Solar System objects after Saturn, the dimmest of the classical planets; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope allowed astronomers to discover the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.

They are planetary-mass moons and among the largest objects in the Solar System. All four, along with Titan, Triton, and Earth's Moon, are larger than any of the Solar System's dwarf planets. The largest, Ganymede, is the largest moon in the Solar System and surpasses the planet Mercury in size (though not mass). Callisto is only slightly smaller than Mercury in size; the smaller ones, Io and Europa, are about the size of the Moon. The three inner moons — Io, Europa, and Ganymede — are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's remaining moons have irregular forms because they are too small for their self-gravitation to pull them into spheres.

There are 97 known moons of the planet Jupiter. 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, colloquially referred to as the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto, all of which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers (or other sexual partners) or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 93 known moons and the rings together comprising just 0.003% of the total orbiting mass.

Of Jupiter's moons, eight are regular satellites with prograde and nearly circular orbits that are not greatly inclined with respect to Jupiter's equatorial plane. The Galilean satellites are nearly spherical in shape due to their planetary mass, and are just massive enough that they would be considered planets if they were in direct orbit around the Sun. The other four regular satellites, known as the inner moons, are much smaller and closer to Jupiter; these serve as sources of the dust that makes up Jupiter's rings. The remainder of Jupiter's moons are outer irregular satellites whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities. The largest of these moons were likely asteroids that were captured from solar orbits by Jupiter before impacts with other small bodies shattered them into many kilometer-sized fragments, forming collisional families of moons sharing similar orbits. Jupiter is expected to have about 100 irregular moons larger than 1 km (0.6 mi) in diameter, plus around 500 more smaller retrograde moons down to diameters of 0.8 km (0.5 mi). Of the 89 known irregular moons of Jupiter, 40 of them have not yet been officially given names.

Xi Zezong (June 6, 1927, Yuanqu, Shanxi – December 27, 2008, Beijing) was a Chinese astronomer, historian, and translator. He was a member of the Chinese Academy of Sciences, and an awardee of the Astronomy Prize.

He identified a possible reference to one of the Galilean moons of Jupiter in the fragmentary ancient works of the 4th-century BC Chinese astronomer Gan De, who may have made observation of either Ganymede or Callisto in summer 365 BC.

The following is a list of Solar System objects by orbit, ordered by increasing distance from the Sun. Most named objects in this list have a diameter of 500 km or more.

• The Sun, a spectral class G2V main-sequence star
• The inner Solar System and the terrestrial planets
  • Mercury
    • Mercury-crossing minor planets
  • Venus
    • Venus-crossing minor planets
      • Zoozve, Venus' quasi-satellite
  • Earth
    • Moon
    • Near-Earth asteroids
      • 2021 PH27
      • Apophis
      • Eros
    • Earth trojans (2010 TK7)
    • Earth-crosser asteroids
      • Earth's quasi-satellites
  • Mars
    • Deimos
    • Phobos
    • Mars trojans
    • Mars-crossing minor planets
  • Asteroids in the asteroid belt, between the orbits of Mars and Jupiter
    • Ceres, a dwarf planet
    • Vesta
    • Pallas
    • Hygiea
    • Europa (asteroid)
    • Davida
    • Sylvia
    • Asteroids number in the hundreds of thousands. For longer lists, see list of exceptional asteroids, list of asteroids, or list of Solar System objects by size.
      • Asteroid moons
  • A number of smaller groups distinct from the asteroid belt
• The outer Solar System with the giant planets, their satellites, trojan asteroids and some minor planets
  • Jupiter
    • Rings of Jupiter
    • Complete list of Jupiter's natural satellites
      • Galilean moons
        • Io
        • Europa (moon)
        • Ganymede
        • Callisto
    • Jupiter trojans
    • Jupiter-crossing minor planets
  • Saturn
    • Rings of Saturn
    • Complete list of Saturn's natural satellites
      • Mimas
      • Enceladus
      • Tethys (trojans: Telesto and Calypso)
      • Dione (trojans: Helene and Polydeuces)
      • Rhea
        • Rings of Rhea (largely disproven)
      • Titan
      • Hyperion
      • Iapetus
      • Phoebe
      • Shepherd moons
    • Saturn-crossing minor planets
  • Uranus
    • Rings of Uranus
    • Complete list of Uranus's natural satellites
      • Miranda
      • Ariel
      • Umbriel
      • Titania
      • Oberon
    • Uranus trojan (2011 QF99)
    • Uranus-crossing minor planets
  • Neptune
    • Rings of Neptune
    • Complete list of Neptune's natural satellites
      • Proteus
      • Triton
      • Nereid
    • Neptune trojans
    • Neptune-crossing minor planets
  • Non-trojan minor planets
    • Centaurs
      • Chiron
      • Chariklo
    • Damocloids
  • Trans-Neptunian objects (beyond the orbit of Neptune)
    • Kuiper-belt objects (KBOs)
      • Plutinos
        • Orcus, a dwarf planet
          • Vanth
        • Pluto, a dwarf planet
          • Complete list of Pluto's natural satellites
            • Charon
        • Ixion
        • Achlys
      • Twotinos
      • Cubewanos (classical objects)
        • Haumea, a dwarf planet
          • Namaka
          • Hiʻiaka
        • Quaoar, a dwarf planet
          • Weywot
        • Makemake, a dwarf planet
          • S/2015 (136472) 1
        • Máni
        • Salacia
          • Actaea
        • Aya
        • Varda
          • Ilmarë
        • Varuna
        • Ritona
        • Arrokoth
    • Scattered-disc objects
      • Gonggong, a dwarf planet
        • Xiangliu
      • Eris, a dwarf planet
        • Dysnomia
      • Chiminigagua
      • (84522) 2002 TC302
      • (87269) 2000 OO67
      • 2015 TH367/V774104
    • Detached objects
      • (612911) 2004 XR190
      • 2012 VP113 (possibly inner Oort cloud)
      • Sedna, a dwarf planet (possibly inner Oort cloud)
    • Oort cloud (hypothetical)
      • Hills cloud/inner Oort cloud
      • Outer Oort cloud

The far side of the Moon, also called the dark side of the Moon, is the hemisphere of the Moon that faces away from Earth; the opposite hemisphere being the near side. Due to tidal locking, the time it takes for the Moon to orbit the Earth once is equal to the time it takes for the Moon to rotate once, thus, the far side of the Moon never fully comes into view from Earth's surface. The far side has sometimes been called the "dark side of the Moon", where "dark" means "unseen" instead of "unilluminated" – despite a common misconception that the dark side of the Moon is so-called because it never receives light, each location on the Moon experiences two weeks of sunlight while the opposite location experiences night. The far side is actually more reflective than the near side, as it lacks the large areas of darker maria surface.

About 18 percent of the far side is occasionally visible from Earth due to oscillation and to libration. The remaining 82 percent remained unobserved until 1959, when it was photographed by the Soviet Luna 3 space probe. The Soviet Academy of Sciences published the first atlas of the far side in 1960. The Apollo 8 astronauts were the first humans to see the far side in person when they orbited the Moon in 1968. Compared to the near side, the far side's terrain is rugged, with a multitude of impact craters and relatively few flat and dark lunar maria ("seas"), giving it an appearance closer to other barren places in the Solar System such as Mercury and Callisto. It has one of the largest craters in the Solar System, the South Pole–Aitken basin.

A multi-ringed basin (also a multi-ring impact basin) is not a simple bowl-shaped crater, or a peak ring crater, but one containing multiple concentric topographic rings; a multi-ringed basin could be described as a massive impact crater, surrounded by circular chains of mountains resembling rings on a bull's-eye. A multi-ringed basin may have an area of many thousands of square kilometres.

An impact crater of diameter bigger than about 180 miles (290 km) is referred to as a basin.