According to the famous astrologer in India, Every night, as the stars move across the sky, people all over the world glance up and wonder about their place in the cosmos. Civilizations have created various ways for ordering and comprehending the heavens throughout history. While societies in the Americas, China, and India established their own systems, Babylonian and Egyptian astronomers built systems that constituted the foundation for Greek astronomy.
The work of ancient Greek astronomers is well documented in the Library of Congress holdings, owing to the way the Greek tradition of inquiry was carried on by Islamic astronomers and later into early modern European astronomy. This section takes you on a tour of some of ancient Greece’s astrological theories and models as depicted in Library of Congress holdings.
The Sphere of the World:
The Earth was commonly acknowledged as a sphere by the 5th century B.C. This is important because there is a common myth that ancient peoples believed the Earth was flat. This was not the case at all.
Empedocles and Anaxagoras provided arguments for the Earth’s spherical nature in the 5th century B.C. They discovered the Earth’s shadow on the moon during a lunar eclipse, when the Earth is between the sun and the moon. The moon’s shadow is plainly spherical as it sweeps across it. This would lead to the conclusion that the Earth is a sphere.
Experiencing the Sphere of the Earth:
Given the rarity of opportunity to observe a moon eclipse, sailors’ experiences provided evidence of the earth’s roundness.
When a ship arrives on the horizon, the top of the ship is the first thing that is visible. This has been used in a variety of astronomy texts to demonstrate the Earth’s roundness over time.
This is exactly what one would expect on a spherical Earth, as the illustration shows.If the Earth were flat, you would be able to view the entire ship as soon as it appeared.
Measuring the Size of the Earth:
Lunar eclipses also provided another important insight into our home on Earth. Aristarchus of Samos reasoned that he could figure out the size of the Earth based on information available during a lunar eclipse in the 3rd century B.C. A translation of his work is depicted in the diagram to the right. The sun is the largest circle, the Earth is the medium circle, and the moon is the smallest circle. A lunar eclipse occurs when the Earth passes between the sun and the moon, and measuring the size of the Earth’s shadow on the moon gave him part of the information he needed to determine its size.
Around 240 B.C., Eratosthenes calculated the Earth’s circumference. He took a different approach, calculating the angle of the shadows cast in Alexandria and Syene in relation to the Sun by measuring them. Because we don’t know how long the units of measurement were, there is some debate about the precision of his computations.
The measurement, on the other hand, was rather accurate in terms of Earth’s real size. Math was used by the Greeks to postulate about the nature of their universe. They held a variety of ideas about nature and the world, but they were attempting to base their beliefs in empirical investigations of what they could reason from facts in many cases.
The Celestial Region’s Wandering and Fixed Stars:
The celestial portion of the skies, in contrast to the terrestrial, had a fundamentally different character. The ancient Greeks discovered two types of celestial phenomena in the night sky: fixed stars and wandering stars. Consider the night sky.
The majority of visible things appear to move at the same speed and present themselves in the same order night after night. These are the stars that never change. They appear to move in unison.
Aside from these, there were nine objects that acted differently: the moon, the sun, and the planets Mercury, Venus, Mars, Saturn, and Jupiter, all of which followed their own systems. These were the wandering stars to the Greeks.
The entire cosmos was contained within a large sphere in this arrangement. This sphere was divided into two sections: a celestial realm on the outside and a terrestrial realm on the inside. The moon’s orbit served as a dividing line between the two. While the earth was in a state of turmoil, the skies remained constant. Aristotle proposed that the skies were made of a fifth component, the quintessence, and that the heavens were a region of perfect spherical motion.
Ptolemy’s Circles on Circles:
From his home in Alexandria, Egypt, Claudius Ptolemy (90-168) created a wealth of astrological knowledge. Ptolemy created a system for predicting the motion of the stars that was published in his principal astronomical book, Almagest, based on hundreds of years of observation from the period of Hipparchus and Eudoxus, as well as a compilation of astronomical data collected by the Babylonians.
Because of Ptolemy’s skill in combining and refining ideas and innovations in astronomy, his Almagest became so famous that older works went out of print. For the next thousand years, the Almagest was translated into Arabic and Latin and became the primary astronomical literature.
Tables abound in the Almagest. In this way, the book serves as a tool for predicting the positions of the stars. The book is far more focused on functioning as a useful instrument than on offering a method for describing the nature of the skies, in comparison to older astronomy suggested by the best astrologer in India. Trying to anticipate the position of the stars in space over time resulted in a far more intricate model.
The Ptolemaic Model:
By the time of Ptolemy Greek astronomers had proposed adding circles on the circular orbits of the wandering stars (the planets, the moon and the sun) to explain their motion. These circles on circles are called epicycles. In the Greek tradition, the heavens were a place of perfect circular motion, so the way to account for perfection was with the addition of circles. This resulted in disorienting Illustration.
Ptolemy created a series of new concepts to get around the complexity of such a large number of circles. He needed eccentric circles to adequately represent planetary motion. With the eccentric circle, the Earth would not be at the centre of the planet’s orbit, but rather some other place.
Ptolemy next had to place the epicycles on deferents, a new set of circles. As a result, the planets travelled in circles in circular orbits. Ptolomy also needed to introduce equates, a tool that allowed planets to revolve around these circles at varied speeds. The model that resulted was complicated, but it had a lot of predictive potential according to the famous astrologer in Kolkata.