Early Heliocentric Models

By: Jasmine Ghafour

994527268

 

            Not only were heliocentric universes based on the positioning of celestial objects, but also on the distances and sizes between them. Many of the early thinkers used their ideas of a heliocentric universe to try to find out the relations of Earth to the Moon, the Sun and other planets. Relations such as distances and sizes were some of the things thinkers such as Aristarchus, Seleucus, and Copernicus were questioning. They used each others ideas and observations, to try and find a better understanding of the Universe. However, heliocentric universes, a universe based on the Sun being in the centre, weren’t always accepted in mainstream society. Early scholars who suggested a heliocentric universe didn’t try to urge others into believing their theories, but eventually, with the influence of Copernicus, the heliocentric universe theory was a revolution.

 

            Heraclides is said to have been the first to propose an idea of a heliocentric universe, even though it wasn’t on purpose. He was a well respected philosopher of his time. He came from a high-class family, and he studied with Aristotle in the Academy. He was nominated to be the leader of the prestigious school, but lost by a few votes to another one of the students from the Academy. Though he didn’t claim to be a believer in the heliocentric universe, scholars now believe that he suggested it without really knowing. He stated that Mercury and Venus orbit around the Sun, and that sometimes Venus appears to be higher than the Sun, and at other times it appears to be lower than the Sun. This suggests that they orbit the Sun and therefore not the Earth, but Heraclides doesn’t state that he rejects the geocentric universe. He in fact did believe in the geocentric universe, and he was the first to have said that the Earth rotates on its axis. It’s possible that he believed while the other planets orbit the Sun, all of them still orbit around the Earth, which is an idea known to be suggested later by Brahe.

 

            The first man to create an idea about a heliocentric universe, and publicly believe in it, was Aristarchus. He was born in Samos, and lived from 310-230 B.C. He believed that the stars were stationary, like other philosophers and scholars, and he also claimed that the Earth rotated on its axis, which then also orbited the Sun. He described the Universe as the stars being at the outer most spheres, because they were immobile and compared against. Then the planets went in order from slowest at the outside, which was Saturn, Jupiter, Mars, and then Earth. But because Venus and Mercury were thought to have orbited at the same rate, Aristarchus was unsure of how they were placed in the Universe with accordance to the Sun. Then there would be the Moon which was the lowest celestial object, because it eclipsed all the other planets and the Sun, which orbited around the Earth.  He tried to answer the question of how big the Moon and the Sun really were, and how it relates according to distance, compared to the Earth. He used geometric methods, to help him find the size and the distances between the Earth, Moon and Sun. One of these theories is the Lunar Dichotomy method, shown below in Figure 1.

 

 

 

      M

          90°

                                           3°       S 

     87°   

    E

 

Figure 1.

M = Moon, S = Sun, E = Earth

 

In his model, he is trying to show the relations between the Moon, the Earth and the Sun. He believed that he determined the distance between these three celestial objects:

            1. L EMS = 90°

            2. L MES = 87°

            4. Therefore according to trigonometry: L MSE=3°

3. 18:1 < ES: EM <20:1

 

Aristarchus also formed principals that help explain the ratios and distances of the Sun, the Moon and the Earth:

 

1. The Moon receives its light from the Sun.

2. The Earth is positioned as a point in the center of the sphere in which the Moon moves

3. When the Moon appears to us halved, the great circle which divides the dark and bright portions of the Moon is in the direction of our eye.

4. When the Moon appears to us halved, its angular distance from the Sun is less than a quadrant by one-thirtieth part of a quadrant. (One quadrant = 90 degrees, which means its angular distance is less than 90 by 1/30th of 90, or 3 degrees, and is therefore equal to 87 degrees.) (This assigned value was based on Aristarchus' observations.)

5. The breadth of the earth's shadow is that of two moons.

6. The moon subtends one fifteenth part of a sign of the Zodiac. (The 360 degrees of the celestial sphere are divided into twelve signs of the Zodiac each encompassing 30 degrees, so the moon, therefore, has an angular diameter of 2 degrees.)

 

(Principals taken from: http://www.mlahanas.de/Greeks/Astronomy.htm )

 

Aristarchus’ geometry was correct, but his actual numbers aren’t. It has been figured out that instead of the angle MES being 87°, it is in fact 89.5°, and because of this mistake, it led to other mathematical errors. His numbers of understanding the size of the Moon are fairly accurate, but his numbers of understanding the size of the Sun are incorrect. He did, however, believe that if he knew the angles between the Moon, the Sun and the Earth, then he could also know the ratios between the Moon, the Sun and Earth. This is where the numbers 18:1 < ES: EM <20:1 came from. By this ratio, it states that the distance between the Earth and the Sun, is greater than 18 times, but it is less than 20 times the distance from the Earth to the Moon. Once he hypothesized on the ratios, he then tried to find the diameters of these celestial objects. Aristarchus is also known for another three of his more famous principals, explaining the ratios of the Sun, Moon and Earth:

 

1.     The distance of the Sun from the Earth is greater than 18 times, but less than twenty times the distance of the Moon from the Earth

2.     The diameter of the Sun has the same ratio (greater than 18 but less than twenty) to the diameter of the moon.

3.     The diameter of the Sun has the diameter of the Earth a ratio greater than 19 to 3, but less than 43 to 6

 

(Principals taken from: http://www.mlahanas.de/Greeks/Astronomy.htm )

 

He claimed that the Sun and the Moon were the same diameter. And to prove this, he used the event of an eclipse. He created the Eclipse Diagram:

 

(Picture taken from: http://www.perseus.tufts.edu/GreekScience/Students/Kristen/Aristarchus.html )

 

It is understandable to see why he would believe that the Sun and the Moon were the same diameter during a solar eclipse, but his reason doesn’t suffice during a lunar eclipse. From this diagram, he believes that the Sun’s radius is 6 times larger than Earth’s, and that Earth is about 3 times bigger than the Moon. But, as stated earlier, Aristarchus’ numbers were incorrect, but his ideas and models were a definite breakthrough in Astronomy.

 

Another man who did believe in the heliocentric universe was Seleucus. He was a Greek philosopher, who was born in Seleucia, Babylon. He was born in 190 B.C., but the date of his death is uncertain.  He agreed with Aristarchus about the formation of the planets, and his mathematical ideas. Little is known about Seleucus, however. His ideas were thought of as impious and that’s why many of his ideas will probably never be known, since there aren’t many records or publishing’s of his work or on much of his life. Though we don’t know too much about him, we do know about one of his theories, which he is famous for; he connected the tides of the Indian Ocean with the Moon. He believed that the Moon and the tides were controlled by one another.

 

The idea, that all the celestial bodies orbited the Sun, caused a lot of aggression between other scholars and philosophers. Unlike Plato and Aristotle, Aristarchus’ idea of the Universe was based on mathematics, and it didn’t revolve around the gods. With the Hellenistic period taking over the beliefs of Greek scholars and philosophers, the ideas of both Aristarchus and Seleucus were rejected. Their ideas were thought of as impious, because they rejected that the gods had any control or any means of how the Universe worked. Both of their ideas weren’t looked upon as worthy of being thought about among many scholars, or taught in any of the schools. Unfortunately, because their ideas were regarded as impious, many of their works and writings were lost.

 

The idea of a heliocentric universe hadn’t been reconsidered until Nicolaus Copernicus, born in Poland, used the ideas and models of Aristarchus. Copernicus was born in 1473 and died 1543. He studied Law at the University of Bologna in Cracow, but he lived with the Professor of Astronomy during his studies, which is where his interest of Astronomy may have started. He used mathematical elements, and physics to help him create his model of the universe, and he believed that the planets moved in epicycles, an idea he derived from Ptolemy’s Equant model. But unlike other scholars, Copernicus wasn’t trying to refine the ideas already created from Ptolemy. He instead wanted to completely transform the ‘modern’ idea of the geocentric universe, and bring a new idea that hadn’t been introduced since Aristarchus. His first book was published in 1512, ‘Commentariolus’, and it contained his pre-mature ideas of the heliocentric universe. In this book he believed that the ideas of Ptolemy weren’t correct, because they didn’t explain the motion of the planets thoroughly. In ‘Commentariolus’ he stated:

 

  1. The heavenly bodies do not all move around the same centre;
  2. The earth is not the centre of the universe, only of the moon's orbit and of terrestrial gravity;
  3. The sun is the centre of the planetary system and therefore of the universe;
  4. Compared to the distance of the fixed stars, the earth's distance from the sun is small;
  5. The daily motion of the firmament is due to the earth's rotation on its own axis;
  6. The apparent annual motion of the sun is due to the fact that the earth, like other planets, revolves around the sun
  7. The apparent "stations and retrogressions" of the planets are due to the same cause.

 

 

His idea of the complete universe was published in his book called ‘De Revolutionibus Orbium Coelestium’. This book contained the mathematical and a physics perspective, as well as cosmology, to help decipher the layout of the universe. In this book, one of his theories explained why it appeared that Mars and Venus were seen closer to the Sun, because they orbit closer to the Sun compared to Earth; and why Mars appears to move backwards, since Earth has a shorter orbit around the Sun, it overtakes Mars’ orbit, and compared to the stars which are fixed, it looks as if it’s traveling backwards.

 

Because the modern belief at the time was strongly in favour of a geocentric universe, Copernicus was afraid that his work would be lost, just like the writings and ideas of Aristarchus and Seleucus, since his ideas were also looked upon as impious. He had a student, Rheticus, whom he explained his works to, and it was through Rheticus that Copernicus’ book ‘De Revolutionibus Orbium Coelestium’ was published. This work of Copernicus wasn’t released until the year of his death. He feared he would get into trouble with the church since his ideas suggested that no God or gods had anything to do with the movements of any of the Planets, the Sun, the Moon or the Earth. His writings were clever, when he explained why the Sun is in the centre of the universe. He described in his writings that other philosophers and scholars looked upon the Sun as something eternal, and lively, so he subconsciously questioned, why shouldn’t the Sun be in the centre? Plato, for example, in his writings talked about the Sun being the ultimate Good, the truest Form. When approached in this perspective, writing about the Sun being the highest Form, or something that is of true importance, it doesn’t seem as if the Sun being the centre of the universe is necessarily impious. Because of reasoning’s like these brought forward by Copernicus, his ideas had become more accepted, and were taught at many schools in the 15th and 16th century.

 

Although Aristarchus’ heliocentric universe wasn’t regarded as something of importance, he started the revolution. Copernicus was said to have taken his ideas about the heliocentric universe from Aristarchus, and that he valued his ideas. In one of his writings, Copernicus had referenced his own ideas to the original ideas of Aristarchus, but then had later gotten rid of those passages. Through out history, Copernicus’ ideas were then taken another step forward by scholars such as Brahe, Kepler, and Newton. His ideas were used as a stepping stone to further advance the idea of a heliocentric universe, which is now widely believed in modern times.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Works Cited:

 

  1. http://www.astro.utoronto.ca/~cclement/a210/copernicus.html
  2. http://eaa.iop.org/full/eaa-pdf/eaa/3998.html
  3. http://eaa.iop.org/full/eaa-pdf/eaa/3439.html
  4. http://eaa.iop.org/full/eaa-pdf/eaa/1921.html
  5. Measuring the Universe. Albert Van Helden. University of Chicago Press. Chicago: 1986.
  6. http://scienceworld.wolfram.com/biography/Copernicus.html
  7. http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Heraclides.html
  8. http://www.reference.com/browse/wiki/Nicolaus_Copernicus
  9. http://www.mlahanas.de/Greeks/Astronomy.htm
  10. http://www.perseus.tufts.edu/GreekScience/Students/Kristen/Aristarchus.html