Evolution of Astronomical Models: From Ancient Cosmos to Newton

Aristotle's Cosmic Conception

Aristotle, in the fourth century BC, distinguished two regions of the universe:

The Terrestrial Region (Below)

• Earth occupies the center of the universe.
• All terrestrial bodies are formed by combining four elements: earth, water, air, and fire.
• Earth is the heaviest element and tends downwards, while fire tends upwards.

The Celestial Region (Above)

• It consists of transparent, concentric spheres surrounding the Earth.
• In each sphere, one of the heavenly bodies (Sun, Moon, Venus, etc.) is located.
• The outermost sphere contains the fixed stars.

Ptolemy's Geocentric System

Developed by Ptolemy (2nd Century AD), this system placed Earth at the center of the universe and described the movements of celestial bodies using Earth as a fixed reference point.

Everyday Observations Supporting Geocentrism

• The Sun rises, moves across the sky, and sets every day. Similarly, the stars appear to revolve around the sky daily, with Earth occupying a central position in the universe.
• The planets follow a certain path, their movement being the result of two described circular motions (epicycles and deferents).

Copernicus's Heliocentric System

Proposed by Nicolaus Copernicus in the sixteenth century, this system placed the Sun at the center of the universe. Earth and other planets revolve around the Sun. The stars are in fixed positions at distances from the Sun much larger than those of the planets. Earth also rotates on its axis with a period of one day.

The scientific community finally accepted the heliocentric model by the end of the 17th century.

Geocentric vs. Heliocentric Models

• Geocentric Model: For two thousand years, this model was favored due to its agreement with daily observations and its correspondence with Aristotle's philosophy and physics.
• Heliocentric Model: Proposed by Copernicus, this model explained the same phenomena as the geocentric system, but also justified other observations, such as the variable brightness of Venus and Mercury.

Kepler's Laws of Planetary Motion

Kepler's First Law: Elliptical Orbits

Planets orbit the Sun in elliptical paths, with the Sun located at one of the foci of the ellipse.

Kepler's Second Law: Equal Areas

The position vector from the Sun to a planet sweeps out equal areas in equal intervals of time.

Kepler's Third Law: Orbital Periods

The square of the orbital period (T) of any planet is directly proportional to the cube of the semi-major axis (r) of its orbit around the Sun. Mathematically, T² = Kr³, where K is a constant.

Newton's Law of Universal Gravitation

Formulated by scientist Isaac Newton, this law states that any two masses attract each other with a force possessing these characteristics:

• It is universal: it occurs between all bodies.
• It is always presented in pairs of action and reaction.
• It is always an attractive force.
• Its direction is along the line joining the centers of the masses.
• Its intensity is expressed by Newton's Law of Universal Gravitation.

The Newtonian Synthesis

Isaac Newton was able to explain many natural phenomena as manifestations of gravitational attraction, including:

• The movement of celestial bodies.
• The periodic appearance of comets.
• The falling of objects on Earth.
• The tides.