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Introduction to Astronomy Assignment
Directions: Read the following article on the history of Astronomy. Answer the questions that follow on a
separate sheet of paper that includes your name and the date.
A Brief History of Astronomy
The very earliest people, as early as six million years ago, knew much more about the moon and the planets and
the stars than most people do today. That's because they lived mainly outside, without electric lights blocking
out the sky, and they saw every night how the moon and the planets moved.
By about 3500 BC (and maybe long before that), people thought of these moving things in the sky as living
beings - gods, with their own human-like personalities. If the moon and the planets were gods, they could affect
the lives of people, and so the Mesopotamians and the Egyptians began to chart the movements of the planets
and the moon to try to predict the future. They identified hundreds of constellations of stars, drew star maps,
and created the idea of horoscopes and the signs of the Zodiac. These early astronomers all thought that the
earth was the center of the universe, and that the moon, the sun, the planets and the stars all went around and
around overhead in the sky.
Around 600 BC, the Greek astronomer Thales rejected this idea that the moon and the stars were gods. Instead,
Thales suggested that the earth was a round ball, and that the moon was lit by light reflecting from the sun. If
the earth was round, then you could think of the moon and planets and stars and sun as going all the way
around the earth. In 585 BC, Thales used this idea to become the first astronomer to successfully predict an
eclipse of the sun. By about 430 BC, Anaxagoras had followed up on Thales' ideas to show exactly what caused
eclipses.
Two hundred years later, about 250 BC, Eratosthenes calculated the circumference of the earth, and shortly
after that Aristarchus figured out that the earth went around the sun, instead of the other way around, by
considering the curved shadow of the earth on the moon during an eclipse of the moon. Aristarchus also figured
out that the sun had to be a lot bigger than either the earth or the moon, and that the stars must be much,
much farther away than the moon or the sun. Even though Aristarchus was right, though, most scientists
thought he must be wrong - how could the stars really be that far away? It just seemed unlikely.
By 130 AD, Buddhist travellers from India had apparently brought news of these new Greek ideas to China,
where Zhang Heng knew that the moon was a ball lighted by the sun, and understood eclipses. Sadly, meanwhile
the Greeks had rejected this whole line of thought, and Roman astronomers like Ptolemy had gone back to
thinking that the sun and stars went around the earth, rather than think that the stars were so far away.
About 500 AD, the Indian astronomer Arya Bhata added the new idea that the earth spun around on its axis to
make day and night. But Arya Bhata also went back to the earlier idea that the earth was at the center of the
universe and the moon and the planets, the sun and the stars all moved around the sky. Even so, Arya Bhata did
some important mathematical work on sines and cosines, and on time-pieces like water-clocks, that would help
to measure the distances to the planets and stars.
A Chinese star chart from the 600s AD shows even some very faint stars that are hard to see with just your eyes.
About 900 AD, the Islamic astronomer Al Razi built on Arya Bhata's work to show that the sun was bigger than
the earth and the moon was smaller than the earth. Soon afterward, Islamic glassworkers used medical research
on the way the human eye worked to figure out how to to make glass lenses that would focus the heat of the
sun in one spot. By 1100 AD, Al Ghazali was able to understand again what caused eclipses of the sun and the
moon.
About 1260 AD, another Islamic astronomer, Al Tusi, used an observatory to make very accurate measurements
of the movements of the planets. Al Tusi also realized that the Milky Way was really not a cloud but a lot of very
far away faint stars. In the 1470s, the Ottoman astronomer Ali Qushji revived the idea that the earth went
around the sun.
The renaissance came to astronomy with the work of Nicolaus Copernicus, who, in 1514, published a small book
proving that the earth actually went around the sun, as Aristarchus had thought more than a thousand years
earlier. He proposed a heliocentric system, in which the planets revolved around the Sun and not the Earth. His
work provided a full mathematical discussion of his system, using the geometrical techniques that had been
traditional in astronomy since before the time of Ptolemy. Still many people - most people - did not believe
Copernicus, and his cause had to be defended in the later 1500s by Galileo, and again in the 1700s by Isaac
Newton.
Galileo was considered the father of observational astronomy. He was among the first to use a telescope to
observe the sky and after constructing a 20x refractor telescope he discovered the four largest moons of Jupiter
in 1610. This was the first observation of satellites orbiting another planet. He also found that our Moon had
craters and observed (and correctly explained) sunspots. Galileo noted that Venus exhibited a full set of phases
resembling lunar phases. Galileo argued that these observations supported the Copernican system and were, to
some extent, incompatible with the favored model of the Earth at the center of the universe.
During the late renaissance came Colonial American Astronomy, which can be traced to the time when the
English began colonizing in the New World during the sixteenth century. They brought with them their interest
in astronomy. At first, astronomical thought in America was based on Aristotelian philosophy, but interest in the
new astronomy began to appear in Almanacs as early as 1659. Colonial astronomers applied the scientific
method to their own work and despite their limited resources; they instigated momentum for further
astronomical research in what would later become the United States.
In the 19th century it was discovered that, when decomposing the light from the Sun, a multitude of spectral
lines were observed (regions where there was less or no light). Experiments with hot gases showed that the
same lines could be observed in the spectra of gases, specific lines corresponding to unique elements. It was
proved that the chemical elements found in the Sun were also found on Earth.
During the 20th century spectroscopy (the study of these lines) advanced, especially because of the advent of
quantum physics that was necessary to understand the observations. Modern astronomical discoveries include
dark matter, black holes, expoplanets and gamma ray bursts. With the use of redshifting measurements, the
discovery of cosmic background radiation, and the age of the globular clusters of ancient stars in galaxies,
Astronomers have even been able to determine the age of the universe; which is currently estimated at 13.75
billion years. Today, Astronomers continue to make new and astonishing discoveries and who knows what the
future may bring!!
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Questions
1. In this article you read about many astronomers including; Thales, Aristarchus, Al Tusi, Arya Bhata,
Copernicus, and Galileo, all who made significant discoveries throughout history. Chose one astronomer
to conduct further research on then, write a paragraph summarizing their significance in the history of
Astronomy.
2. Which of the discoveries discussed in the article do you think was most important to the study of
Astronomy? Explain why you feel this way.
3. Why do you feel it is important for scientists to continue to study Astronomy? Would you consider
becoming an astronomer? Why or why not?
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