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. In addition the day was divided into seven-hour intervals, each ruled by one of the planets, although the order was staggered (see below). The Ptolemaic system used in Greek astronomy placed the planets by order of proximity to Earth in the then-current geocentric model, closest to furthest, as the Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. مقال رئيسيs: Planetary hours and Names of the days of the week The modern Sun symbol, pictured as a circle with a dot (☉), first appeared in the Renaissance. Ī 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. Bianchini's planisphere, produced in the 2nd century, 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. Maunder finds antecedents of the planetary symbols in earlier sources, used to represent the gods associated with the classical planets. 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. The written symbols for Mercury, Venus, Jupiter, and Saturn have been traced to forms found in late Greek papyri. 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. The astrological symbols for the classical planets appear in the medieval Byzantine codices in which many ancient horoscopes were preserved. مقال رئيسيs: Astrological symbols and Planet symbols Therefore only five of the seven classical planets remain recognized as planets, alongside Earth, Uranus and Neptune. Today the term planet is used considerably differently, with a planet being defined as a natural satellite directly orbiting the Sun ( or other stars) and having cleared its own orbit. Through the use of telescopes other celestial objects like the classical planets were found, starting with the Galilean moons in 1610. Greek astronomers such as Geminus and Ptolemy often divided the seven planets into the Sun, the Moon, and the five star-like astra planeta. These classical planets were recorded during classical antiquity, introducing the term planet, which means originally "wanderer" in Greek (πλάνης planēs and πλανήτης planētēs), expressing the fact that these objects move across the celestial sphere relative to the fixed stars. They are from brightest to dimmest: the Sun, the Moon and the five star-like classical planets, the astra planeta ( Venus, Jupiter, Mars, Mercury and Saturn). Visible to humans on Earth there are seven classical planets (the seven luminaries). A classical planet is an astronomical object that is visible to the naked eye and moves across the sky and its backdrop of fixed stars (the common stars which seem still in contrast to the planets).
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