As a species, we humans are curious about our surrounding environment, thus we have always tried to explain our world and the heavens above it. Perhaps the very early efforts to model our Universe, date back to the prehistoric era, since in many caves, markings, drawings and various symbols that represent astronomical objects have been found. Throughout our history we have tried to explain our world using two models. The first is known as the geocentric model, which assumes that the Earth (flat, or spherical), sits at the centre of the Universe, while the second is the heliocentric model, which sets the Sun at the centre of the Universe. Thus, in this article the main historical views on our Universe are presented.
The Geocentric Model - Earth center Universe
The first model of the Universe came from the ancient Babylonians. According to this model, the Earth sits at the center of the Universe and the Sun, Moon, planets and all stars revolve around the heavens and Earth. This model is known as the geocentric model (see Figure 1), and it was based in two concepts:
- The Sun appears to revolve around Earth once per day. The planets and the Moon have their own motions, but they also appear to revolve around Earth once per day. The stars appear to be fixed on a celestial sphere and they rotate on a daily basis with respect to an axis through the geographic poles of the Earth.
- The Earth is stationary, since it seems not to move from the perspective of an earthbound observer.
The geocentric model was also favoured among Ancient Greek Astronomers. The main difference from the Babylonian model, was that the Earth was considered to be a sphere. Ptolemy’s model from his book Almagest, became the accepted view of the Universe. According to Ptolemy, a spherical Earth sits at the center of the Universe, a belief that was considered valid until the 16th century.
The Heliocentric Model - Sun center Universe
The first heliocentric model was suggested by the Aristarchus of Samos (c. 310 – 230 BC),
when he placed the then-known planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn) in their correct orbits around the Sun. The model of Aristarchus, was not accepted, since the removal of the Earth as center of the Universe was considered something that places mankind in an inferior position in the Universe. During the Dark Era in Europe, any revision on scientific vies was considered to be heretic, so it took centuries for the heliocentric model to resurface on the scientific world. Thus, Nicholas Copernicus is known as the father of heliocentrism through his book De Revolutionibus Orbium Coelestium in 1543. Copernicus brought back the heliocentric model of Aristarchus, since he also considered that the planets have circular orbits around the Sun.
A couple of decades later, in 1609 Johannes Kepler based on observations of planetary motion derived three empirical laws, which confirmed the heliocentric model and the orbital paths of the planets.
Kepler's laws of planetary motion
The following are known as Kepler’s laws for planetary motion and they state:
- Each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse.
- The straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time.
- The square of a planet’s orbital period is directly proportional to the cube of the semi-major axis of its orbit (the maximum distance between a planet and the Sun).
Newton's law of universal gravitation
According to Newton’s theory, between two masses, there is an attractive force between them and this force is proportional to the inverse of the distance squared. In the figure below an illustration of the gravitational force is shown together with the equation that the gravitational law obeys.
Note that: F1 and F2 is the attractive force between the two bodies, m1 and m2, are the masses of the two bodies in kilograms, r is the distance between them is meters, while G is the universal gravitational constant and has a value of 6.674×10−11 m3⋅kg−1⋅s−2. The unit that is used to measure force is known as Newton, in honour of Sir Isaac Newton.
Newton’s law of gravitation was of great importance, since Kepler’s law that were based on observations only, where able to be explained also with Physics.
Advantages of the Heliocentric Model -Sun center Universe
Of course today we all know that the Sun does not sit at the center of the Universe, since it is one of the billions of stars in our Galaxy, which in turn is one galaxy out of the billions in the Universe. Thus, the heliocentric model is not an accurate description of the Universe, but still it can explain better:
- The motion of the planets and their observed brightness variation, due to distance variation. As it can be seen on the image below on the left, the heliocentric Solar System and Kepler’s first law can explain the apparent retrograde motion of planets, a thing that the geocentric model fails to do.
- The parallax of objects in the sky. Parallax is the difference in the apparent position of an object when viewed from two different lines of sight. Parallax is measured by the angle, or semi-angle of inclination between those two lines. On the right hand side on the image above, a visualisation shows clearly the concept of parallax and how it can be explained only with the heliocentric model.
- The seasons in our planet and how is it possible that the two hemispheres have always the opposite season. An orbiting Earth around the Sun, together with the inclination that the Earth’s orbit can explain the seasonal variations perfectly, a thing that the geocentric model fails to do.
Based on the above, it is not surprise that the heliocentric model was favoured and it became undoubtedly, the model of choice for our Universe initially, and nowdays for our Solar System. It was also a great step in our understanding of the Universe. Finally, we should always remember the following. A theory with certain limitations is not complete. This is why we can not explain at the moment everything in the Universe. But a theory that needs to treat every issue separately (planetary motion, seasons, etc), is just wrong.
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