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Download viewEratosthenes. Born around 276 BCE; died around 194 BCE. He lived in northern Africa, which was at that time part of the Roman Empire. Eratosthenes was an ancient

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Eratosthenes

Born around 276 BCE; died around 194 BCE. He lived in northern Africa, which was at that time part of the Roman Empire.

Eratosthenes was an ancient Greek writer, geographer, music theorist, mathematician, astronomer, poet, teacher, and librarian. (Quite an overachiever, huh? In fact, he was considered to be the inventor of geography!)

Since Eratosthenes was involved in so many different fields, his critics claimed he wasnt the best at any particular discipline. They made up a derisive nickname for him, Beta, which is the second letter of the Greek alphabet, implying that he was always second best. His supporters had a different nickname, though. They called him Pentathlos, after the Olympic athletes who were so talented they could compete in five different Olympic events.

Eratosthenes is known for his many accomplishments, including calculating the tilt of the earths axis, devising the first calendar with a Leap Day, and making the first map that showed lines of longitude and latitude. However, Eratosthenes is most famous for something pretty amazing, considering the times in which he lived. He was the first man known to have calculated the Earth's circumference. And, surprisingly, considering his only tool was a primitive device for measuring angles, he was only off by a very small percentage.

Eratosthenes is not in the textbook, but I have added him because his accomplishments illustrate the fact that contrary to much modern misunderstandings people of the western world understood that the earth is round since ancient times at least the 6th Century BCE! How did they know? They made inferences based on their observations (evidence!):

For example, when a ship approaches over the horizon, it doesnt appear all at once. First you see the top of the mast, then more and more of the ship. This shows the ship is approaching over a curved surface.

So, just how did Eratosthenes measure the earths circumference over 2,000 years ago?

Eratosthenes had been told that at noon on the June Solstice, if someone looked down into a deep well in a city called Syene (now Aswan, Egypt), located on the Tropic of Cancer, their reflection blocked the reflection of the sun on the water at the bottom of the well. This indicates the suns rays fall exactly vertically at that date and time in that location. Meanwhile, in Alexandria, which was 500 miles northwest of Syene and was where Eratosthenes was living, he measured the suns on that same date and time and found that they fall about 7 degrees from the vertical. He correctly assumed the Sun's distance to be very great and concluded that because of this, the suns rays are practically parallel by the time they reach Earth.

Since 7 degrees is approximately 1/50th of a circle(which has 360 degrees), and assuming that Alexandria and Syene were on the same line of longitude, Eratosthenes concluded that thedistance from Alexandria to Syene must therefore be 1/50th of the earths circumference. (See diagram, below.) The distance between these two cities was reported to be 5,000 stadia (singular, stage), an ancient unit of distance.

He rounded the result to a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. Some claim Erathostenes used the Egyptian stade of 157.5 meters, which would imply a circumference of 39,690km, an error of 1.6%, but at that time, most people in the Greek Empire would have used the Attic stade, which is equivalent to 185 meters. This would imply a circumference of 46,620km, an error of 16.3% -- still pretty impressive! We know now that the earth is not a perfect sphere, but it is obvious that ancient Greeks such as Eratosthenes had a fairly accurate understanding of its shape and definitely did not believe it was flat.

At the age of 81, Eratosthenes suffered an inflammation of the eyes, which rendered him blind. It is said that he became so depressed at no longer being able to read or to observe the beauty and wonders of nature that he committed suicide by self-starvation after one year of sightlessness.

Ptolemy (Claudius Ptolemaeus)

(90 168)

Even though other ancient Greek astronomers, including Aristarchus (310 230 BCE) and Hipparchus (190 120 BCE), Ptolemy was so influential that his insistence on his geocentric model of the known universe dominated western thought for almost 1500 years. In fact, the geocentric model he proposed is often referred to as the Ptolemaic System.

One of the reasons that Ptolemys model was so compelling is that people could make accurate predictions based on the math he used.

Since in reality, the solar system (along with the stars visible from Earth, the known universe at that time), is in actuality heliocentric, the planets sometimes seem to change directions when seen from the earth. (In fact, this is how the planets earned their name. Planetes means wanderer in Greek. Unlike the fixed stars, which seemed to rotate the Earth in very reliable circles, the planets appeared to wander about the sky. Note: The planets past Saturn were unknown at this time in history.) This is because Earth in its orbit sometimes passes up the slower moving, farther planets, which also have a longer path to travel. Venus and Mercury, closer to the sun than Earth and faster moving, sometimes pass us, leading to the same illusion of backward motion. Ptolemy devised an elaborate system of small circles which he termed epicycles, to explain this apparent motion. (See diagram, below.)

Three main ideas that Ptolemy and others believed must be true (and are included in his model) are as follows:

1. All motion in the heavens is uniform, perfectly circular motion.

2. All objects in the heavens are made from perfect material and are shaped in perfect spheres. In addition, their properties, such as brightness, were permanent and never changing.

3. The Earth is at the center of the Universe.

Not much is known about Ptolemys personal life, but we do know he had a major influence on astronomical thought.

Nicolaus Copernicus is the Latinized name of Polish astronomer and mathematician Mikolaj Kopernik. Born in 1473, he is credited with the heliocentric model of the solar system. However, it is important to note he was not the first person to figure out that the sun is at the center of what was then thought to be the entire universe. However, he was the first to calculate the math that was needed to explain the model.

Copernicuss model did have several shortcomings. Like the astronomers before him, he assumed the planets orbit in perfect circles. Since they do not, the only way he could make his math fit celestial observations was to include epicycles, as Ptolemy before him had done.

Remember, at this time, the majority of astronomers continued to accept Ptolemys geocentric model. Another big problem was that the Holy Roman Empire was the law of the land. Claiming the sun was at the center of the universe was deemed heresy by the Church. Copernicus was reluctant to incite controversy and therefore did not publish his work until shortly before his death in 1543.

Galileo Galilei

(1564 1642)

The terms physicist, astronomer, mathematician, engineer, and philosopher can all be used to describe Galileo Galilei, commonly referred to by his first name only. He was truly an amazing individual and his list of scientific accomplishments is astounding. As Stephen Hawking has said, "Galileo, perhaps more than any other single person, was responsible for the birth of modern science."

Galileo originally studied medicine, intending to become a physician, but while at university he found himself drawn to mathematics and physics.

While it is true he formulated many of the foundational ideas about gravity and motion, and is even referred to as the father of modern astronomy, Galileo did not invent the telescope, as is often mistakenly believed. No one is certain who the first person to make a telescope was, but a German eyeglass maker, Hans Lipperhey, was the first to apply for a patent on a telescope, in 1608.

Having heard of this wonderful new contraption that could make distant objects appear closer, Galileo set about constructing his own telescope without ever actually seeing one. He also figured out how to increase the magnification, from only three times to as great as 30 times.

Galileo has long been believed to be the first person to turn the telescope to the sky and use it for astronomical purposes. Recently, it has been made known that an Englishman named Thomas Harriot viewed the moon through a telescope several months before Galileo, but Harriot did not publicize his findings. In contrast, Galileo published a book called Starry Messenger in 1610 which outlined his discoveries. Here is a reproduction of one his actual drawings of the moon (left) and an artists sketch of Galileo using his telescope (right):

Some of the phenomena Galileo observed flew in the face of accepted thought in the 1600s. His observations of craters and mountains on the moon revealed that it was not a smooth, perfect sphere, as had been assumed. Similarly, his discovery of sunspots showed the sun was not perfect, either. Witnessing four small satellites (moons) revolving around the planet Jupiter was proof that not everything in the heavens revolves around Earth. Most telling of all, the phases that the planet Venus goes through simply could not occur unless Venus was traveling around the sun, and at a closer orbit than Earth.

If Ptolemys model were correct, we would only see Venus as either a waxing or waning crescent of the same size (left). However, Galileo observed Venus pass through an entire set of phases of different sizes (right). Galileos observations proved that the sun i

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