Classical planet

A classical planet is an astronomical object that can be seen with the naked eye moving across the sky. These objects look like wandering stars because they move against the backdrop of the fixed stars. There are seven classical planets, listed from brightest to dimmest: the Sun, the Moon, Venus, Jupiter, Mercury, Mars, and Saturn.

Ancient Greek astronomers such as Geminus and Ptolemy wrote about these classical planets during classical antiquity. They called them planēs or planētēs, meaning 'wanderers' in Greek, because these objects travel across the celestial sphere compared to the fixed stars.

With the help of telescopes, scientists discovered more objects in space, beginning with the Galilean moons in 1610. Today, the word planet has a new meaning: it describes a natural satellite that orbits the Sun (or another star) and has cleared its area of other objects. Only five of the seven classical planets are still called planets, along with Earth, Uranus, and Neptune.

History

Main article: Babylonian astronomy

The ancient Babylonians identified seven special objects in the sky that moved across the stars. These were the Sun, the Moon, Jupiter, Venus, Saturn, Mercury, and Mars. They were seen as important celestial bodies.

Main article: Mandaean cosmology

In the beliefs of the Mandaean people, these same seven planets were also important. They were thought to belong to a powerful figure called Ruha. The names of these planets were borrowed from older languages and traditions.

Symbols

Main articles: Astrological symbols and Planet symbols

The symbols for the classical planets, like the Sun and Moon, have been used for thousands of years. Early versions of these symbols showed pictures of gods linked to each planet. For example, Mercury was shown with a caduceus, and the Sun was shown as a circle with rays. Over time, these symbols changed and evolved, leading to the symbols we recognize today. The modern symbol for the Sun, a circle with a dot in the center, appeared during the Renaissance.

Planetary hours

Main articles: Planetary hours and Names of the days of the week

In ancient times, people believed that the Sun, Moon, and five visible planets — Mercury, Venus, Mars, Jupiter, and Saturn — moved around Earth. They thought the day could be split into seven parts, with each part ruled by one of these planets.

This idea helped give us the names of the days of the week. For example, in many languages, the days are named after these planets. In English, the days come from old Germanic gods, but they still match up with the planets, like Thursday from Thor, who is linked to Jupiter.

Alchemy

Further information: Astronomical symbols and Alchemical symbol

In alchemy, each classical planet was linked to one of the seven metals known in ancient times. Alchemists thought that the symbols for these metals and planets were the same. They also believed that other metals were variations of these seven.

Alchemy was closely connected to ancient astrology, especially in the Western World. Astrologers used the idea of the four classical elements to help understand their charts. Each of the seven planets known to ancient people was thought to "rule" a certain metal:

• The Sun rules Gold
• The Moon rules Silver
• Mercury rules Quicksilver/Mercury
• Venus rules Copper
• Mars rules Iron
• Jupiter rules Tin
• Saturn rules Lead

Contemporary astrology

Main article: Planets in astrology

See also: Astrological sign § Dignity and detriment, exaltation and fall

Main article: Navagraha

Indian astronomy and astrology recognize seven visible planets, including the Sun and Moon, and also consider two additional invisible planets.

Main article: Wufang Shangdi

See also: Names of the days of the week § East Asian tradition

Chinese astronomy and astrology also recognize seven visible planets, including the Sun and Moon. Chinese astrology developed during the Han dynasty.

Naked-eye planets

Main article: Planetae

Mercury and Venus can only be seen during twilight because they are closer to the Sun than Earth is. Venus is very bright and easy to spot, while Mercury is harder to see because it stays close to the Sun. Mars looks its brightest when it is directly opposite the Sun in the sky, which happens about every twenty-five months. Jupiter and Saturn are big planets, but because they are far from the Sun, they do not shine as brightly. Jupiter is often the third brightest object in the night sky after the Moon and Venus. Saturn’s rings can make it look even brighter, though we cannot see the rings without a telescope.