The History of Cosmology
By Keri Lofftus
World Ancestry Staff
Published 13 June 2008

Cosmology is directly linked to astronomy. It is the study of the universe and its origins. Astronomy is considered to be the oldest of the natural sciences where it resembled what we today think of as astrology. Early astronomy was mostly concerned with recording the positions of the Sun, Moon, stars, and planets for both religious and agricultural purposes. Ancient civilizations saw the different celestial objects as gods who had influence over weather, the changing of the season, and the fortunes of humans. They used the changing positions of the Sun and Moon to create calendars, which were essential to early agricultural societies because of the need to plant their crops at the right time of year.

Babylonian astronomy refers to the astronomy that developed in Mesopotamia (modern-day Iraq) in the ancient civilizations of Sumer, Assyria, and Babylon. The most important discovery made by the Babylonian astronomers is that almost all astronomical phenomena are periodic. Using their extensive observations, they were eventually able to predict the rising and setting of the visible planets. They also discovered the pattern to the changing length of daylight Earth experiences as we revolve around the sun. The founding of the Neo-Babylonian Empire under the reign of Nabonassar began a new era in Babylonian astronomy in 747 BCE. It is during this time that increased observational records allowed for the discovery of the 18-year saros cycle, which can predict the eclipses of the Sun and the Moon. The most well known astronomer from the age is Seleucus of Seleucia (190–150 BCE), known mostly from the writing of Plutarch, a Greek historian. Seleucus was one of the first to propose the heliocentric theory that the Earth rotated on its own axis and revolved around the Sun, in contrast to the common geocentric (Earth is the center of the universe) theory of the times. He probably got some of these ideas from Aristarchus of Samos, a Greek astronomer active in the 3rd century BCE. Plutarch claimed that Seleucus had proved the heliocentric theory, but the evidence is unknown to modern scholars. Seleucus also deduced that the Moon was the cause of the tides, noticing that the variation in the tides correspond to the phases of the moon, although he did theorize that the Earth’s atmosphere played an intermediate role.

Indian astronomy encompasses a period from the 3rd century BCE to the 16th century CE on the Indian subcontinent. Ancient Indian astronomy used sidereal calculations. A sidereal period is the time it takes a celestial object to orbit the Sun once in relation to the stars. Around 500 CE, Aryabhata proposed his system of astronomy which had the Earth rotating about an axis (although whether it was a heliocentric theory is still debated). He made very accurate approximation of the Earth’s circumference and diameter. Aryabhata’s work with eclipses eventually enabled him to predict almost the exact times they would occur and helped him deduce that the light from the Moon and other planets was actually reflected light from the Sun. Also, he was the first to claim that the planets orbited the Sun in the shape of an ellipse. Another famous Indian astronomer was Brahmagupta (598-668 CE). His text on astronomy, the Brahmasphutasiddhanta, was the first to use algebra in the calculation and had detailed equations to predict the movements of the planets and conjunctions (where two celestial objects seem to come very close together in the night sky). Following in the mathematical footsteps of Brahmagupta, another Indian astronomer, Bhaskara, calculated the period of Earth’s orbit to 9 decimal places.

Compared to other cultures, the Chinese made magnificent recordings of astronomical phenomena. Astronomy flourished without much outside influence between the 4th century BCE and the 12th century CE. The first famous astronomer out of ancient China is Gan De, the first person to create a star catalogue. It is also claimed that he was the first to see one of the moons of Jupiter in 364 BCE, 1300 years before Galileo’s observations. Another famous Chinese astronomer, Shen Kuo (1031–1095), wrote about the meridian measurement between the North Star (Polaris) and true north, an invaluable concept for navigation with magnetic compasses. Shen Kuo also argued for a spherical Earth based on his observations of lunar eclipses and how the shadow that the Earth casts on the Moon is circular. However, Chinese astronomy is most famous not for the discoveries by its astronomers, but by the detailed observations they made, such as determining that Jupiter takes 12 years to orbit the sun (the modern value is about 11.86 years). They were the first to record a solar eclipse (2137 BCE), a solar prominence and two novas (1400 BCE), Halley’s Comet (613 BCE), a sunspot (28 BCE), a supernova (185 CE), and a meteor shower (687 CE). The ancient Chinese were the first to accurately determine the Spring Equinox around 1100 BCE, and Chinese astronomy is also famous for its observations of the Crab Nebula, a supernova that exploded in 1054 CE. It wasn’t discovered by Western astronomers until the 1700s.

Much of the basis for Western astronomy came from the Greeks and Romans, even though their approach to the study of the cosmos was sometimes less scientific than other civilizations. The largest influence Greek astronomy had on modern astronomy was the names for the different heavenly bodies, including the constellations. From the Romans, we get the names of the 8 planets. It was the Greeks who came up with the first models of how our solar system works. Eudoxus of Cnidus was the first to attempt a mathematical model which could predict the motion of the planets. While technically incorrect, his model did make somewhat accurate approximations. His model was the first formulation of the geocentric theory, where the Earth was at the center of the heavens and all the other planets (which included the Moon and the Sun) rotated around it in their own celestial spheres. However, the flaws in Eudoxus’ model motivated other astronomers to attempt to create their own, more precise, theories. The Greek astronomer Hipparchus was able to come up with more accurate models for the movement of the Sun and the Moon while still keeping Earth as the center of the solar system. He also discovered the phenomenon of precession, which is the movement of stars caused by the tilt of the Earth’s axis. The most famous astronomer from the period was Ptolemy (87-150 CE). While very little of his theories were his own, borrowing most of his theorems and observations from other astronomers, his model of the solar system was extraordinarily accurate. It is this accuracy which probably helped the geocentric model of the solar system to survive until the 1500s.

During the Middle Ages, Western astronomy slowed almost to a halt. It wasn’t until the Renaissance that any more large astronomical discoveries were made. This new period of scientific progress began with Nicholaus Copernicus (1473-1543) and his heliocentric theory. Although proposed before, Copernicus was the first to back it up with mathematical descriptions. It was the work of another great scientist, Galileo Galilei (1564-1642), whose observations of Jupiter’s four largest moons leant credence to Copernican theory. Galileo also discovered that Venus, like the moon, goes through phases. Another Renaissance astronomer, Johannes Kepler (1571-1630), came up with the three laws of planetary motion by using the observations of Tyco Brahe (1546-1601 CE), which showed that planets orbit in ellipses, do not have uniform speed, and that there is a relationship between a planet’s distance from the Sun and its orbital period. Arguably, however, the greatest astronomer during this time was Sir Issac Newton (1642-1727), whose laws of motion and ideas of gravity explained the movements of both Earthly and heavenly bodies.

However, while Newton’s laws told us much about how gravity worked and could be calculated, it wasn’t until the dawn of the 20th century that we knew what gravity actually was. Albert Einstein (1879-1955) is known for his famous equation E=mc² (1905) and for finally explaining to the world that gravity was actually the warping of space-time by massive objects, such as our Sun. In 1929, the astronomer Edwin Hubble discovered that most galaxies he viewed were traveling away from our own at extremely high velocities. This observation led him to realize that, since the universe was expanding, it must have originated from a single point in space and time. We now know this theory as the Big Bang. Hubble’s discoveries allowed later scientists to accurately determine the age of our universe to be approximately 13.7 billion years old.

Modern astronomy has become much more complex in the past 50 or so years. Telescopes are now built to examine all parts of the light spectrum, not just visible light. With these telescopes, astronomers are able to understand galactic, stellar, and planetary formation. Arno Penzias and Robert Wilson discovered the Cosmic Microwave Background Radiation, which revealed many intriguing details about the early universe before matter as we know it had been formed. The work of Stephen Hawking and Roger Penrose has greatly expanded our knowledge of black holes, one of the most unusual phenomena in the universe. And during their work the United States put the first man on the Moon. In addition, we’ve been able to construct two Earth-orbiting space stations: the Mir and the International Space Station. Mir is no longer in orbit, meeting its demise in in the first quarter of 2001, when it was deliberately taken offline and de-orbited in a phenomenal display of atmospheric reentry. More recently the Phoenix Mars Lander settled on the Red Planet’s surface in 2008. Many mysteries are left for the current and upcoming astronomers to solve, such as the nature of so-called dark matter, the possible fate of our universe, and whether human-kind is just one of many intelligent beings.

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