2.2 Ancient Astronomy
3 min read•june 12, 2024
Ancient astronomers laid the groundwork for modern astronomy. From Babylonian calendars to Mayan observatories, early cultures tracked celestial events, linking them to agriculture and religion. These practices shaped our understanding of time and space.
Greek thinkers made significant strides in understanding Earth's shape and size. They used logical reasoning and simple observations to conclude Earth was spherical, paving the way for more advanced astronomical concepts and measurements.
Ancient Astronomical Practices and Earth's Properties
Ancient astronomical practices
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- Babylonians developed a based on Moon and Sun cycles, recorded detailed planetary motion observations, and used a base-60 number system for calculations
- Egyptians aligned pyramids and temples with astronomical events (rising of Sirius), created a solar calendar based on Nile River flooding, and used a tool for cardinal directions
- Chinese recorded observations of comets, novae, and supernovae, developed a lunisolar calendar, and linked astronomical events to political power ()
- Maya constructed precise calendars based on Sun, Moon, and Venus cycles, built observatories and temples aligned with astronomical events, and connected cycles to agriculture and religious ceremonies
- Ancient astronomers used various tools to measure and predict celestial events, including the for determining the positions of celestial bodies
Greek reasoning for spherical Earth
- Lunar eclipses show Earth's shadow on the Moon is always circular, which can only be cast by a spherical object
- Ships disappearing bottom-first over the horizon suggests Earth's surface is curved as the hull disappears before the sails
- Changing positions of stars in the sky as one travels north or south indicates Earth's surface is curved
Greek method for Earth's size
- observed the Sun directly overhead in Syene (Aswan) on the summer and measured the Sun's angle in Alexandria on the same day
- Calculated Earth's circumference using the angle and distance between Syene and Alexandria
- Assumed Earth is a sphere, Syene and Alexandria are on the same meridian, and the Sun's rays are parallel
- Used a (vertical stick) to measure the Sun's angle and cast shadows
Precession and Ptolemy's Geocentric Model
Precession in astronomical observations
- is Earth's slow, conical rotational axis motion over ~26,000 years caused by the Sun and Moon's gravitational pull on Earth's equatorial bulge
- Results in the apparent shift of celestial poles and star positions over time
- Affects timing of equinoxes and solstices, causing them to occur earlier each year
- Leads to the gradual change of the pole star over time (currently , but will change in the future)
Ptolemy's geocentric model and significance
- 's placed Earth at the universe's center, with the Sun, Moon, planets, and stars orbiting it
- Planets move on small epicycles, which move on larger deferents, explaining observed
- The model was widely accepted for over 1,400 years and provided a basis for predicting celestial object positions
- Influenced astronomical thinking until the gained acceptance in the 16th century
- Demonstrated the importance of creating mathematical models to explain observations
Celestial Coordinate Systems and Measurements
Celestial sphere and coordinate systems
- The is an imaginary sphere surrounding Earth, onto which celestial objects are projected
- The is the projection of Earth's equator onto the
- The is the apparent path of the Sun's motion on the celestial sphere
- The is a band of constellations along the , traditionally used in astrology
Astronomical measurements
- The (AU) is a unit of length roughly equal to the average distance between Earth and the Sun, used for measuring distances within the solar system
Key Terms to Review (36)
Aristotle: Aristotle was an ancient Greek philosopher whose works covered a broad range of subjects, including astronomy. He proposed a geocentric model of the universe, where Earth is at the center and all celestial bodies revolve around it.
Astrolabe: An astrolabe is an ancient astronomical instrument used for navigation, timekeeping, and surveying. It was a critical tool in the development of early astronomy and played a significant role in the exploration and mapping of the world during the ancient and medieval periods.
Astronomical unit: An astronomical unit (AU) is a unit of distance defined as the average distance between Earth and the Sun, approximately 149.6 million kilometers (93 million miles). It is commonly used to describe distances within our solar system.
Astronomical Unit: The astronomical unit (AU) is the standard unit of length used to measure distances within our solar system. It is defined as the average distance between the Earth and the Sun, which is approximately 150 million kilometers or 93 million miles.
Celestial equator: The celestial equator is an imaginary projection of Earth's equator into space. It divides the sky into the northern and southern celestial hemispheres.
Celestial Equator: The celestial equator is an imaginary line in the celestial sphere that corresponds to the Earth's equator, dividing the sky into northern and southern hemispheres. It serves as a key reference point for understanding the motions of celestial objects and their positions in the night sky.
Celestial sphere: The celestial sphere is an imaginary sphere that extends infinitely into space with Earth at its center, used to map the positions of stars and planets. Astronomers use it to simplify the complex motions of celestial bodies for observation and navigation.
Celestial Sphere: The celestial sphere is an imaginary, three-dimensional sphere that surrounds the Earth, upon which all celestial objects and events appear to be projected. It serves as a conceptual framework for understanding the motions and positions of objects in the sky, and is a fundamental concept in the study of astronomy.
Copernican: The Copernican model, also known as the heliocentric model, is an astronomical theory that places the Sun, rather than the Earth, at the center of the universe. This revolutionary concept, proposed by Nicolaus Copernicus in the 16th century, challenged the prevailing Ptolemaic geocentric model, which had been the accepted view for over a thousand years.
Deferent: In the context of ancient astronomy, a deferent is a circular path around the Earth that was used to explain the apparent motion of a planet in the night sky. It was a key component of the geocentric model of the universe developed by ancient Greek astronomers.
Ecliptic: The ecliptic is the apparent path that the Sun traces out in the sky over the course of a year as observed from Earth. It also represents the plane of Earth's orbit around the Sun.
Ecliptic: The ecliptic is the apparent path that the Sun appears to take across the sky over the course of a year, as viewed from Earth. It is the projection of Earth's orbit around the Sun onto the celestial sphere, and it serves as a reference plane for many important astronomical phenomena.
Epicycle: An epicycle is a small circle whose center moves around the circumference of a larger circle (deferent). It was used in the Ptolemaic model to explain retrograde motion of planets.
Epicycle: An epicycle is a geometric model used in ancient astronomy to explain the apparent motion of planets in the sky. It involves a smaller circular orbit, called an epicycle, superimposed on a larger circular orbit, called a deferent, to account for the complex and sometimes retrograde motion of the planets as observed from Earth.
Equinox: An equinox is an astronomical event that occurs twice a year, around March 20-21 and September 22-23, when the sun is directly above the equator, and the length of day and night are approximately equal. This phenomenon is a crucial concept in understanding ancient astronomy, the seasons, and the calendar.
Eratosthenes: Eratosthenes was an ancient Greek mathematician, astronomer, and geographer who is best known for his remarkably accurate calculation of the circumference of the Earth. He is considered one of the most influential scholars of the Hellenistic period, making significant contributions to various fields of study.
Geocentric Model: The geocentric model is the astronomical model in which the Earth is at the center of the universe, with all other celestial bodies orbiting around it. This model was the prevailing view of the cosmos in many ancient civilizations and was the accepted model of the universe for centuries before being challenged and eventually replaced by the heliocentric model.
Gnomon: A gnomon is a vertical object, such as the raised part of a sundial, whose shadow is used to indicate the time of day. It is a fundamental component of ancient astronomical timekeeping and measurement tools.
Heliocentric Model: The heliocentric model is an astronomical model that places the Sun, rather than the Earth, at the center of the solar system. This model, which was proposed by Copernicus and later supported by Galileo, revolutionized our understanding of the universe and challenged the long-held geocentric view.
Hipparchus: Hipparchus was an ancient Greek astronomer and mathematician who is often considered the founder of trigonometry. He made significant contributions to the understanding of celestial bodies and their movements.
Lunar eclipse: A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon. This can only happen during a full moon when the Sun, Earth, and Moon are aligned in a straight line or close to it.
Lunar Eclipse: A lunar eclipse occurs when the Earth passes directly between the Sun and the Moon, blocking the Sun's light from reaching the Moon. This causes the Moon to appear dark or reddish in color, as the Earth's atmosphere refracts and scatters the Sun's light, casting a shadow on the lunar surface.
Lunisolar Calendar: A lunisolar calendar is a calendar that combines elements of both a solar calendar, which is based on the length of the year as determined by the Earth's revolution around the Sun, and a lunar calendar, which is based on the phases of the Moon. This type of calendar attempts to synchronize the months with the Moon's cycles while keeping the years aligned with the solar year.
Mandate of Heaven: The Mandate of Heaven is a Chinese political and religious concept that legitimizes the rule of the emperor. It suggests that the emperor's authority is of divine origin and that his rule is justified as long as he rules justly and maintains the prosperity of the kingdom.
Merkhet: A merkhet is an ancient astronomical instrument used by ancient civilizations, particularly in Egypt and Mesopotamia, to track the movement of stars and determine the time of night. It played a crucial role in the development of early astronomy and timekeeping methods.
Polaris: Polaris, also known as the North Star, is a prominent star in the northern celestial hemisphere. It holds great significance in the context of ancient astronomy, Earth's sky, the seasons, and the brightness of stars.
Precession: Precession is the slow, conical motion of Earth's rotation axis, which causes the celestial poles to trace out circles in the sky over a period of approximately 26,000 years. This phenomenon affects the position of stars and constellations over long time periods.
Precession: Precession is the gradual shift in the orientation of an object's rotational axis in relation to a fixed reference frame. This phenomenon occurs in various celestial bodies, including Earth, and has significant implications for the study of astronomy, timekeeping, and the calendar.
Ptolemy: Ptolemy was a prominent ancient Greek astronomer, mathematician, and geographer who lived in Alexandria, Egypt during the 2nd century AD. He is best known for his comprehensive model of the universe, which placed the Earth at the center and described the motions of the Sun, Moon, and planets.
Pythagoras: Pythagoras was an ancient Greek philosopher and mathematician known for his contributions to mathematics, astronomy, and music theory. He is renowned for the Pythagorean theorem and his belief that numbers are fundamental to understanding the universe.
Retrograde motion: Retrograde motion is the apparent backward movement of a planet against the background of stars. This phenomenon occurs when Earth, which moves faster in its orbit, overtakes and passes an outer planet.
Retrograde Motion: Retrograde motion refers to the apparent backward or westward movement of a celestial body across the sky, relative to the fixed stars, as observed from Earth. This phenomenon is observed for certain planets in our solar system and can provide important insights into the dynamics of planetary motion.
Solstice: The solstice is the point in Earth's orbit where the Sun reaches its highest or lowest position in the sky at noon. This results in the longest day (summer solstice) or the longest night (winter solstice) of the year.
Solstice: A solstice is an astronomical event that occurs twice a year, marking the longest and shortest days of the year. It is a point in time when the sun reaches its northernmost or southernmost position in the sky, relative to the Earth's equator.
Zodiac: The zodiac is a belt-shaped region in the sky that extends approximately 8-9 degrees on either side of the ecliptic, where the apparent paths of the sun, moon, and planets can be observed. It is divided into twelve equal parts, each associated with a constellation.
Zodiac: The zodiac is a celestial coordinate system that divides the sky into 12 equal parts, each associated with a specific constellation. It is a fundamental concept in both ancient astronomy and astrology, serving as a framework for understanding the motions of the Sun, Moon, and planets relative to the fixed stars.