Classical planet

A classical planet is an astronomical object that you can see moving in the sky without any special tools. These objects look like wandering stars because they move against the backdrop of the fixed stars. There are seven classical planets. They are, 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 long ago. They called them planēs or planētēs, meaning 'wanderers' in Greek, because these objects move across the celestial sphere.

With telescopes, scientists found more objects in space, starting with the Galilean moons in 1610. Today, the word planet means a natural satellite that goes around 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 saw seven special objects moving in the sky. These were the Sun, the Moon, Jupiter, Venus, Saturn, Mercury, and Mars. They thought these were important stars.

Main article: Mandaean cosmology

For the Mandaean people, these same seven planets were also special. They believed these planets belonged to a powerful figure called Ruha. The names of these planets came from old 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 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 we now use the symbols we see 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 the Sun, Moon, and five planets — Mercury, Venus, Mars, Jupiter, and Saturn — moved around Earth. They thought the day could be split into seven parts. Each part was ruled by one of these planets.

This idea helped give us the names of the days of the week. 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 the symbols for these metals and planets were the same. They also believed that other metals were types 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 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 objects in the sky. These include the Sun, the Moon, and five bright planets. They also think about two invisible objects.

Main article: Wufang Shangdi

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

Chinese astronomy and astrology also recognize seven visible objects in the sky. These include the Sun, the Moon, and five bright planets. Chinese astrology grew 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. 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. This 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.