Classical planet in the context of "Fixed star"

Geocentrism is a superseded astronomical model description of the Universe with Earth at the center. It is also known as the geocentric model, often exemplified specifically by the Ptolemaic system. Under most geocentric models, the Sun, the Moon, stars, and planets all orbit Earth. The geocentric model was the predominant description of the cosmos in many European ancient civilizations, such as those of Aristotle in Classical Greece and Ptolemy in Roman Egypt, as well as during the Islamic Golden Age.

Two observations supported the idea that Earth was the center of the Universe. First, from anywhere on Earth, the Sun appears to revolve around Earth once per day. While the Moon and the planets have their own motions, they also appear to revolve around Earth about once per day. The stars appeared to be fixed on a celestial sphere rotating once each day about an axis through the geographical poles of Earth. Second, Earth seems to be unmoving from the perspective of an earthbound observer; it feels solid, stable, and stationary.

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.

The Wǔfāng Shàngdì (Chinese: 五方上帝 "Five Regions' Highest Deities" or "Highest Deities of the Five Regions"), or simply Wǔdì (五帝; "Five Deities") or Wǔshén (五神; "Five Gods") are, in Chinese canonical texts and common Chinese religion, the fivefold manifestation of the supreme God of Heaven (天; Tiān, or equivalently 上帝; Shàngdì). This theology dates back at least to the Shang dynasty. Described as the "five changeable faces of Heaven", they represent Heaven's cosmic activity which shapes worlds as 壇; tán, "altars", imitating its order which is visible in the starry vault, the north celestial pole and its spinning constellations. The Five Deities themselves represent these constellations. In accordance with the Three Powers (三才; Sāncái) they have a celestial, a terrestrial and a chthonic form. The Han Chinese identify themselves as the descendants of the Red and Yellow Deities.

They are associated with the five colors, the five phases of the continuous creation (五行; Wuxing), the five classical planets of the Solar System and the five constellations rotating around the celestial pole, the five sacred mountains and five directions of space (their terrestrial form), and the five Dragon Gods (龙神; Lóngshén) who represent their mounts, that is to say the material forces they preside over (their chthonic form). They have also been defined simply as five special forms of the worship of the God of Heaven, different "accesses" or perspectives, suitable for different situations, to serve Heaven.

The celestial spheres, or celestial orbs, were the fundamental entities of the cosmological models developed by Plato, Eudoxus, Aristotle, Ptolemy, Copernicus, and others. In these celestial models, the apparent motions of the fixed stars and planets are accounted for by treating them as embedded in rotating spheres made of an aetherial, transparent fifth element (quintessence), like gems set in orbs. Since it was believed that the fixed stars were unchanging in their positions relative to one another, it was argued that they must be on the surface of a single starry sphere.

In astrology, planets have a meaning different from the astronomical understanding of what a planet is. Before the age of telescopes, the night sky was thought to consist of two similar components: fixed stars, which remained motionless in relation to each other, and moving objects/"wandering stars" (Ancient Greek: ἀστέρες πλανῆται, romanized: asteres planetai), which moved relative to the fixed stars over the course of the year(s).

To the Ancient Greeks who learned from the Babylonians, the earliest astronomers/astrologers, this group consisted of the five planets visible to the naked eye and excluded Earth, plus the Sun and Moon. Although the Greek term planet applied mostly to the five 'wandering stars', the ancients included the Sun and Moon as the Sacred 7 Luminaires/7 Heavens (sometimes referred to as "Lights",) making a total of 7 planets. The ancient Babylonians, Greeks, Persians, Romans, Medieval Christians, and others thought of the 7 classical planets as gods and named their 7 days of the week after them. Astrologers retain this definition of the 7 classical planets today.