A cosmos of such enormous extent required a leap of imagination that most ancient philosophers were not prepared to make, so they retreated to the safety of the Earth-centered view, which would dominate Western thinking for nearly two millennia.
The Greeks not only knew Earth was round, but also they were able to measure its size. His method was a geometric one, based on observations of the Sun. The Sun is so distant from us that all the light rays that strike our planet approach us along essentially parallel lines.
To see why, look at Figure 2. Take a source of light near Earth—say, at position A. Its rays strike different parts of Earth along diverging paths. From a light source at B, or at C which is still farther away , the angle between rays that strike opposite parts of Earth is smaller. The more distant the source, the smaller the angle between the rays. For a source infinitely distant, the rays travel along parallel lines.
Figure 2: Light Rays from Space. The more distant an object, the more nearly parallel the rays of light coming from it. Of course, the Sun is not infinitely far away, but given its distance of million kilometers, light rays striking Earth from a point on the Sun diverge from one another by an angle far too small to be observed with the unaided eye. As a consequence, if people all over Earth who could see the Sun were to point at it, their fingers would, essentially, all be parallel to one another.
The same is also true for the planets and stars—an idea we will use in our discussion of how telescopes work. Eratosthenes was told that on the first day of summer at Syene, Egypt near modern Aswan , sunlight struck the bottom of a vertical well at noon. This indicated that the Sun was directly over the well—meaning that Syene was on a direct line from the center of Earth to the Sun. And the measurement of the angle in Alexandria, he realized, allowed him to Figure out the size of Earth.
Alexandria had been measured to be stadia north of Syene. The stadium was a Greek unit of length, derived from the length of the racetrack in a stadium. Figure 3. How Eratosthenes Measured the Size of Earth.
It is not possible to evaluate precisely the accuracy of Eratosthenes solution because there is doubt about which of the various kinds of Greek stadia he used as his unit of distance. Even if his measurement was not exact, his success at measuring the size of our planet by using only shadows, sunlight, and the power of human thought was one of the greatest intellectual achievements in history. Perhaps the greatest astronomer of antiquity was Hipparchus , born in Nicaea in what is present-day Turkey.
He erected an observatory on the island of Rhodes around BCE, when the Roman Republic was expanding its influence throughout the Mediterranean region. There he measured, as accurately as possible, the positions of objects in the sky, compiling a pioneering star catalog with about entries. He designated celestial coordinates for each star, specifying its position in the sky, just as we specify the position of a point on Earth by giving its latitude and longitude. He also divided the stars into apparent magnitudes according to their apparent brightness.
This rather arbitrary system, in modified form, still remains in use today although it is less and less useful for professional astronomers. By observing the stars and comparing his data with older observations, Hipparchus made one of his most remarkable discoveries: the position in the sky of the north celestial pole had altered over the previous century and a half. Hipparchus deduced correctly that this had happened not only during the period covered by his observations, but was in fact happening all the time: the direction around which the sky appears to rotate changes slowly but continuously.
If the north celestial pole is wobbling around, then Earth itself must be doing the wobbling. If you have ever watched a spinning top wobble, you observed a similar kind of motion. Figure 4: Precession. Just as the axis of a rapidly spinning top wobbles slowly in a circle, so the axis of Earth wobbles in a 26,year cycle. Today the north celestial pole is near the star Polaris, but about years ago it was close to a star called Thuban, and in 14, years it will be closest to the star Vega.
Because our planet is not an exact sphere, but bulges a bit at the equator, the pulls of the Sun and Moon cause it to wobble like a top. As a result of this motion, the point where our axis points in the sky changes as time goes on. While Polaris is the star closest to the north celestial pole today it will reach its closest point around the year , the star Vega in the constellation of Lyra will be the North Star in 14, years.
The last great astronomer of the Roman era was Claudius Ptolemy or Ptolemaeus , who flourished in Alexandria in about the year Today, it is our main source of information about the work of Hipparchus and other Greek astronomers. Hipparchus, not having enough data on hand to solve the problem himself, had instead amassed observational material for posterity to use.
Ptolemy supplemented this material with new observations of his own and produced a cosmological model that endured more than a thousand years, until the time of Copernicus.
As we watch the planets from our vantage point on the moving Earth, it is a little like watching a car race while you are competing in it. Ancient sailors used stars and constellations to guide their travels. Polynesians, for example, sailed among the Pacific Ocean islands by watching stars. To explain why planets seemed to change direction, Ptolemy used old calculations by Hipparchus to understand planetary motion.
Hipparchus worked about B. For instance, al-Battani working about A. If it hadn't been for Arabs, Greek science would have been lost.
One side of an astrolabe contained a detailed star map. Miscellaneous: Polaris is today's north star. It was not in the same position 4, years ago as it is today, and would not have helped the pyramid builders. In July , a star could be seen blazing in the heavens.
The Book of Fixed Stars was the most in-depth description of the night sky available, and it was also one of the first star catalogs to feature illustrations to make reading easier. The Arabic names of many stars would be retained as the book became influential in the Western world, and, to this day, most bright stars still have names derived from Arabic. He believed that each star had some influence over a specific aspect of the Universe, and that their precise movements would influence human behavior and natural events.
As Islamic star catalogs, astrological texts, and translations of the work of Greek philosophers became available in Latin, a new age of learning and discovery began across Europe. An interest in classical philosophy and science was rekindled, and the Renaissance began.
This period would see new astronomers rise up and challenge the centuries-old geocentric theory, aided by the invention of the telescope. Astronomers would see the Universe in far more detail than ever before. A little over years ago, Nicolaus Copernicus came up with a radical way of looking at the Universe. His heliocentric system put the Sun helio at the center of our system. He was not the first to have this theory. Earlier starwatchers had believed the same, and, in fact, Copernicus cited Aristarchus of Samos as an inspiration, but it was Copernicus who brought it to the world of the Renaissance and used his own observations of the movements of the planets to back up his idea.
His ideas, including the revelation that the Earth rotates on its axis, were too different for most of the scholars of his time to accept. They used only parts of his theory. Those who did study his work intact often did so in secret. They were called Copernicans. Born in Pisa, Italy, approximately years after Copernicus, Galileo became a brilliant student with an amazing genius for invention and observation. He had his own ideas on how motion really worked, as opposed to what Aristotle had taught, and devised a telescope that could enlarge the visibility of objects up to eight times their original size.
After a later upgrade, it could enlarge objects up to 20 times. He would be the first to observe that the Milky Way was actually a group of stars, rather than clouds in the night sky. He was able to use this telescope to prove the truth of the Copernican system of heliocentrism. He published his observations which went against the established teaching of the Church.
He was brought to trial and, although he made a confession of wrongdoing, he was still kept under house arrest for the rest of his life. But it was too late to lock away the knowledge that Galileo shared.
Other scientists, including Sir Isaac Newton and Johannes Kepler, seized its importance and were able to learn even more about the ways of the world and the heavens beyond. These early scientists' legacies continue to this day. As time goes on, we use our instruments, science, math, reasoning, and creativity to learn more about the secrets of the Universe. In this way, we are directly linked to the astronomers of times past who gave us direction to discover more about the dances of the planets and the nature of the stars.
Makers of Science Volume one of this reference set has information on Aristotle, Copernicus, and Galileo. Recentering the Universe An page round-up of how these early scientists' work changed everything about people understood their world. Articles on many early astronomers can be can be found in our online databases: Biography in Context and Encyclopaedia Britannica English, Spanish, and children's versions. Aristotle Leads the Way Captures the life and times of Aristotle, the ancient Greek philosopher who studied a wide range of subjects and helped shape early scientific beliefs.
In our young adult collection. The Great Thinker Stresses the importance of Aristotle's intellectual discoveries on the future of science. Biography for Kids: Aristotle A short online biography of Aristotle good for upper elementary grades that touches on some important points. Copernicus An enjoyable page biography of the 16th-century Polish scientist that includes activities on retrograde motion, the solar system, parallax, and an astrolabe.
Nicolaus Copernicus The center of the universe -- A childhood in Poland -- A long-term student -- In the service of the church -- Look to the stars -- The Copernican theory -- Late in life -- A scientific revolution -- Late in life -- Life at a glance. The History News in Space Uses a newspaper format to take a look at developments that led from the ideas of Copernicus and other early scientists to the technological advances that enabled man to venture to the moon and beyond.
Galileo A biography of 17th-century Italian astronomer and physicist Galileo that includes related activities for readers. Galileo for Kids This biography has experiments and activities as well as his life story.
0コメント