Evolution of Astronomical Theories: From Antiquity to Einstein
Evolution of Astronomical Theories
This document outlines the evolution of astronomical theories from ancient philosophers to modern scientists.
Anaxagoras
Anaxagoras proposed that the sun is a fiery metallic sphere. He also explained eclipses and the phases of the moon.
Thales of Miletus
Thales of Miletus is credited with predicting a solar eclipse.
Aristotle
Aristotle believed that the Earth was at the center of a sphere, with the stars fixed in place. He stated that the Earth is fixed and immobile. The sun, moon, and five visible planets moved in circular paths within their own spheres, each nested within the celestial sphere.
Claudius Ptolemy
Ptolemy theorized that the heavens were spherical and rotating, with the Earth at the center. He also believed the Earth was spherical and stationary. Ptolemy proposed that planets moved in two circular motions: an epicycle (a smaller circular path) around a deferent (a larger circular path around the Earth).
Copernicus’s Model
Copernicus proposed that heavenly motions are eternal and uniform, either circular or composed of multiple cycles (epicycles). He posited that the sun, not the Earth, occupied the center of the universe, with the moon revolving around the Earth and the planets revolving around the sun. He also suggested that the stars were distant, fixed objects not orbiting the sun, and that the Earth rotated on its axis.
Galileo Galilei
Galileo discovered the phases of Venus, indicating that it revolved around the sun. He also discovered four satellites orbiting Jupiter, demonstrating that not all celestial bodies orbited the Earth.
Kepler’s Laws of Planetary Motion
- Planets move in elliptical orbits, with the sun at one focus.
- A line drawn from the sun to a planet sweeps out equal areas in equal times.
- The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit: (R3 / T2) = constant.
Law of Universal Gravitation
All bodies in the universe attract each other with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G * (m1 * m2) / r2
Albert Einstein’s Theories
Special Relativity
At speeds approaching the speed of light, space contracts in the direction of motion. Time dilates (slows down) as speed increases. Inertia increases with speed.
General Theory of Relativity
- Gravity and acceleration have the same effect, known as the principle of equivalence.
- Space-time is warped by massive objects, with greater mass causing greater warping.
Edwin Hubble’s Discoveries
Hubble made two significant discoveries: some nebulae were galaxies composed of stars like our own (the Andromeda Galaxy was the first studied), and the universe is expanding. He found that galaxies are receding from us, with more distant galaxies receding faster.
The Big Bang Theory
In 1970, scientists concluded that between 10 and 15 billion years ago, all matter in the universe was concentrated in an extremely hot, dense state. This state expanded rapidly in a “Big Bang,” then cooled, giving rise to the universe as we know it, including matter, space, and time.