Classical planet

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In antiquity the classical planets were the seven non-fixed objects visible in the sky. The classical planets were, therefore, the Sun and Moon and the five planets Mercury, Venus, Mars, Jupiter, and Saturn. The name planet comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky with the other stars. They called these objects asteres planetai, or wandering stars. Together they form the seven classical planets or 7 planets of the ancients,^[1] as well as the names of the seven days of the week - Sun-day, Moon-day, Saturn-day, and in Latin, 'Martis' (Mars, Tuesday), 'Mercurii' (Wednesday), 'Iovis' (Jupiter, Thursday) and 'Veneris' (Venus, Friday).

Mercury and Venus are only visible in twilight hours as their orbits are interior to the Earth's orbit. The third brightest object in the sky, Venus is the most prominent planet. Mercury is more difficult to see due to its proximity to the Sun. Lengthy twilight and an extremely low angle at maximum elongations make optical filters necessary to see Mercury from extreme polar locations.^[2] Mars is at its brightest when it is in opposition, which occurs approximately every twenty-five Earth-months. Jupiter and Saturn are the largest of the five planets, but are farther from the sun, and therefore receive less sunlight. Nonetheless, Jupiter is often the next brightest object in the sky after Venus. Saturn's luminosity is often enhanced by its rings, which reflect light back toward the Earth to varying degrees depending on their inclination to the ecliptic; however, the rings themselves are not visible to the naked eye from the Earth. Uranus and sometimes the asteroid Vesta are visible to the naked eye in principle on very clear nights, but unlike the true naked-eye planets they are always less luminous than several thousands of stars, and as such, do not stand out enough for their existences to be noticed without the aid of a telescope.

History

Babylonian astronomy

Babylonians grouped the stars in companies of seven. References are made to the seven Tikshi, the seven Lumashi, and the seven Mashi, which are older than the signs of the Zodiac; so far as can be ascertained these groups were selected from various constellations. When the five planets were identified, they were associated with the Sun and Moon and connected with the chief gods of the Hammurabi pantheon. A bilingual list in the British Museum arranges the sevenfold planetary group in the following order: ^[3]

Symbols

Symbols for the classical planets, zodiac signs, aspects, lots, and the lunar nodes appear in the medieval Byzantine codices in which many ancient horoscopes were preserved.^[5] In the original papyri of these Greek horoscopes, there are found a circle with one ray () for the Sun and a crescent for the Moon.^[6] The written symbols for Mercury, Venus, Jupiter, and Saturn have been traced to forms found in late Greek papyri.^[7] The symbols for Jupiter and Saturn are identified as monograms of the initial letters of the corresponding Greek names, and the symbol for Mercury is a stylized caduceus.^[7] A. S. D. Maunder finds antecedents of the planetary symbols in earlier sources, used to represent the gods associated with the classical planets. Bianchini's planisphere, produced in the 2nd century,^[8] shows Greek personifications of planetary gods charged with early versions of the planetary symbols: Mercury has a caduceus; Venus has, attached to her necklace, a cord connected to another necklace; Mars, a spear; Jupiter, a staff; Saturn, a scythe; the Sun, a circlet with rays radiating from it; and the Moon, a headdress with a crescent attached.^[9] A diagram in Johannes Kamateros' 12th century Compendium of Astrology shows the Sun represented by the circle with a ray, Jupiter by the letter zeta (the initial of Zeus, Jupiter's counterpart in Greek mythology), Mars by a shield crossed by a spear, and the remaining classical planets by symbols resembling the modern ones, without the cross-mark seen in modern versions of the symbols.^[9] The modern sun symbol, pictured as a circle with a dot (☉), first appeared in the Renaissance.^[6]

Week-day names

The Ptolemaic system used in Greek astronomy placed the planets in order, closest to Earth to furthest, as the Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. In addition the day was divided into 7-hour intervals, each ruled by one of the planets, although the order was staggered (see below).

The first hour of each day was named after the ruling planet, giving rise to the names and order of the Roman seven-day week. Modern Latin-based cultures, in general, directly inherited the days of the week from the Romans and they were named after the classical planets—for example in Spanish Miércoles = Mercury, in French Mardi = Mars.

The modern English days of the week were inherited from gods of the old Germanic Norse culture—Thursday = Thor (Jupiter), Friday = Frige (Venus). It can be correlated that the Norse gods were attributed to each Roman planet and its god, probably due to Roman influence rather than coincidentally by the naming of the planets.

Alchemy

In alchemy, each classical planet (Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn) was associated with one of the seven metals known to the classical world (silver, mercury/quicksilver, copper, gold, iron, tin and lead respectively). As a result the alchemical glyphs for the metal and associated planet coincide. Alchemists believed the other elemental metals were variants of these seven (e.g. zinc was known as "Indian tin" or "mock silver" ^[10]).

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  ☾ The symbol for the Moon has, also used to represent silver.

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  ☿ The symbol has been used to represent both the element quicksilver (mercury) and the planet Mercury.

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  ♀ The alchemical symbol for copper, perhaps a stylized mirror, was also the symbol for the goddess and planet Venus.

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  ☉ The Sun symbol has long represented gold.

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  ♂ The symbol for Mars has been used to represent iron.

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  ♃ The alchemical symbol for tin is also used as the glyph for Jupiter.

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  ♄ Saturn is represented by the alchemical symbol for lead.

Some alchemists (e.g. Paracelsus) adopted the Hermetic Qabalah assignment between the vital organs and the planets as follows:^[10]

Western astrology

Astrology: the Thema Mundi shows the naked-eye planets in their domicile

Indian astrology

Indian astronomy and astrology (Jyotiṣa) recognises seven visible planets (including the sun and moon) and two additional invisible planets.

East Asia

For the five true planets, their element's Chinese character, hanzi(汉字), is also part of the names of weekdays in Japanese and Korean, complemented with Sunday and Monday.^[11] However, Chinese and Vietnamese number the days other than Sunday.^[12]

The cycles of the Chinese calendar are linked to the orbit of Jupiter, there being 12 sacred beasts in the Chinese dodecannualar geomantic and astrological cycle, and 12 years in the orbit of Jupiter.^{[citation needed]}

See also

• Antikythera mechanism
• Aspects of Venus
• Definition of planet
• Five elements (Chinese philosophy)
• Geocentric model
• Celestial spheres

References

External links

• The Naked Eye Planets and how to identify them
• Wandering Stars: Movements and Visibility Cycles of the Naked Eye Planets
• Chronology of Discoveries in the Solar System

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