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For the failed IAU planet category proposal, Classical planets, see 2006 definition of planet.

In antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun (and also closest to the Earth); all easily visible without a telescope. They are the Sun, Moon, 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 in relation to the other stars. They called these objects asteres planetai, or wandering stars. Together they form the seven classical planets, 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. Venus is the most prominent planet, being the third brightest object in the sky after the Sun and the Moon. 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 northerly locations like Scandinavia or Alaska.^[1] Mars is at its brightest when it is in opposition to the Earth, 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.

The planets in culture and mythology

In some cultures, the five naked-eye planets are grouped with the Sun and the Moon, called luminaries (meaning "shining object") to form a group of seven heavenly objects (or seven lights of the Earth, starry planets, traditional planets, etc.). Because their existence was known by many cultures throughout the world, since before recorded history, no discoverer is credited for them.

Most Western cultures refer to them by their Latin names in honor of the Roman pantheon and their number probably led to the naming of the seven-day week.

Every culture has woven them into their myths and legends. They are the basis of the practice of astrology. In English, the planets bear names of gods in Roman mythology, a relic of the ancient belief that planets were gods controlling human destiny. They were sometimes considered to be living beings, and myths were created concerning how they came to be, and what they were supposed to be like, based on their movements in the sky, their physical characteristics, and the supposed influences they have on our planet and its inhabitants.

• The Sun was known for its obvious importance in providing life. It also served as literal universal constant in the passage of night and day. Many ancient cultures were able to accurately create calendars from charting the Sun—thus predicting the regional seasons and solar events.
• The Moon and its phases were charted by most cultures, giving rise to lunar calendars. Lunar cycles were used to divide the solar, or seasonal, year into sections called months.
• Mercury was known to have the quickest period, probably leading it to be named for a Roman god associated with haste.
• Venus was identified as being the brightest object in the night sky after the Moon.
• Mars, was often associated with war due to its association with the Roman god and visible redness.
• Jupiter, now known to be the largest planet in the solar system. Whether the Romans knew this or not is not known, but is a likely possibility as Jupiter does appear to outshine most of the other planets, yet it has the second-longest period.
• Saturn, now popularly known for its rings and for being the second largest planet, though the rings being difficult to discern even with modern house telescopes. The Roman naming was probably based on their (correct) assumption that Saturn was the furthest of the classical planets due to the length of its orbital period. Its brightness is slightly less than Jupiter though brighter than most of the other planets. The Romans may have named it for the Titan that was formerly deposed by the now greater Jupiter.

In astrology

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

Astrology was developed when only the naked-eye planets were known. Because they appear to move in cycles about us in the heavens, whereas the "fixed" stars (what we today call the stars) did not, the planets were grouped together with the luminaries and considered to have divinatory significance. At the same time, a distinction was made within this category for the greater importance and brightness of the luminaries (the Sun and the Moon). For more on the significance of naked-eye planets in astrology, see Mercury (astrology), Venus (astrology), Mars (astrology), Jupiter (astrology), and Saturn (astrology).

In Alchemy

In alchemy, each of the seven classical planets (Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn) was associated with one of the seven metals known to the classical world (gold, silver, mercury/quicksilver, copper, 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" ^[2]).

☉ The Sun symbol has long represented gold.	☾ The symbol for the Moon has been used since ancient times to represent silver.	☿ The symbol has been used since ancient times to represent both the element quicksilver (mercury) and the planet Mercury.	♀ The alchemical symbol for copper, perhaps a stylized mirror, was also the symbol for the goddess and planet Venus.	♂ The symbol for Mars has been used since ancient times to represent iron.
♃ The alchemical symbol for tin is also used as the glyph for Jupiter.	♄ 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 ^[2]:

In Babylonia

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]

Days of the week

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.

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 of the Roman planets and their gods, probably due to Roman influence rather than coincidentally by the naming of the planets.

In Judaism

According to the Jewish Encyclopedia and the Jewish historian Josephus, the seven branches of the Jewish Menorah represent the seven heavenly objects.

In the Indian subcontinent

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

In 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. ^[5] However, Chinese and Vietnamese number the days other than Sunday.^[6]

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.

References

See also

• Antikythera mechanism
• Definition of planet
• Five elements (Chinese philosophy)
• Geocentric model

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

The Solar System The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Haumea · Makemake · Eris Moons (Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Haumean · Eridian) · Rings (Jovian · Saturnian · Uranian · Neptunian) Meteoroids · Minor planets (Asteroids (Main belt) · Centaurs · TNOs (Kuiper belt · Scattered disc) ) · Comets (Oort cloud) Related articles: astronomical object, star, planet, dwarf planet, small body and planetary system See also the Solar System's list of objects, sorted by size, the list of minor planets, or the Solar System Portal