Evolution of Astronomical Models and Gravity Concepts
Ptolemy’s Geocentric Models
Alexandra Ptolemy (86-165) established a geometric model where the Earth is fixed, and planets revolve around it. The Sun and Moon orbit Earth in circular paths, while other planets follow complex orbits. Planets move in small circles (epicycles) whose centers trace larger circles around Earth. A field of stars also revolves around Earth.
Heliocentric Models
Aristarchus’ Model
Aristarchus of Samos (310-230 BC) proposed a heliocentric model with the Sun at the center, and Earth, Moon, and other planets orbiting it at different speeds and orbits, all enclosed in a sphere of stars.
Copernican Model
Nicolaus Copernicus (1473-1543) placed the Sun at the center with Earth and other planets in circular orbits. The Moon orbits Earth.
Galileo’s Model
Galileo Galilei (1564-1642) used the telescope to make observations, discovering lunar mountains and valleys, the Milky Way’s composition, sunspots, Venus’ phases, Jupiter’s moons, and Saturn’s rings. He maintained circular orbits.
Kepler’s Laws
Johannes Kepler (1571-1630), using Tycho Brahe’s data, concluded that planetary orbits are elliptical. Kepler’s Laws:
- Planets move in elliptical orbits with the Sun at one focus.
- Planets move with constant areal velocity, sweeping equal areas in equal times.
- The square of the orbital period is proportional to the cube of the orbit’s semi-major axis.
Isaac Newton (1642-1727) explained planetary motion with his Law of Universal Gravitation.
Law of Universal Gravitation
Published in 1687, Newton’s law states that every body attracts every other body with a force proportional to their masses and inversely proportional to the square of the distance between them.
Consequences of Universal Gravitation
- Weight: The gravitational force exerted by Earth on nearby bodies.
- Equilibrium: A body is in equilibrium when the vertical line through its center of gravity passes through its base of support.
Tidal Cycles
Tides are the rise and fall of sea levels caused by the gravitational attraction of the Moon and, to a lesser extent, the Sun.
Celestial Body Motion
Planets orbit the Sun due to the Sun’s gravitational force, enabling predictions about their speed, position, and orbital period.
Weightlessness
Weightlessness occurs when a body experiences no weight, typically in the absence of gravity. It can be achieved in orbiting space stations where opposing forces counteract gravity.
Forces and Fluid Pressures
Fluids (liquids and gases) change particle positions easily. Liquids fill the bottom of their containers, while gases expand to fill their containers regardless of shape.
Archimedes’ Principle
A body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced.