There seems to be something basic about the number seven in nature. There are the seven colors of the rainbow, and the seven notes of the musical scale. Western science recognizes seven chief glands in the human body, called the endocrine glands; while Eastern yoga systems speak of the seven chakras, or centers of force, in man's subtle body.
Medieval astrology has its seven Ancient and Sacred planets: Saturn, Jupiter, Mars, Sun, Venus, Mercury, and the Moon or Luna. There are the seven visible heavenly bodies which make up our solar system, the planets Uranus, Neptune and Pluto not being visible to the naked eye. Strictly speaking, of course, the Sun and Moon are not planets, and are more correctly described as the two luminaries; but the term 'seven sacred planets' was used for convenience. Everything upon Earth is under the governance, astrological speaking, of these seven influences.
This is why we have the seven-day week. Each day is ruled by one of the planets. The Sun rules Sunday, the Moon Monday, Mars Tuesday, Mercury Wednesday, Jupiter Thursday, Venus Friday, and Saturn Saturday. These rulerships go back to the days of ancient Chaldea and Babylon.
Also, in the Northern Hemisphere, we have the most conspicuous constellation of the night sky, the Plough, or the Great Bear (Ursa Major), which points to the North Star or the Pole star, and which consists of a group of seven bright stars.
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