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Eratosthenes World view the horizon, you would have to look down, not across. For a long time, people have known that Earth is round, despite How Do We Know Earth our everyday experience of Earth’s Is Round? surface. Some people think Christopher Columbus proved Earth’s shape. Throughout history, some peo- They believe that he sailed to the ple have thought that Earth is flat. New World in order to show that It is easy to understand why. When Earth was round. This is not true. you walk to school, you do not By the time of his voyage in 1492, feel that you are walking a curved most educated people already path. When standing on the shore knew that Earth was round. This of an ocean, the horizon appears misunderstanding about Colum- level. You also always look straight bus’s voyage was popularized in ahead to see the horizon. If Earth an 1828 book by Washington Ir- is curved, some argue, in order to ving called A History of the Life and Eratosthenes World N GREECE 400 mi Athens 400 km Aegean Sea Rhodes M editerranean Sea Alexandria Cairo N i l e SAHARA R R e . d S EGYPT e a Syrene Geography News Network 12/08/17 ©2017 continued > – 1 – from previous page > Voyages of Christopher Columbus. Around 100 years after Aristotle, Today, most historians agree that a Greek scholar named Eratosthe- Irving’s story was incorrect. nes not only proved that Earth is Today, we can see Earth is round round, but he also calculated its from photos taken by satellites that circumference, or how far around it show Earth in space. However, by is. Eratosthenes wanted to make a 500 b.c.e., the ancient Greeks knew map of the world. However, an ac- that Earth was round. They did not curate map required that he deter- need spacecraft to know this. At mine Earth’s size. How could he do first, their thought on Earth’s shape this without walking the distance was based on their ideas about art, himself? not science. The Greeks believed At noon each year on or near June that the most perfect shape is a 21, summer starts in the northern sphere. Therefore, they concluded hemisphere. At noon, the sun is that Earth must be round. Howev- directly overhead at 23.5 degrees er, the Greeks were able to support north latitude. Eratosthenes heard this belief. The Greek philosopher of a well in the Egyptian town of Aristotle (384-322 b.c.e.) reinforced Syrene. On noon on or near June 21 the idea that Earth is round through of each year, because Syrene hap- scientific observation. pened to sit at about the right lati- He accomplished this by look- tude, the sun illuminated the entire ing at ships on the horizon. When bottom of the well without casting standing on a beach, look for ships any shadows. This proved that at approaching shore. If Earth is flat, that moment, the sun was located a ship would at first be a blurry directly overheard of the well. speck. Then, as the ship came Next, Eratosthenes thought closer, you would see it come into about the properties of circles peo- focus as a whole ship. You could ple already knew, related to our un- make out the sails and the decks derstanding of a circle having 360 at the same time. However, when degrees. One theory of how we Aristotle watched an approaching originally learned this is from Su- ship, he saw its sails first. Then as it merian astrologers in around 2400 came nearer, he next saw the low- b.c.e. They studied the paths of est body of the ship, the hull. Be- stars and planets in an attempt to cause he saw the tallest parts of a predict future events. When they ship first, and not the entire ship all viewed the sun’s path over the at once, he knew it must be coming course of a year, they found that it toward him over a curved surface, took about 360 days for the sun to so the Earth must be round. complete one circuit of its appar- If you ever see an eclipse of the ent movement in the sky. For each moon, you can make another of day, the astrologers noted how far Aristotle’s observations. A lunar the sun had traveled since the last eclipse occurs when Earth travels day. The distance the sun moves in between the sun and the moon in one day along this circular route is space. When this occurs, the sun’s called a degree. Since the sun ap- light causes Earth to cast its shad- peared to take 360 days to make ow onto the moon’s surface. Aris- a full circuit, Sumerian astrologers totle noticed that this shadow was divided the circle into 360 equal always round. This gave more evi- units, called degrees. dence that Earth is a sphere. On June 21, Eratosthenes applied continued > – 2 – from previous page > what the Sumerians learned. He around the Equator, Eratosthenes placed a stick in the ground in Alex- was actually very close! andria. At noon, he measured the Scientists repeat other scientists’ angle cast by the stick’s shadow. As experiments to see if their findings he knew, the sun cast no shadow are reliable and accurate. Eventual- at the well in Syrene. However, Era- ly, a scholar named Posidonius re- tosthenes found that his stick cast peated Eratosthenes’s experiment. a shadow of 7.2 degrees. He knew Instead of the sun, Posidonius that all circles are 360 degrees. measured the position of the star When he divided the 360 degrees Canopus above the horizon at two of a circle by the 7.2 degrees of the locations of different latitude: the stick’s shadow, he discovered that island of Rhodes, and Alexandria. In the angle of the stick was 1/50 of a Posidonius ‘s experiment, he used complete circle. Now, if Eratosthe- the distance between those two nes multiplied the distance from Al- places to calculate the circumfer- exandria to Syrene by 50, he could ence of the entire world. calculate Earth’s circumference. Unfortunately, Posidonius had Unfortunately, in ancient times bad data. Both his measurement of measuring distance was not easy. the star position and his figure for Distances were often measured the land distance were inaccurate. using the amount of time it took a However, the two errors balanced caravan of travelers and camels to each other out, so his first calcula- travel from one place to another. tion for the circumference was fair- However, this figure could change ly close to what Eratosthenes had depending on how fast a camel found. Later though, Posidonius walked or whether or not a camel found a more precise land distance traveled in a straight line. In order measurement and re-calculated the to get an accurate number, Eratos- circumference using that. This time thenes hired bematists. Bematists he had a mix of good and bad data, were people trained to walk with so his final conclusion of 18,000 equal-length steps. By multiply- miles was much smaller than Era- ing the length of their steps by the tosthenes’s 24,000 miles. number of steps they took, a be- Did Posidonius show that Eratos- matist could tell how far apart two thenes was wrong? No, but Posido- places were. The bematists found nius’s 18,000-mile figure was includ- that Syrene and Alexandria were ed in a popular work on geography 5,000 stadia apart. Historians are published in the second century not sure how long one stadion is, c.e. As a result, most people used but they think it is probably some- Posidonius’s measurement instead where between 500 and 600 feet. of Eratosthenes’s. One famous per- If a stadion was 500 feet, Earth’s son who used Posidonius’s mea- circumference would have mea- surement for Earth’s circumference sured 24,000 miles. If, however, a was Christopher Columbus. Using stadion was 600 feet, the circumfer- Posidonius’s circumference, Colum- ence would have measured 29,000 bus concluded that Earth was small miles. This means that Eratosthe- enough to sail around. Historians nes calculated that Earth was any- now wonder that if Columbus had where from 24,000 to 29,000 miles used Eratosthenes’ larger number, around. Since we now know that whether he would have set sail at Earth measures about 24,900 miles all. – 3 – from previous page > CRITICAL THINKING SKILLS PRACTICE ACTIVITIES AND QUESTIONS Explaining: What are two ways that Aristotle showed that Earth is round and not flat? Summarizing: How did Sumerian astrologers contribute to the under- standing of circles? Defining: Who are bematists? What role did they play in Eratosthenes’s calculation of the circumference of Earth? Contrasting: Contrast Eratosthenes’s and Posidonius’s findings on the circumference of Earth. Why did their conclusions vary? Brought to you by Geography News Network. December 8, 2017 #265. CLASSROOM RESOURCES http://www.newsweek.com/neil-degrasse-tyson-flat-earthers-723765 https://spaceplace.nasa.gov/menu/play/ http://www.independent.co.uk/news/uk/home-news/summer-solstice- 2017-first-day-of-summer-when-date-how-axis-northern-hemisphere-a7798986. html https://www.smithsonianmag.com/science-nature/how-eclipse-anxiety-helped- lay-foundation-modern-astronomy-180963992/ http://www.montereyinstitute.org/courses/DevelopmentalMath/COURSE_ TEXT2_RESOURCE/U07_L2_T3_text_final.html SOURCES http://www.businessinsider.com/the-experiment-the-ancient-greeks-used-to- show-the-earth-isnt-flat-2017-12 https://www.csmonitor.com/USA/2011/1010/Christopher-Columbus-Five- things-you-thought-you-knew-about-the-explorer/MYTH-Columbus-set-out-to- prove-the-earth-was-round https://www.popsci.com/10-ways-you-can-prove-earth-is-round#page-2 https://www.aps.org/publications/apsnews/200606/history.cfm http://web.archive.org/web/20130520192640/http://www.wonderquest.com/ circle.htm Larry Marotta: Larry Marotta is a writer and editor with 20 years of experience in the K-12 educational publishing industry.
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