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Chapter 4
The Origin of
Modern Astronomy
Guidepost
The previous chapters gave you a modern view of what you see in the sky,
and now you are ready to understand one of the most sweeping
revolutions in human thought: the realization that we live on a planet.
In this chapter, you will discover how astronomers of the Renaissance
overthrew an ancient theory and found a new way to understand Earth.
Here you will find answers to four essential questions:
• How did the ancients describe the place of the Earth?
• What is the real contribution of Islamic Astronomy?
• How did Copernicus change the place of the Earth?
• Why was Galileo condemned by the Inquisition?
• How did Copernican astronomers solve the puzzle of planetary motion?
Guidepost (continued)
This astronomical story will introduce you to the origin of
modern astronomy, and it will help you answer an important
question about science.
• How do we know? How do scientific revolutions occur?
Outline 1
I. The Roots of Astronomy
A. The Astronomy of Greece
B. The Ptolemaic Universe
II. Islamic Astronomy
A. Growth of Islamic Astronomy
B. Major Schools of Astronomy (Baghdad, Cairo, Andalusia,
Maragha, Samarqand)
C. Instruments, Observatories
III. The Copernican Revolution
A. Copernicus the Revolutionary
B. De Revolutionibus
C. Galileo the Defender
Outline 2
IV. The Puzzle of Planetary Motion
A. Tycho the Observer
B. Tycho Brahe's Legacy
C. Kepler the Analyst
D. Kepler's Three Laws of Planetary Motion
I. The Roots of Astronomy
Ancient Greek Astronomers (1)
• Unfortunately, there are no written documents
about the significance of stone and bronze age
monuments.
• First preserved written documents about ancient
astronomy are from ancient Greek philosophy.
• Greeks tried to understand the motions of the sky
and describe them in terms of mathematical (not
physical!) models.
Ancient Greek Astronomers (2)
Models were generally wrong because they were based on
wrong “first principles”, believed to be “obvious” and not
questioned:
1. Geocentric Universe: Earth at the Center of the Universe
2. “Perfect Heavens”: Motions of all celestial bodies described by
motions involving objects of “perfect” shape, i.e., spheres or
circles
Greeks assumed the Earth
was not moving because
they did not observe
parallaxes in the sky.
(Animation - 1)
Stellar Parallax
Later refinements (2nd century B.C.)
• Hipparchus: Placing the Earth away from the centers of the
“perfect spheres”
• Ptolemy: Further refinements, including epicycles
Epicycles
Introduced to explain
retrograde (westward)
motion of planets
(Animation - 2)
The Ptolemaic system was considered the “standard model” of
the Universe until the Copernican Revolution.
II. Islamic Astronomy
Islamic Astronomy
• Major impetus came from religious observances:
Presented an assortment of problems in mathematical
astronomy.
•
Religious customs presenting problems in spherical
astronomy:
• Crescent Moon: start of Islamic month
• Direction of Qibla: praying toward Mecca
• Proper time for the five daily prayers.
Golden Names, Forgotten Golden Civilization…..
Al-Abhari, Adnan Al-Ainzarbi, Ali Ibn Issa, Ibn Amajur, Al-Amili, Al-Ashraf,
Al-Asturlabi, Ibn Bajjah, Al-Bakhnayqi, Al-Balkhi, Banu-Mussa, Abu’l
Barakat, Ibn Al-Fasi, Al-Battani, Al-Biruni, Al-Bitruji, Al-Jaghmayni, AlFarghani, Al-Farisi, Al-Fasi, Al-Fazari, Abbas Ibn Firnas, Habash AlHasisb, Al-Hajjaj Ibn Yussuf, Al-Hamdani, Hamid Ibn Ali, Ibn al-Haytham,
Ibnrahim Ibn Sinan, Jabir Ibn Aflah, Al-Jawahri, Al-Jayani, Ibn al-Jazuli, AlKabisi, Qadhi Zadeh Al-Rumi, Al-Karari, Al-Kashi, Al-Katibi, Al-Kawashi,
Al-Katibi, Al-Khalili, Al-Haraki, Al-Khayyami, Al-Khazin, Al-Khazini, AlKhujandi, Al-Khwarizmi, Al-Kuhi, Al-Kushdji, Kashyar Ibn Labban, Ibn alAl-Abudi, Al-Maghribi, Al-Mahani, Ibn Al-Majdi, Al-Majriti, Al-Maksi, AlMarrakushi, Abu-Ma’shar, Al-Misri, Al-Mizzi, Abu Nasr, Al-Nayrizi, Qaysar,
Ibn Al-Qunfudh, Al-Qusantini, Ibn al-Raqqam, Ibn al-Rashidi, Al-Razi, IbnRushd, Ibn al-Saffar, Al-Saghani, Ibn Sa’id, Abu’l Salt, Ibn Sina , Ibn alShatir, Al-Samarqandi, Ibn Al-Samh, Sinad Ibn Ali, Ibn al-Sarraj,Al-Shirazi,
Al-Sijri, Sinan Ibn Thabit, Al-Sufi, Thabit Ibn Qurrah, Al-Tusi, Ulugh Begh,
Umar Ibn al-Farkhan, Abu al-Uqul, Al-Urdi, Abu’l Wafa, Yahya, Yaqub Ibn
Tariq, Ibn Yunus, Al-Zarqali,
Major Schools of Islamic Astronomy (1)
• Baghdad
• Started from the reign of Caliph al-Mansur (754-775)
• Under Harun ar-Rashid and al-Ma’mun, the school of Baghdad
produced its most important works.
•
Old theories were revised, Ptolemy’s errors were rectified, and
the astronomical tables of the Greeks corrected.
•
Most Important Astronomical Works:
• Detailed study of the exact duration of the solar year (alBattani)
• Prediction of solar and lunar eclipses (al-Battani)
• Theory of the lunar crescent visibility.
• Al-Battani’s Zij, considered as the most important
astronomical works between the time of Ptolemy and that of
Copernicus.
Major Schools of Islamic Astronomy (2)
• Cairo:
• Founded by the illustrated Ali Ibn Yunus, inventor of the
pendulum, and the solar quadrant.
•
Most Important Astronomical Works:
• “Al-Hakimi Zij”, astronomical tables edited by Ibn Yunus,
used in Cairo into the 19th century.
• More than 100 solar observations and planetary
conjunctions are reported in the Hakimi Zij.
• Optical treatises of Hassan Ibn al-Haytham, which served
as the basis for the works of Roger Bacon and Kepler.
Summary of Past Lecture
• Many cultures around the world observed the sky and marked important
alignments
• But, having no written language, left no detailed records of their
astronomical beliefs.
• Greek astronomy, derived in part from Babylon and Egypt, is better
known because written documents have survived.
• Greek astronomers believed firmly in two principles:
– Geocentric universe: Earth center of the whole universe
– Perfect heavens: Heavens must be made of spheres in uniform motion
• Ptolemy Universe: Added epicycles to account for the retrograde motion
• Muslim astronomers:
− Revised old theories.
− Rectified Ptolemy’s errors.
− Corrected the astronomical tables of the Greeks.
•
School of Baghdad:
– Best work: Al-Battani’s Zij, considered as the most important astronomical
works between the time of Ptolemy and that of Copernicus.
• School of Cairo:
– Best work: “Al-Hakimi Zij”, astronomical tables edited by Ibn Yunus, used in
Cairo into the 19th century.
Think and Answer…(Animation - 2)
1. Ptolemy's model of the universe
a.
b.
c.
d.
was heliocentric.
included elliptical orbits.
contained epicycles.
all of the above
2. The purpose of using epicycles to explain the motion of the planets
in the night sky was to account for
a.
b.
c.
d.
prograde motion.
Mercury and Venus’ limited angular distance from the Sun.
retrograde motion.
non-uniform speed of the planets in their orbits.
Scientific Argument…. (Animation - 1)
• Why did the Greeks believed firmly in the geocentric
system?
Major Schools of Islamic Astronomy (3)
• Maragha:
•
•
Started with Nasir al-Din al-Tusi by the construction of the
Maragha observatory in 1259.
•
Responsible for the revival of Islamic science in all fields.
•
Astronomical activities influenced the observatories of Samarqand
and Istanbul, and also those in the West.
Most Important Astronomical Works:
• Compilation of the “Ilkhani Zij” displaying the results of 12 years of
observations at the Maragha observatory.
Major Schools of Islamic Astronomy (4)
• Samarqand:
•
•
•
•
•
Started with the foundation of a “Madrasa” by Ulugh Begh in 1420.
Lecturers included the most famous Muslim mathematicians and
astronomers (Kadi Zada and al-Kashi) brought under one roof.
Thousands of students were lectured and were assisted by hundred of
scientists.
An Observatory was built to fulfill the task of completing the educational
needs of the students in astronomy.
Most Important Astronomical Works:
•
Compilation of Ulugh Begh’s Zij displaying the results of 20 years
of observations at the Samarqand observatory.
•
Yearly movements of Saturn, Jupiter, Mars, and Venus were
known to within the limits of two to five seconds of those of modern
times.
C. Instruments, Observatories
Sundials
A huge armillary sphere
Huge armillary sphere (Ibn Battutah Mall – Dubai)
Maragha Observatory
Ruins of the entrance of the Samarqand Observatory
Astrolabe
Quadrant
III. The Copernican Revolution
The Copernican Revolution
Nicolaus Copernicus (1473 – 1543):
Heliocentric Universe (Sun in the Center)
Copernicus’ New (and Correct) Explanation for the
Retrograde Motion of the Planets
Retrograde
(westward)
motion of a
planet occurs
when the Earth
passes the
planet.
(Animation - 3)
This made Ptolemy’s epicycles unnecessary.
Galileo Galilei (1594 – 1642)
• Invented the modern view of science: Transition
from a faith-based “science” to an observation-based
science
• Greatly improved on the newly invented telescope
technology, (But Galileo did NOT invent the
telescope!)
• Was the first to meticulously report telescope
observations of the sky to support the Copernican
Model of the Universe
Major Discoveries of Galileo
• Moons of Jupiter (4 Galilean moons)
• Rings of Saturn
(What he really saw)
Major Discoveries of Galileo (2)
• Surface structures on the moon; first estimates
of the height of mountains on the moon
Major Discoveries of Galileo (3)
• Sun spots (proving that the sun is not perfect!)
Major Discoveries of Galileo (4)
• Phases of Venus (including “full Venus”),
proving that Venus orbits the sun, not the Earth!
IV. The Puzzle of Planetary Motion
Tycho Brahe (1546 – 1601)
• High precision observations of the
positions of stars and planets
• Measurement of the nightly motion of a “new
star” (a supernova) showed no parallax
• Evidence against Aristotelian belief of
“perfect”, unchangeable heavens
Tycho Brahe’s Legacy
New World model
• Still geocentric (Earth in the center of
the sphere of stars)
• Sun and Moon orbit Earth;
Planets orbit the sun.
Johannes Kepler (1571 – 1630)
• Used the precise observational tables of
Tycho Brahe to study planetary motion
mathematically.
• Found a consistent description by
abandoning both
1. Circular motion and
2. Uniform motion
• Planets move around the sun on elliptical
paths, with non-uniform velocities.
Kepler’s Laws of Planetary Motion
1. The orbits of the planets are ellipses with the
sun at one focus.
c
Eccentricity e = c/a
(Animation - 4)
Eccentricities of Ellipses
1)
2)
e = 0.02
3)
e = 0.1
e = 0.2
5)
4)
e = 0.4
e = 0.6
Eccentricities of Planetary Orbits
Orbits of planets are virtually
indistinguishable from circles:
Earth: e = 0.0167
Most extreme example:
Pluto: e = 0.248
Planetary Orbits (2)
2. A line from a planet to the sun sweeps
over equal areas in equal intervals of time.
(Animation - 5)
Planetary Orbits (3)
3. A planet’s orbital period (P) squared is
proportional to its average distance from the
sun (a) cubed:
Py2 = aAU3
(Py = period in years;
aAU = distance in AU)
Scientific Argument…….
Why would one say the Copernican hypothesis was correct
but the model inaccurate?
Chapter Summary
• Greek astronomy was based on a geocentric universe.
• Islamic astronomy revised the Greek astronomy and
introduced the concept of observatory.
• Renaissance astronomers refuted the geocentric model,
and supported the heliocentric model.
• Kepler’s three empirical laws explain the motion of
planets in the heliocentric model.