Historical models of the Solar System

Summary

Approximate sizes of the planets relative to each other. Outward from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Jupiter's diameter is about 11 times that of the Earth's and the Sun's diameter is about 10 times Jupiter's. The planets are not shown at the appropriate distance from the Sun.

The historical models of the Solar System began during prehistoric periods and is updated to this day. The models of the Solar System throughout history were first represented in the early form of cave markings and drawings, calendars and astronomical symbols. Then books and written records then became the main source of information that expressed the way the people of the time thought of the Solar System.

New models of the Solar System are usually built on previous models, thus, the early models are kept track of by intellectuals in astronomy, an extended progress from trying to perfect the geocentric model to eventually using the heliocentric model of the Solar System. The uses of the Solar System model began as a time source to signify particular periods during the year and also a navigation source which is exploited by many leaders from the past.

Astronomers and great thinkers of the past were able to record observations and attempt to formulate a model that accurately interprets the recordings. This scientific method of deriving a model of the Solar System is what enabled progress towards more accurate models to have a better understanding of the Solar System that we are in.

Prehistoric astronomy

Until the twenty-first century, astronomy was generally thought to begin with Hipparchus of Nicaea (d. c. 120 BC), who was credited with discovering the precession of the equinoxes.[1] Ancient cave paintings that monitor the equinoxes and solstices were found from as early as 40 000 years ago by researchers in the University of Edinburgh and Kent.[2] These events were used as an annual time reference in Çatalhöyük during 7000 BCE:

Jeguès Wolkiewiez found that 122 out of 130 ice age Paleolithic caves she visited were aligned to equinoxes and solstices.[2] The researchers concluded that these were calendar experts who used these astronomical events to know when to begin their rituals.[2] This highlights their knowledge of the Sun and Moon's positioning which is the foundation of formulating a model of the Solar System.

Also, according to NASA, the first cave markings of a lunar cycle were made by the Aurignacian Culture in 32 000 BCE.[3] These cave markings are thought of as calendars that helped the people contain the cycles of the Solar System which is a method of keeping track of time. In Lascaux caves, there were many cave drawings in which a dot is in the middle and 11 to 14 dots are drawn around the centre dot which archaeologists date back to as early as 15 000 BCE.[4] Alexander Marshack, Professor of Paleolithic Archaeology at Harvard University's Peabody Museum, concluded that these dots represent lunar cycles.[4]

Early astronomy

The Nebra Sky Disc

The Nebra Sky Disc, dating back to the Bronze Age in 1600 BCE, is a bronze disc that represents the cosmos, consisting of a crescent moon, the Sun, 32 stars and 3 arcs.[5] According to Professor Dr. Wolfhard Schlosser, the most correct interpretations of the sky disc of Nebra are that the 32 points represent the stars and the arcs on the edges of the disk making an 82 degree angle indicates the sunset and sunrise during summer and winter.[6]

Babylonian interpretation

Babylonians thought the universe revolved around heaven and Earth.[7] They used methodological observations of the patterns of planets and stars movements to predict future possibilities such as eclipses.[8] Babylonians were able to make use of periodic appearances of the Moon to generate a time source - a calendar. This was developed as the appearance of the full moon was visible every month.[9] The 12 months came about by dividing the ecliptic into 12 equal segments of 30 degrees and were given zodiacal constellation names which was later used by the Greeks.[10]

Chinese theories

The Chinese had multiple theories of the structure of the universe.[11] The first theory is the Gaitian (celestial lid) theory, mentioned in an old mathematical text called Zhou bei suan jing in 100 BCE, in which the Earth is within the heaven, where the heaven acts as a dome or a lid. The second theory is the Huntian (Celestial sphere) theory during 100 BCE.[11] This theory claims that the Earth floats on the water that the Heaven contains, which was accepted as the default theory until 200 AD.[11] The Xuanye (Ubiquitous darkness) theory attempts to simplify the structure by implying that the Sun, Moon and the stars are just a highly dense vapour that floats freely in space with no periodic motion.[12]

Greek astronomy

Since 600 BCE, Greek thinkers noticed the periodic fashion of the Solar System. Many theories were announced during this period.[13] Parmenides claimed that the Solar System is spherical and moonlight is actually a reflection of sunlight.[13]

This is the geocentric model of the Solar System with the Earth at the centre. The above image is also a representation of Eudoxus's proposal.

Anaxagoras suggested that the Moon is closer to the Earth than the Sun, comets are formed by collisions of planets and that the motion of planets is controlled by the nous (mind).[13] Pythagoras's students thought the motion of planets is caused by a fire at the centre of the universe that powers them and Earth orbits that fire. They also claimed that the Moon, the Sun and planets orbit the Earth.[14]

Eudoxus, in around 400 BCE, introduced a technique to describe the motion of the planets called the method of exhaustion.[15] Eudoxus reasoned that since the distances of the stars, the Moon, the Sun and all known planets do not appear to be changing, they are fixed in a sphere in which the bodies move around the sphere but with a constant radius and the Earth is at the centre of the sphere.[16] Eudoxus emphasised that this is a purely mathematical construct of the model in the sense that the spheres of each celestial body do not exist, it just shows the possible positions of the bodies.[17] However, Aristotle modified Eudoxes' model by supposing the spheres are real.[18] He was able to articulate the spheres for most planets, however, the spheres for Jupiter and Saturn crossed each other. Aristotle solved this complication by introducing an unrolled sphere.[18] Aristotle also tried to determine whether the Earth moves and concluded that all the celestial bodies fall towards Earth by natural tendency and since Earth is the centre of that tendency, it is stationary.[18]

Around 360 BCE when Plato proposed his idea to account for the motions. Plato claimed that circles and spheres were the preferred shape of the universe and that the Earth was at the centre and the stars forming the outermost shell, followed by planets, the Sun and the Moon.[19] However, this did not suffice to explain the observed planetary motion. During the period 127 to 141 AD, Ptolemy deduced that the Earth is spherical based on the fact that not everyone records the solar eclipse at the same time and that observers from the North can not see the Southern stars.[20] Ptolemy attempted to resolve the Planetary motion dilemma in which the observations were not consistent with the perfect circular orbits of the bodies. Ptolemy proposed a complex motion called Epicycles.[21] Epicycles are described as an orbit within an orbit. For example, looking at Venus, Ptolemy claimed that it orbits the Earth, and as it orbits the Earth, it also orbits the original orbit which is demonstrated in the illustration to the right. Ptolemy emphasised that the epicycle motion does not apply to the Sun. This model were Earth is positioned at the centre of the Solar System, is known as the Geocentric model.

The epicycles of the planets in orbit around Earth (Earth at the centre). The blue path-line is the combined motion of the planets orbit around Earth and within the orbit itself. This is Ptolemy's attempt to explain the complex planetary motion.

Medieval Astronomy

Islamic astronomy

The Islamic golden age period in Baghdad, picking off from Ptolemy's work, had more accurate measurements taken followed by interpretations. In 1021, Ibn Al Haytham adjusted Ptolemy's geocentric model to his specialty in optics in his book Al-shukuk 'ata Batlamyus which translates to "Doubts about Ptolemy".[22] Ibn al-Haytham claimed that the epicycles Ptolemy introduced are inclined planes, not in a flat motion which settled further conflicting disputes.[23] However, Ibn Al Haytham agreed with the Earth being in the centre of the Solar System at a fixed position.[24]

Nasir al-Din, during the 13th century, was able to combine two possible methods for a planet to orbit and as a result, derived a rotational aspect of planets within their orbits.[25] Copernicus arrived to the same conclusion in the 16th century.[22] Ibn al-Shatir, during the 14th century, in an attempt to resolve Ptolemy's inconsistent lunar theory, applied a double epicycle model to the Moon which reduced the predicted displacement of the Moon from the Earth.[26] Copernicus also arrived at the same conclusion in the 16th century.[27]

Chinese Astronomy

In 1051, Shen Kua, a Chinese scholar in applied mathematics, rejected the circular planetary motion. He substituted it with a different motion described by the term ‘willow-leaf’. This is when a planet has a circular orbit but then it encounters another small circular orbit within or outside the original orbit and then returns to its original orbit which is demonstrated by the figure on the right.[28]

Up to Newton

Copernicus's Heliocentric Model

Nicholas Copernicus, in reflecting about Ptolemy and Aristoles interpretations of the Solar System, believed that all the orbits of the planets and Moon must be a perfect uniform circular motion despite the observations showing the complex retrograde motion.[29] During the 16th century, Nicholas Copernicus introduced a new model which was consistent with the observations and allowed for perfect circular motion. This is known as the Heliocentric model where the Sun is placed at the centre of the Solar System and the Earth is, like all the other planets, orbiting it. The heliocentric model also resolved the varying brightness of planets problem.[30] Copernicus also supported the spherical Earth theory with the idea that nature prefers spherical limits which are seen in the Moon, the Sun, and also the orbits of planets.[31] Copernicus believed that the universe had a limit, a spherical limit.[31] Copernicus contributed further to practical Astronomy by producing advanced techniques of observations[32] and measurements and providing instructional procedure.[33]

A visual representation of the Earth-orbiting around the Sun in an elliptical orbit.

Kepler's model

In 1609, Johannes Kepler, using his teacher's (Tycho Brahe) accurate measurements, noticed the inconsistency of a heliocentric model where the sun is exactly in the centre. Instead Kepler developed a more accurate and consistent model where the sun is located not in the centre but at one of the two foci of an elliptic orbit.[34] Kepler derived the three laws of planetary motion which changed the model of the Solar System and the orbital path of planets. The three laws of planetary motion are:

  1. All planets orbit the Sun in elliptical orbits (image on the left) and not perfectly circular orbits.[35]
  2. The radius vector joining the planet and the Sun has an equal area in equal periods.[36]
  3. The square of the period of the planet (one revolution around the Sun) is proportional to the cube of the average distance from the Sun.[37]
[37]

where a is the radius of the orbit, T is the period, G is the gravitational constant and M is the mass of the Sun. The third law explains the periods that occur during the year which relates the distance between the Earth and the Sun.[38]

Galileo's discoveries

With the help of the telescope providing a closer look into the sky, Galileo Galilei proved the heliocentric model of the Solar System. Galileo observed the phases of Venus's appearance with the telescope and was able to confirm Kepler's first law of planetary motion and Copernicus's heliocentric model.[39] Galileo claimed that the Solar System is not only made up of the Sun, the Moon and the planets but also comets.[40] By observing movements around Jupiter, Galileo initially thought that these were the actions of stars.[41] However, after a week of observing, he noticed changes in the patterns of motion in which he concluded that they are moons, four moons.[41]

Newton's interpretation

After all these theories, people still did not know what made the planets orbit the Sun. Until the 17th century when Isaac Newton introduced The Law of Universal Gravitation. He claimed that between any two masses, there is an attractive force between them proportional to the inverse of the distance squared.[42]

[43]

where m1 is the mass of the Sun and m2 is the mass of the planet, G is the gravitational constant and r is the distance between them.[43] This theory was able to calculate the force on each planet by the Sun, which consequently, explained the planets elliptical motion.[44]

See also

References

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