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The history of Gravity



A long time, scientists tried to understand the motion of celestial bodies. During the Middle Ages, scientists and philosophers believed in the geocentric astronomical model, where the Earth was the center of the Universe, with the planets, the Sun and other celestial bodies moving around it. Nicolaus Copernicus (1473 – 1543) proposed the heliocentric model. This model displaces the Earth as the center of universe and place the Sun in it and the Earth and others known planets moving around of Sun. Copernicus’ shocked society and the church, which condemned his model.  However, Copernicus got support of great astronomers and scientists such as Galileo Galilei and Johannas Kepler.

Solar System

Scheme of Solar System

Galileo Galilei (1564 – 1642), using a brand new invention, the telescope, developed observational astronomy. This astronomy division is concerned in getting data of celestial bodies to create a model or theory. With the telescope, Galileo could observe celestial objects and phenomena that were not seemed before. Galileo’s discovers, such asVenus’ phases and Jupiter’s Moons, gave support to Copernicus’ model. 

In 1610, Galileo observed that Venus has a full set of phases like the phases of the Moon. It was contradictory to geocentric model where Venus should not have a full lit from the perspective of the Earth.  Actually, Venus phases are result of the orbit of Venus around the Sun inside of Earth orbit. That way, when Venus is between Sun and Earth, it is full shadowy. Then Venus is partially illuminated when it moves in its orbit until it becomes fully lit when it is on the opposite side of the Earth orbit. In sequence, the shadowy is covering Venus when it moving from the opposite side of Earth orbit to the position between Sun and Earth. Thus, Venus has a complete set of phase when complete its orbit around the Sun.

Venus' Phases

The possible Venus Phases on the Geocentric Model

The possible Venus Phases on the Heliocentric Model
Jupiter Moons

Galilean Moons and Jupiter

In the same year, Galileo observed with his telescope four objects moving near the planet Jupiter. After analyzing data of their full period of moving, he concluded that actually these four objects are orbiting the Jupiter as moons. This was unacceptable by the geocentric model where all celestial body should just orbit Earth. Today, we know that Jupiter has about sixty moons and the four biggest was the ones observed by Galileo, called nowadays Galilean Moons.

Even with these and other proves of heliocentric model, many astronomers still could not believe it. They condemned the Galileo telescope as evil instrument that showed unreal image of the sky.

Another support to heliocentric model came with Johannes Kepler (1571 – 1630).  He calculated the orbit of the planets around the Sun, with astronomic data of Tycho Brahe (1546 – 1601), a famous astronomer of his time.  Kepler created the three laws of planetary motion that get his name: Kepler’s laws.

The Kepler’s laws are:

1.      “The orbit of every planet is an ellipse with the sun at a focus.”

2.      “A line joining a planet and the sun sweeps out equal areas during equal intervals of time.”

3.      “The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.”

2nd Kepler Law
Illustration of the 2nd Kepler law

The last doubts about the heliocentric model were removed years later by Isaac Newton (1643 – 1727).  Based on Galileo’s and Kepler’s works, Newton published “Principia” in 1687. In this book, Newton posed the theory of Gravity, in which the force that makes planets to move around the Sun is the same force that makes object to fall in the Earth: force of gravity. In his theory, Newton deduced gravity is a force of mutual interaction of body with mass and this force is inversely proportional to the square of the distance between objects.

The heliocentric model was established by Newton but there were some question about the gravity, for example, its action at a distance and immediately action.  Even Newton had doubts about the gravity action at a distance. How can massive objects attract each other at distance without mediation of anything?  And how can attraction force between them be immediately without a time to action?


Isaac Newton

Later, in 1915, Albert Einstein (1879 – 1955) published the general theory of relativity, in which gravity is not a force but it is a consequence of the curvature of space-time. Thus, massive body creates a curve in the space-time then inertial trajectory that was straight lines became curved. These inertial trajectories are called geodesics. An object can inertial follow a geodesic without an interaction of forces. As consequence, heavy objects create a “big” curvature on space-time that makes other object fall towards them by a geodesic. If an object has extremely big mass even the light will suffer a noticeable deflection. This object is called of black hole.