As the stars move across the sky each night people of the world have looked up and wondered about their place in the universe. Throughout history civilizations have developed unique systems for ordering and understanding the heavens. Babylonian and Egyptian astronomers developed systems that became the basis for Greek astronomy, while societies in the Americas, China and India developed their own.
Ancient Greek astronomers' work is richly documented in the collections of the Library of Congress largely because of the way the Greek tradition of inquiry was continued by the work of Islamic astronomers and then into early modern European astronomy. This section offers a tour of some of the astronomical ideas and models from ancient Greece as illustrated in items from the Library of Congress collections.
By the 5th century B.C., it was widely accepted that the Earth is a sphere. This is a critical point, as there is a widespread misconception that ancient peoples thought the Earth was flat. This was simply not the case.
In the 5th century B.C., Empedocles and Anaxagoras offered arguments for the spherical nature of the Earth. During a lunar eclipse, when the Earth is between the sun and the moon, they identified the shadow of the Earth on the moon. As the shadow moves across the moon it is clearly round. This would suggest that the Earth is a sphere.
Given that opportunities for observations of a lunar eclipse do not come along that often, there was also evidence of the roundness of the earth in the experiences of sailors.
When a ship appears on the horizon it's the top of the ship that is visible first. A wide range of astronomy texts over time use this as a way to illustrate the roundness of the Earth. As the image suggests this is exactly what one would expect on a spherical Earth. If the Earth were flat, it would be expected that you would be able to see the entire ship as soon as it became visible.
Lunar eclipses also allowed for another key understanding about our home here on Earth. In 3rd Century B.C., Aristarchus of Samos reasoned he could figure out the size of the Earth based on information available during a lunar eclipse. The diagram at the right illustrates a translation of his work. The large circle is the sun, the medium circle is the Earth and the smallest circle is the moon. When the Earth is in-between the sun and the moon it causes a lunar eclipse and measuring the size of the Earth's shadow on the moon provided part of the information he needed to calculate its size.
Eratosthenes estimated Earth's circumference around 240 B.C. He used a different approach, measuring the shadows cast in Alexandria and Syene to calculate their angle relative to the Sun. There is some dispute on the accuracy of his calculations as we don't know exactly how long the units of measure were. The measurement however was relatively close to the actual size of the Earth. The Greeks were applying mathematics to theorize about the nature of their world. They held a range of beliefs about nature and the world but they were, in many cases, working to ground those beliefs in an empirical exploration of what they could reason from evidence.
In the tradition of Plato and Empedocles before him, Aristotle argued that there were four fundamental elements, fire, air, water and earth. It is difficult for us to fully understand what this meant as today we think about matter in very different terms. In Aristotle's system there was no such thing as void space. All space was filled with some combination of these elements.
Aristotle asserted that you could further reduce these elements into two pairs of qualities, hot and cold and wet and dry. The combination of each of these qualities resulted in the elements. These qualities can be replaced by their opposites, which in this system become how change happens on Earth. For example, when heated, water seemingly turns steam which looks like air.
In Aristotle's Cosmology, each of these four elements (earth, water, fire and air) had a weight. Earth was the heaviest, water less so, and air and fire the lightest. According to Aristotle the lighter substances moved away from the center of the universe and the heaver elements settled into the center. While these elements attempted to sort themselves out, to achieve this order, most of experience involved mixed entities.
While we have seen earth, fire, air and water, everything else in the world in this system was understood as a mixture of these elements. In this perspective, transition and change in our world resulted from the mixing of the elements. For Aristotle the terrestrial is a place of birth and death, based in these elements. The heavens are a separate realm governed by their own rules.
In contrast to the terrestrial, the celestial region of the heavens had a fundamentally different nature. Looking at the night sky the ancient Greeks found two primary kinds of celestial objects; the fixed stars and the wandering stars. Think of the night's sky. Most of the visible objects appear to move at exactly the same speed and present themselves in exactly the same arrangement night after night. These are the fixed stars. They appear to move all together. Aside from these were a set of nine objects that behaved differently, the moon, the sun and the planets Mercury, Venus, Mars, Saturn and Jupiter each moved according to a different system. For the Greeks these were the wandering stars.
In this system the entire universe was part of a great sphere. This sphere was split into two sections, an outer celestial realm and an inner terrestrial one. The dividing line between the two was the orbit of the moon. While the earth was a place of transition and flux, the heavens were unchanging. Aristotle posited that there was a fifth substance, the quintessence, that was what the heavens were made of, and that the heavens were a place of perfect spherical motion.
In Aristotle's words, "In the whole range of time past, so far as our inherited records reach, no change appears to have taken place either in the whole scheme of the outermost heaven or in any of its proper parts." It's important to keep in mind that in Aristotle's time there simply were not extensive collections of observational evidence. Things that looked like they were moving in the heavens, like comets, were not problematic in this model because they could be explained as occurring in the terrestrial realm.
This model of the heavens came with an underlying explanation. The celestial spheres were governed by a set of movers responsible for the motion of the wandering stars. Each of these wandering stars was thought to have an "unmoved mover" the entity that makes it move through the heavens. For many of the Greeks this mover could be understood as the god corresponding to any given entity in the heavens.
Claudius Ptolemy (90-168) created a wealth of astronomical knowledge from his home in Alexandria, Egypt. Benefiting from hundreds of years of observation from the time of Hipparchus and Eudoxus, as well as a set of astronomical data collected by the Babylonians, Ptolemy developed a system for predicting the motion of the stars that was published in his primary astronomical work, Almagest. Ptolemy's success at synthesizing and refining ideas and improvements in astronomy helped make his Almagest so popular that earlier works fell out of circulation. Translated into Arabic and Latin the Almagest became the primary astronomy text for the next thousand years.
The Almagest is filled with tables. In this sense the book is a tool one can use to predict the locations of the stars Compared to earlier astronomy the book is much more focused on serving as a useful tool than as presenting a system for describing the nature of the heavens. Trying to accurately predict the place of the stars over time resulted in creating a much more complicated 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 illustrations.
To escape the complicated nature of this extensive number of circles, Ptolomy added a series of new concepts. To accurately describe planetary motion, he needed to use eccentric circles. With the eccentric circle the center of the planets orbit would not be Earth but would instead be some other point. Ptolemy then needed to put the epicycles on another set of circles called deferents. So the planets moved on circles that moved on circular orbits. Ptolomy also needed to introduce equants, a tool that enabled the planets to move at different speeds as they moved around these circles. The resulting model was complex, but it had extensive predictive power.
Ptolemy came to represent a mathematical tradition, one focused on developing mathematical models with predictive power. Aristotle came to be known for putting forward the physical model of the heavens. Ptolemy was also interested in deploying his model of the heavens to describe its physical reality. However, his most important work was the mathematical models and data he used for predicting the motion of heavenly bodies. For a long time his name was synonymous with the model of the heavens.
Continued here:
- Students, teachers craft software to make astronomy accessible to the blind - UChicago News [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- [ 3 May 2017 ] NASA probe finds Saturn ring gap emptier than predicted News - Astronomy Now Online [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- Dark matter may be fuzzier than we thought - Astronomy Magazine [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- How a hidden population of pulsars may leave the Milky Way aglow - Astronomy Magazine [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- Local astronomy club offers peek at the heavens - Scranton Times-Tribune [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- Astronomers confirm nearby star a good model of our early solar system - Phys.Org [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- Pioneering radio astronomer Harold Weaver dies at age 99 - UC Berkeley [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- If we successfully land on Mars, could we live there? - Astronomy Magazine [Last Updated On: May 3rd, 2017] [Originally Added On: May 3rd, 2017]
- Arizona Technology Council and Arizona Astronomy Consortium ... - Yahoo Finance [Last Updated On: May 4th, 2017] [Originally Added On: May 4th, 2017]
- Hubble images the distant universe through a cosmic lens - Astronomy Magazine [Last Updated On: May 4th, 2017] [Originally Added On: May 4th, 2017]
- Everybody in the lab gettin' TIPSI: NAU astronomy students build camera to track asteroids - NAU News [Last Updated On: May 4th, 2017] [Originally Added On: May 4th, 2017]
- Bad Astronomy - : Bad Astronomy [Last Updated On: May 4th, 2017] [Originally Added On: May 4th, 2017]
- Scientists found a wave of ultra hot gas bigger than the Milky Way - Astronomy Magazine [Last Updated On: May 6th, 2017] [Originally Added On: May 6th, 2017]
- Cassini encounters the 'Big Empty' during its first dive - Astronomy Magazine [Last Updated On: May 6th, 2017] [Originally Added On: May 6th, 2017]
- Harold F. Weaver, pioneer of radio astronomy at UC Berkeley, dies - mySanAntonio.com [Last Updated On: May 6th, 2017] [Originally Added On: May 6th, 2017]
- How to See Jupiter by Day and its Moons by Night using Mobile Astronomy Apps - Space.com [Last Updated On: May 6th, 2017] [Originally Added On: May 6th, 2017]
- Astronomy Picture of the Day [Last Updated On: May 6th, 2017] [Originally Added On: May 6th, 2017]
- Final MTSU Star Party of the semester hosted by physics, astronomy departments - Sidelines Online (subscription) [Last Updated On: May 8th, 2017] [Originally Added On: May 8th, 2017]
- Harold F. Weaver, pioneer of radio astronomy at UC Berkeley, dies - SFGate [Last Updated On: May 8th, 2017] [Originally Added On: May 8th, 2017]
- Astronomy - Wikipedia [Last Updated On: May 8th, 2017] [Originally Added On: May 8th, 2017]
- Rosliston Astronomy Group is asking shoppers to vote for them to win Tesco Bags of Help cash - Burton Mail [Last Updated On: May 9th, 2017] [Originally Added On: May 9th, 2017]
- UW astronomy expert brings eclipse lessons - Gillette News Record [Last Updated On: May 9th, 2017] [Originally Added On: May 9th, 2017]
- Comet 67P is making its own oxygen gas - Astronomy Magazine [Last Updated On: May 9th, 2017] [Originally Added On: May 9th, 2017]
- Graduating UI senior takes 'roundabout' journey to astronomy - Iowa Now [Last Updated On: May 9th, 2017] [Originally Added On: May 9th, 2017]
- Merging galaxies wrap their black holes in dusty shrouds ... - Astronomy Magazine [Last Updated On: May 11th, 2017] [Originally Added On: May 11th, 2017]
- [ 9 May 2017 ] Surprise! When a brown dwarf is actually a planetary mass object News - Astronomy Now Online [Last Updated On: May 11th, 2017] [Originally Added On: May 11th, 2017]
- The newest big thing in radio astronomy - Astronomy Magazine [Last Updated On: May 11th, 2017] [Originally Added On: May 11th, 2017]
- [ 10 May 2017 ] Waves of lava seen in Io's largest volcanic crater News - Astronomy Now Online [Last Updated On: May 11th, 2017] [Originally Added On: May 11th, 2017]
- The wild wild worlds: a guide to the weirdest planets in the Milky Way - Astronomy Magazine [Last Updated On: May 11th, 2017] [Originally Added On: May 11th, 2017]
- Australian astronomy one of few winners in new budget | Science ... - Science Magazine [Last Updated On: May 11th, 2017] [Originally Added On: May 11th, 2017]
- BC-RNS-VATICAN-ASTRONOMY - Colorado Springs Gazette [Last Updated On: May 13th, 2017] [Originally Added On: May 13th, 2017]
- With eclipse coming, library sets up astronomy series - Glens Falls Post-Star [Last Updated On: May 13th, 2017] [Originally Added On: May 13th, 2017]
- Observatories combine to crack open the Crab Nebula - Astronomy Now Online [Last Updated On: May 13th, 2017] [Originally Added On: May 13th, 2017]
- A new hot Neptune may be a massive water world - Astronomy Magazine [Last Updated On: May 13th, 2017] [Originally Added On: May 13th, 2017]
- Chandra spots a recoiling black hole - Astronomy Magazine [Last Updated On: May 13th, 2017] [Originally Added On: May 13th, 2017]
- Astronomy club hosts Safe Schools members and mentees at fundraiser - Herald and News [Last Updated On: May 14th, 2017] [Originally Added On: May 14th, 2017]
- Astronomy on Tap just one of the fun Tuesday things to do - Austin American-Statesman [Last Updated On: May 17th, 2017] [Originally Added On: May 17th, 2017]
- Citizen scientists are invited to help find supernovae - Astronomy Magazine [Last Updated On: May 17th, 2017] [Originally Added On: May 17th, 2017]
- Assoc. astronomy professor named new director of Echols Scholars Program - University of Virginia The Cavalier Daily [Last Updated On: May 17th, 2017] [Originally Added On: May 17th, 2017]
- Music, astronomy collide at multimedia Bienen performance - The Daily Northwestern [Last Updated On: May 17th, 2017] [Originally Added On: May 17th, 2017]
- What's Going on August 21st | Astronomy.com - Astronomy Magazine [Last Updated On: May 17th, 2017] [Originally Added On: May 17th, 2017]
- Astronomers claim first evidence of PARALLEL UNIVERSE - 'there could be BILLIONS more' - Express.co.uk [Last Updated On: May 18th, 2017] [Originally Added On: May 18th, 2017]
- Could the Closest Extrasolar Planet Be Habitable? Astronomers Plan to Find Out - Universe Today [Last Updated On: May 18th, 2017] [Originally Added On: May 18th, 2017]
- [ 18 May 2017 ] Hubble spots moon around third largest dwarf planet News - Astronomy Now Online [Last Updated On: May 18th, 2017] [Originally Added On: May 18th, 2017]
- See a moving global view of Ceres at opposition - Astronomy Magazine [Last Updated On: May 18th, 2017] [Originally Added On: May 18th, 2017]
- Fireworks Galaxy sets off its 10th supernova in a century - Astronomy Magazine [Last Updated On: May 18th, 2017] [Originally Added On: May 18th, 2017]
- NASA invites scientists to submit ides for Europa lander - Astronomy Magazine [Last Updated On: May 18th, 2017] [Originally Added On: May 18th, 2017]
- Don't miss Jupiter's moons and Great Red Spot during May - Astronomy Now Online [Last Updated On: May 20th, 2017] [Originally Added On: May 20th, 2017]
- Researchers find a tiny moon around a large unnamed dwarf planet - Astronomy Magazine [Last Updated On: May 20th, 2017] [Originally Added On: May 20th, 2017]
- [ 19 May 2017 ] Icy ring around Fomalhaut observed in new wavelength News - Astronomy Now Online [Last Updated On: May 20th, 2017] [Originally Added On: May 20th, 2017]
- The weird star that totally isn't aliens is dimming again | Astronomy ... - Astronomy Magazine [Last Updated On: May 20th, 2017] [Originally Added On: May 20th, 2017]
- Astronomers create the largest map of the universe | Astronomy.com - Astronomy Magazine [Last Updated On: May 20th, 2017] [Originally Added On: May 20th, 2017]
- Astronomy: HoLiCOW! Measuring speed of universe expansion is no easy task - The Columbus Dispatch [Last Updated On: May 22nd, 2017] [Originally Added On: May 22nd, 2017]
- Dr. Rangi Mtmua hopes to revive Mori astronomy - Mori Television [Last Updated On: May 22nd, 2017] [Originally Added On: May 22nd, 2017]
- Astrofest teaches about astronomy and physics - Universe.byu.edu [Last Updated On: May 23rd, 2017] [Originally Added On: May 23rd, 2017]
- Bad Astronomy | Astronomers find a moon for a distant, frigid world ... - Blastr [Last Updated On: May 23rd, 2017] [Originally Added On: May 23rd, 2017]
- Merging white dwarfs may create most of our galaxy's antimatter ... - Astronomy Magazine [Last Updated On: May 23rd, 2017] [Originally Added On: May 23rd, 2017]
- Astronomers know TRAPPIST-1h's orbit - Astronomy Magazine [Last Updated On: May 23rd, 2017] [Originally Added On: May 23rd, 2017]
- A familiar galaxy with a new surprise: Two supermassive black holes - Astronomy Magazine [Last Updated On: May 23rd, 2017] [Originally Added On: May 23rd, 2017]
- Astronomers Spot Bright New Object near Cygnus A Galaxy's ... - Sci-News.com [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- Volunteers help astronomers find star that exploded 970 million ... - Phys.Org [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- Rocketing off to (cyber) space - Astronomy Magazine [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- Mice born from freeze-dried space sperm are doing OK - Astronomy Magazine [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- NASA's mission to a planetary core has been moved up - Astronomy Magazine [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- Astronomy: An all-American eclipse : Nature : Nature Research - Nature.com [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- 25 things to bring to the eclipse | Astronomy.com - Astronomy Magazine [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- A star turned into a black hole before Hubble's very eyes - Astronomy Magazine [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- Astronomy r/Astronomy - reddit.com [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- Astronomy News & Current Events | Sky & Telescope [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- Astronomy (magazine) - Wikipedia [Last Updated On: May 26th, 2017] [Originally Added On: May 26th, 2017]
- [ 27 May 2017 ] Jupiter surprises in first trove of data from NASA's Juno mission News - Astronomy Now Online [Last Updated On: May 28th, 2017] [Originally Added On: May 28th, 2017]
- Juno results offer tantalizing hints of Jupiter's secrets - Astronomy Magazine [Last Updated On: May 28th, 2017] [Originally Added On: May 28th, 2017]
- Study: Female Astronomers are Cited Less Frequently - The Atlantic - The Atlantic [Last Updated On: May 28th, 2017] [Originally Added On: May 28th, 2017]
- Astronomy Guide to the rest of the Memorial Day Weekend - AccuWeather.com (blog) [Last Updated On: May 28th, 2017] [Originally Added On: May 28th, 2017]
- The Lunar Reconnaissance Orbiter was hit by a meteoroid and lived - Astronomy Magazine [Last Updated On: May 30th, 2017] [Originally Added On: May 30th, 2017]
- Predicting eclipse crowds: More astrology than astronomy - Bend Bulletin [Last Updated On: May 30th, 2017] [Originally Added On: May 30th, 2017]
- Artist's Stunning New Exhibit Celebrates Harvard's 'Hidden' Female Astronomers - Space.com [Last Updated On: May 30th, 2017] [Originally Added On: May 30th, 2017]
- Astronomy tour to visit several SWI libraries next week - The Daily Nonpareil [Last Updated On: May 30th, 2017] [Originally Added On: May 30th, 2017]
- South Africa participates in international astronomy programme - Creamer Media's Engineering News [Last Updated On: May 30th, 2017] [Originally Added On: May 30th, 2017]
- Space geeks: Astronomy Night on the Mall is Friday and it's free - Washington Post [Last Updated On: May 30th, 2017] [Originally Added On: May 30th, 2017]