Solar system

The solar system is the retinue of objects gravitationally bound to our Sun. Traditionally, it is said to consist of nine planets and their 156 (at last count) moons; however a large number of other objects, including asteroids, meteoroids, planetoids and comets orbit the Sun along with them.

Discovery of the solar system

The planets out to Saturn were known to ancient astronomers, who observed the wandering of these objects against the apparently fixed pattern of stars. Venus and Mercury were each identified as single objects despite the difficulty of connecting "evening" and "morning stars". It was also identified that the two non-pointlike objects, the sun and the Moon, moved across the same fixed background. However knowledge of the nature of these celestial drifters was entirely speculative and largely incorrect.

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The nature and structure of the solar system were long misperceived, for at least two reasons:

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• The Earth was considered stationary, and the motion of objects in the sky was therefore taken at face value: the sun was thought to orbit the Earth, for example (This conception of the universe, in which the Earth is at the center, is called the Geocentric model; geos means "Earth" in Greek).
• Many solar system objects and phenomena cannot be perceived at all without technical aid.

Over the last several hundred years, conceptual and technological advances have helped us understand the solar system much better.

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The first and most fundamental of the conceptual advances was the Copernican Revolution, which proposed that the planets orbit the sun—models of the solar system with the sun in the center are called heliocentric (helios meaning "Sun" in Greek). Despite the name, the most striking (and then-controversial) Copernican realization was not that the sun was central but that the Earth was peripheral, orbital: planets had been considered merely points in the sky, but if the Earth itself was a planet, perhaps the other planets were, like Earth, huge solid spheres.

Related Topics:
Copernican - Heliocentric

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Philosophically, there were a number of objections to heliocentrism:

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• If the Earth is moving, what force keeps the air from flying off into space?
• The Earth is made of heavy rock. Heavy rock moves down. Down in a sphere means the centre. The planets are ephemeral and light, so they are above. How can Earth be a planet?
• If the Earth is mobile, then why do we not observe parallax in the stars (the stars appearing to shift in relation to further objects due to the change in position)?

The subsequent invention of the telescope gave the principal technological advance on discovering the solar system, with Galileo's improved version of the telescope rapidly giving benefit in terms of discovering satellites of other planets, especially Jupiter's four major satellites. This showed that all objects in the universe did not orbit the Earth. However, perhaps Galileo's most important discovery was that the planet Venus has phases like the Moon, proving that it must orbit the Sun.

Related Topics:
Venus - Phases

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Then, in 1687, Isaac Newton devised his law of universal gravitation which explained the force that both kept the Earth moving through the heavens and also kept the air from flying away.

Related Topics:
1687 - Isaac Newton - Law of universal gravitation

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Finally, in 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, proving conclusively that the Earth was in motion.

Related Topics:
1838 - Friedrich Wilhelm Bessel - 61 Cygni

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