1.2: Origins of Astronomy

Astronomy around the World

Astronomy has been connected to human life since people first looked up at the sky. Tracking the motions of the Sun, Moon and stars had practical applications. The Sun provided light and defined the day. The Moon sometimes provided light at night for those brave enough to venture into the dark. The Moon was also useful for tracking the passage of time since it regularly completed a cycle, with regular occurrences of full moons. As societies developed, they used the positions of the Sun and Moon to track the passage of seasons, aiding in the development of agriculture. For example, ancient Egyptians used the stars to predict the flooding of the Nile river, their main source of water for crops. The positions of the stars were also used for navigation, from simple tracking of a pole star like Polaris to following the stars across the ocean.

At the same time, the objects in our sky also have deep spiritual meaning. There are countless cultural stories describing the origin of the Sun, Moon, and stars. It would take an entire new textbook to describe a fraction of these belief systems. This section will review some ancient calendars and a few examples of how cultures interacted with the sky, with some information in the Further Exploration box. Do not worry about any terms that you are unfamiliar with, they will be covered in later chapters.

Early Calendars

Early cultures created calendars based on the objects in the sky, including building structures to aid in the measurement of time. Figure \(\PageIndex{1}\) includes Stonehenge, a complex array of stones, ditches, and holes arranged in concentric circles, located about 13 kilometers from Salisbury in southwest England. Carbon dating and other studies show that Stonehenge was built during three periods ranging from about 2800 to 1500 BCE. Some of the stones are aligned with the directions of the Sun and Moon during their risings and settings during the summer and winter solstices, to track the seasons.

Figure \(\PageIndex{1}\) : Stonehenge. This ancient stone monument in southwest England is aligned with the positions of the Sun and Moon at the summer and winter solstices, allowing early cultures to track the seasons. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{1}\).

The Maya in Central America built an observatory that the Spanish later named El Caracol, the snail, after its shell-like structure. In Figure \(\PageIndex{2}\), the observatory was located in a tower so that astronomers could have a clear view of the sky above the surrounding trees. The Maya developed a calendar that not only tracked the passage of time, but also predicted the future positions of objects in the sky.

Figure \(\PageIndex{2}\) : El Caracol. This circular tower at Chichen Itza in the Yucatan gave Maya astronomers a clear view above the surrounding trees to track and predict the positions of celestial objects. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{2}\).

The Western calendar derives from a long history of timekeeping beginning with the Sumerians, dating back to at least the second millennium BCE, and continuing with the Egyptians and the Greeks around the eighth century BCE. These calendars led, eventually, to the Julian calendar, introduced by Julius Caesar, which approximated the year at 365.25 days. The Romans created the leap year to have one extra day every 4 years, bringing its length to 366 days, making the average length of the year in the Julian calendar 365.25 days.

Australian Aboriginal Astronomy

Australian Aboriginal astronomy has been passed down orally, through ceremonies, and in their artwork of many kinds. The astronomical systems passed down allowed them to use them as a practical means for creating calendars and for navigating across the continent and waters of Australia. There is a diversity of astronomical traditions in Australia, each with its own particular expression of cosmology. However, there appear to be common themes and systems between the groups. Many of the constellations were given names based on their shapes, while others, such as Emu in the Sky, describe the dark patches rather than the points lit by the stars. Contemporary Indigenous Australian art often references astronomical subjects and their related lore.

Ancient Chinese Astronomy

Astronomy in China has a long history stretching from the Shang dynasty, being refined over a period of more than 3,000 years. The ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categorized in the twenty-eight mansions have been found on oracle bones unearthed at Anyang, dating back to the mid-Shang dynasty. The core of the "mansion" (宿 xiù) system also took shape around this period, by the time of King Wu Ding (1250-1192 BCE).

Detailed records of astronomical observations began during the Warring States period (fourth century BCE). They flourished during the Han period (202 BCE - 220 CE) and subsequent dynasties with the publication of star catalogues. Chinese astronomy was equatorial, centered on close observation of circumpolar stars. Some elements of Indian astronomy reached China with the expansion of Buddhism after the Eastern Han dynasty (25-220 CE), but most incorporation of Indian astronomical thought occurred during the Tang dynasty (618-907 CE), when numerous Indian astronomers took up residence in the Chinese capital Chang'an, and Chinese scholars, such as the Tantric Buddhist monk and mathematician Yi Xing, learned the Indian system. Chinese astronomers recorded comets, meteors, and dark spots on the Sun. They also kept careful records of stars that were normally too faint to see but suddenly flared up to become visible to the unaided eye for a few weeks or months.

Early Greek and Roman Cosmology

Ancient Greek astronomers knew the Earth was round since the time of Pythagoras, a philosopher and mathematician who lived 2500 years ago. He believed circles and spheres to be perfect forms and suggested that Earth should therefore be a sphere. As evidence that the gods liked spheres, the Greeks cited the fact that the Moon is a sphere.

Aristotle (384-322 BCE) described how the progression of the Moon's phases was the result of seeing different portions of the Moon's sunlit hemisphere as the month goes by. He also knew that the Sun had to be farther away from Earth than the Moon because occasionally the Moon passed between Earth and the Sun during a solar eclipse. Aristotle provided evidence that Earth was round, because of the shape of Earth's shadow during a lunar eclipse. Figure \(\PageIndex{3}\) demonstrates that a spherical object always produces a round shadow. If Earth were a disk, for example, there would be some occasions when the sunlight would strike it edge-on and its shadow on the Moon would be a line.

Figure \(\PageIndex{3}\) : Earth's Round Shadow. A lunar eclipse occurs when the Moon passes into and out of Earth's shadow. The curved edge of the shadow crossing the Moon is evidence that Earth is spherical. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{3}\).

Aristarchus of Samos (310-230 BCE) suggested that Earth was moving around the Sun, but Aristotle and most of the ancient Greek scholars rejected this idea. One of the reasons for their conclusion was the thought that if Earth moved about the Sun, they would be observing the stars from different places along Earth's orbit, in a process called stellar parallax. As Earth moved along, nearby stars should shift their positions in the sky relative to more distant stars. In a similar way, we see foreground objects appear to move against a more distant background whenever we are in motion. This meant either that Earth was not moving or that the stars had to be so tremendously far away that the parallax shift was immeasurably small. At the time astronomers were unable to measure stellar parallax, so they assumed that Earth did not move.

Ptolemy's Model of the Solar System

Claudius Ptolemy wrote a large compilation of astronomical knowledge, which today is called by its Arabic name, Almagest, meaning The Greatest. Almagest is a collection of Ptolemy's own work and the work of other Greek astronomers. Ptolemy's most important contribution was a geometric representation of the Solar System that predicted the positions of the planets for any desired date and time. Ptolemy combined new observations with a large collection of previous observations and created the geocentric model, a model of the universe with Earth at the center and the Sun, Moon, and planets orbiting the Earth in perfect circles. The model not only describes the structure of the Solar System, but was also able to accurately predict the future positions of objects in the sky.

One difficulty in the design of the geocentric model was how the planets sometimes reversed course in their movement across the sky, called retrograde motion. Normally, planets move eastward in the sky, relative to the positions of the background stars and constellations. Ptolemy had to include retrograde motion in his model. Also, the Greeks believed that celestial motions had to be circles, so Ptolemy had to construct his model using only circles.

Ptolemy solved the problem of explaining retrograde motions of planets by having each planet move in a small orbit called an epicycle. The center of the epicycle then orbited Earth on a circle called a deferent. When the planet is at position x in Figure \(\PageIndex{4}\) on the epicycle orbit, it is moving in the same direction as the center of the epicycle. From Earth, the planet appears to be moving eastward. When the planet is at y, however, its motion is in the direction opposite to the motion of the epicycle's center around Earth. By choosing the right combination of speeds and distances, Ptolemy succeeded in having the planet moving westward at the correct speed and for the correct interval of time, thus replicating retrograde motion with his model. In order to match the observed motions of the planets, Ptolemy had to center the deferent circles, not on Earth, but at points some distance from Earth. In addition, he introduced uniform circular motion around yet another axis, called the equant point. All of these considerably complicated the model.

Figure \(\PageIndex{4}\) : Epicycles. Each planet orbits on a small circle called an epicycle, which itself orbits Earth on a larger circle called the deferent, offset from an equant point. This complexity let Ptolemy's model preserve circular motion around a stationary Earth while still matching the observed retrograde motion of planets. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Accessible description of Figure \(\PageIndex{4}\).

Islamic Astronomy in the Middle Ages

Medieval Islamic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (9th-13th centuries), and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia, Al-Andalus, and North Africa, and later in the Far East and India. Islamic astronomy played a significant role in the revival of ancient astronomy following the loss of knowledge during the early medieval period, notably with the production of Latin translations of Arabic works during the 12th century.

A significant number of stars in the sky, such as Aldebaran, Altair and Deneb, and astronomical terms such as alidade, azimuth, and nadir, are still referred to by their Arabic names. A large amount of literature from Islamic astronomy remains today, numbering approximately 10,000 manuscripts scattered throughout the world, many of which have not been read or catalogued. Even so, a reasonably accurate picture of Islamic activity in the field of astronomy can be reconstructed. Astronomers of this age built observatories and astronomical instruments such as astrolabes and mechanical calendars. Ibn Firnas created a planetarium in his home that included artificial storm noises and was completely made of glass.

Further Exploration

• Learn how polynesian sailors in the ancient past and today use the stars to navigate the ocean.
• The planetary configurations simulator from Foothill AstroSims allows you to see the usual prograde and occasional retrograde motion of other planets. You can switch back and forth between viewing motion from Earth and Mars (as well as other planets).
• Use the Ptolemaic System simulator from Foothill AstroSims to explore how Ptolemy's system of deferents and epicycles explained the apparent motion of the planets.

Attributions

This page was adapted from "Australian Aboriginal Astronomy," "Chinese Astronomy," and "Astronomy in the Medieval Islamic World" in Wikipedia originally written by Wikipedia, and published under CC BY-SA 4.0.