Origin of Astronomy 

 

4000 - 500 BC Chaldean, Babylonians, Egyptian, Chinese, Indian, ...
Grouped stars into constellations.
A 523 BC tablet contains the 12 constellations of the Zodiac.
Babylonians divided circumference into 360 degrees; 1 degree into 60 minutes; 1 minute into 60 seconds. They believed stars controlled human destiny.

 
624 - 548 BC Thales
Founded Ionian School of Philosophers. Greek science begins.
Convinced that universe is understandable in terms of simple rules. Rejected superstition.
Suggested the earth floats in a vast ocean.
 
610 - 547 BC Anaximander
Earth isolated, and unsupported, in space.
 
560 - 480 BC Pythagoras of Samos
Discovered relationship between rational numbers and musical intervals. This was a momentous discovery because it showed that Nature could be described in terms of number-series and number-ratios. The goal of the Philosophos, the Lover of Wisdom, was to discover these rules through deep reflection.  
Pythagoras was convinced that number was the basis of all of creation. 
Tied religion, mind, body and science into a unified body of thought.
Music related to astronomy and both to mathematics; emotional experience became enriched and deepened by intellectual insight. Aesthetic delight and reason were one.
Decided upon spiritual grounds that the earth was spherical--the perfect shape.

 

450 - 400 BC Philolaus
Student of Pythagoras
Explained diurnal motion of Earth with the hypothesis that the earth revolved about a central point in space in 24 hours. At the center was the central fire that illuminates the Sun and the Moon. Greece was always turned away from the central fire, and between it and the earth was the Antichton--or the counter-earth.
Around the central fire revolved nine bodies:
Antichton
Earth
Moon
Sun (a window in the celestial sphere)
the five planets
sphere of stars beyond which was a wall of fire

428 - 348 BC Plato
Space is infinite containing a finite, spherical, universe at the center of which lies the earth.
Universe is divide into 9 concentric spheres, which turn about the earth. But this theory didn't work well--it couldn't explain the motion of the planets (vagabond stars--or wanderers).
Plato had a hostile attitude towards astronomy. He declared "the visible world to be just a dim and distorted copy of the real world of ideas." So there was no point observing the visible world. This sentiment led to a casting aside of the great Pythagorean program, the uncovering of the underlying mathematical rules of Nature. 
Plato declared, on purely philosophical grounds, that the shape of the Earth must be a perfect sphere and that all motion must in  perfect circles, at constant speed. This held up the Pythagorean program for almost 20 centuries! 
 
388 - 315 BC Herakleides
Earth rotates about an axis.
Mercury and Venus orbit the Sun--but the Sun orbits the Earth.
 
384 - 322 BC Aristotle
Born in Stagira, Chalcidice, Macedonia.
Orphaned at an early age--joined Plato's Academy in 367 BC--spent 20 years studying. Described, by Plato, as the Nous (intelligence personified) of the academy.
Tutored young prince who would become Alexander the Great (the founder the city of Alexandria in 332 BC).
Aristotle returned to Athens and founded the Lyceum. Believed, like Pythagoras, that mathematics was the key to a true understanding of Nature. Codified the rules of logic.
Promoted idea of circular motion into a dogma of astronomy along with  geocentrism (Earth-centered universe).

 

310 BC Aristarchus of Samos
Last of the great Pythagoreans.
Put Sun at the center (heliocentric model). This model re-discovered by Copernicus 17 centuries later.
Alas Aristarchus had no students and followers and so his ideas all but died with him.
 
276 - 194 BC Eratosthenes of Alexandria
Determined circumference of the Earth. 
Alexandria to Syene: 500 miles. 
At the Summer Solstice the Sun is vertical at Syene but at Alexandria it is 7.25 degrees south of the vertical. 
Therefore, circumference = (360/7.25) * 500 miles.

 

190 - 120 BC Hipparchus
Considered the greatest astronomical observer of antiquity. From his precise observations he became convinced that Aristotle was wrong about circular motion. He, therefore, created a more accurate model of planetary motion. He introduced epicycles into the geocentric model. The model of Hipparchus became the accepted model of the universe for the next 1600 years.

 

 
47 BC Roman Empire at its height
Era of Cleopatra, Julius Caesar and Mark Anthony.
30 AD Era of Jesus of Nazareth. Roman conquest of the Middle East.

100 - 170 AD Ptolemy (Claudius Ptolemeus)
Wrote great textbook: The Almagest. Remained on the European astronomy best-seller list for 1500 years!

 

642 AD Fall of Alexandria
After a 14-month siege by Arabian troops, Alexandria was all but destroyed. The great library was burned. 
The West enters a period of stagnation, called the Dark Ages, that lasted until about 1400 AD. The knowledge of Greek astronomy was passed on to the Arabs, via India, who preserved this knowledge and added to it. 
In 760 AD, Islamic leaders in Baghdad sponsored translations of old Greek texts. 
Around 1000 AD the Islamic empire spread to Spain and Greek-Arabian science began to enter the West via Spain, and very gradually around 1400 AD the West arose from its long period of darkness. 
The re-awakening began with Nicolaus Copernicus (1473-1543), born in Poland, who re-discovered Aristarchus' heliocentric model of the universe.

Origin of Modern Astronomy

   
The Dark Ages
This is a period of Western history characterized by

terror and despair, in populations oppressed, famished, and wretched to a degree almost unimaginable today. To the miseries of constant war, political and social disintegration, there was added the dreadful affliction of inescapable, mysterious, and deadly disease. Mankind stood helpless as though trapped in a world of terror and peril against which there was no defense.

from The Sleepwalkers by Arthur Koestler

Age of Reason and Un-reason

Two forces were in direct conflict: one sought to maintain the status quo of ideas, dictated largely by the Roman Church, the other struggled to bring free rational thought to the fore.
1000 - Islamic Empire reaches Spain. The awakening of the Western mind begins.
1452 - Birth of Leonardo da Vinci.
1473 - Birth of Nicolaus Copernicus, author of
On the Revolutions of the Heavenly Spheres.
1492 - Western discovery of the Americas by Columbus.
1519 - Death of Leonardo da Vinci.
1543 - Death of Copernicus.
1546 - Birth of Tycho Brahe.
1564 - Birth of Galileo.
1571 - Birth of Johannes Kepler.
1600 - Burning of Giordano Bruno by the Holy Office in Campo Dei Fiori in Rome (after 6 years of imprisonment by the Inquisition) for, amongst other indiscretions, his belief in an infinite universe containing an infinite number of worlds.
1601 - Death of Tycho Brahe.
1616 - Galileo censured by the Holy Office.
1618 - The Counter-Reformation begins.
1619 - Enslavement of Africans, by Europeans, begins in the Americas.
1665 - Discovery of laws of motion by Newton.
1687 - Newton publishes his Principia.

 

Timeline 
Nicolaus Copernicus (1473-1543)
Born in Poland, entered the University of Cracow in 1491, where he studied mathematics. Received a doctoral degree in canon law from the University of Ferrara.
His great contribution was to take seriously the sentiment expressed by William of Ockham (14th century): "it is vain to do with more what can be done with fewer" and use it as a principle for (re-)establishing a heliocentric (sun-centered) model of the universe. The sentiment expressed by Ockam has come to be known as 
Ockham's razor: Given several competing theories, or hypotheses, one should favor the simplest.

Copernicus was a rather reluctant revolutionary. In fact, his motivation for a new model of the universe was merely to correct certain defects in the geocentric model. He wanted to find another way of arranging the plethora of epicycles that would allow for uniform circular motion. 
Copernicus wanted a simpler model. He was less concerned about finding an accurate one. Indeed, his heliocentric model was far less accurate in its prediction of the motion of planets than the rival geocentric model. This was largely because of Copernicus' insistence on circular orbits; the "perfect" orbit. But Copernicus preferred his model because it was aesthetically pleasing and explained retrograde motion in a very natural and simple way. But because it was inaccurate in its predictions, it took another century before the idea of a sun-centered universe was accepted by astronomers.
His great work De Revolutionibus,  which expounded his revolutionary ideas, was published in 1543 on the day of his death.

 

Tycho Brahe (1546-1601)
Born in Denmark, studied at the University of Leipzig (1562-1565). His greatest contribution was his realization of the importance of acquiring precise data in order to make progress in science, and his actually acquiring such data on behalf of the science of astronomy.
Observed, and studied, a nova on November 11, 1572 in the constellation of Cassiopeia. Brahe believed he had observed the birth of a new star, hence "nova" which means "new star". Today, however, we know that such events signal, in fact, a massive explosion of a star, rather than its birth.
Under the auspices of Frederick II of Denmark, Brahe established an observatory on the island of Hveen. From 1576 to 1597 Brahe collected a vast amount of precise astronomical data, that would prove profoundly useful to Kepler who became Brahe's assistant (at the latter's invitation) in 1599. (At that time, Brahe was stationed near Prague in the castle of Benatky).
 
Johannes Kepler (1571-1630)
From a long and careful consideration of the data collected by Brahe, Kepler established the Laws of Planetary Motion.
Planets move in ellipses with the Sun at one focus.
The radius vector sweeps out equal areas in equal times.
The square of the period (p) is proportional to the cube of the mean radial distance (a) from the sun:
p2 = constant a3
 
Heliocentric model, with elliptical orbits.
1618 - Published his 3rd law in his book The Harmony of the World. Done after 16 years of very hard work. A few days later the counter-reformation (30-years war) ignited in Europe. 
Kepler was a visionary--he believed that one day there would be spaceships filled with explorers who would not fear the vastness of space.
 
Galileo Galilei (1564-1642)
1610 - Discovered the 
four largest moons of Jupiter. 
Results published in a work entitled The Sidereal Messenger.


Investigated motion by doing experiments.
Introduced the notion of inertia and forces. Inertia is the tendency of objects to resist being accelerated. To accelerate an object we must apply a force on the object.
Discovered that objects fall with constant acceleration. He made an even more profound discovery: the acceleration of falling objects is independent of their inertia. So an elephant and a flea would both fall towards the earth at the same rate (in the absence of air friction.)
Put on trial by Rome for his scientific outlook.
Died eleven months before Isaac Newton was born.

Isaac Newton (1642-1727)
Newton synthesized, into his 4 laws of physics, the law of inertia; the constancy of acceleration of bodies under gravity; the idea of a gravitational force (acting at a distance); the idea that gravitational mass causes gravity and Kepler's 3rd law.
1642 Born Christmas Day, a weak, sickly child. Regarded as an average student at school!
1661 Admitted to Trinity College, Cambridge, England at age 18.
1665 Finished his undergraduate studies (in April). Outbreak of plague in London. University closed; students ordered home by university officials. Newton moved back to his village (Woolsthorpe in Lincolnshire, England).
1666 Great fire of London.
1667 Moved back to Cambridge having invented calculus, determined the nature of light, synthesized his laws of motion and his universal law of gravity:
 
F = -G m M/r2
This mathematical law is universal in that it is presumed to hold true for any two objects in the universe. Here, F is the force between two bodies of gravitational mass m and M, respectively, and r is the distance between them. G is a constant of nature, called Newton's gravitational constant. The negative sign indicates that the force of gravity is attractive. 
Newton's law of motion (for objects with constant inertial mass) is
 
Force = inertial mass ´ acceleration
 
By combining his law of motion with his law of gravity Newton provided a complete explanation of Kepler's laws and, consequently, the motion of all the objects in the solar system.
 
1672 Isaac Barrow, impressed by Newton's incredible achievements, resigned from the Chair of the Lucasian Professor of Mathematics in favor of Newton. Today Professor Stephen Hawkins of black holes fame holds the chair.
1684 Gottfried Wilhelm Leibnitz published his works on calculus.
1686 In a letter to Edmund Halley (of comet fame) Newton admits: ``I can affirm that I gathered it [the law of gravity] from Kepler's theorem about twenty years ago."


1687 Newton's masterpiece published: 
Philosophiae Naturalis Principia Mathematica. 
Paid for by Edmund Halley. 
Sadly, Newton does not acknowledge his dept to Kepler.



1692 Newton suffers a nervous breakdown.
1696 Becomes Warden of the Mint.
1697 Invents new mathematical theory called calculus of variations to solve the brachistochrone problem. (Receives problem from Johann Bernoulli--the Swiss mathematician--on 29-Jan-1697 at 4 PM; solves it by the following morning!)
1704 Newton published his version of calculus. A nasty nationalistic quarrel begins between France and Britain about who invented calculus first. It seems that Newton invented calculus first, but published second! Doubtless, the French would disagree.
1705 Knighted by Queen Anne--becomes Sir Isaac--first scientist to be so honored.


Last updated September 8, 1999 Harrison B. Prosper