Transcript Lecture One (Powerpoint format)

Science 3210 001 : Introduction to Astronomy
Lecture 1 : Introduction and Overview
Robert Fisher
Overview
 I. Introductions
 II. Historical and Cultural Overview of the Origin of Astronomy and
Astrophysics
 II. Scientific Overview
 A. How large is the Universe? How old is it?
 B. How can we learn about the distant universe?
 C. How is life on Earth connected to the life cycles of stars?
What is Astronomy?
 An ancient and universal subject, predating written
records.
 “The study of the motion of celestial bodies.”
“… a trip to the serenity of the soul, to the eternal fusion with the
cosmos, there we feel our own fragility.”
-- Pablo Naruda, “The Heights of Machu Picchu”
Star Trails over Mauna Kea, Hawaii
Stonehenge : Stone Age Tech Observatory
-- c. 2600 BC
Stonehenge Layout
Pyramids at Gaza, Egypt
Layout of Complex at Gaza
Machu Picchu, Peru -- 15th Century AD
Intihuatuna Stone
“Alternative” Explanations of Ancient
Monuments
 From time to time, one will hear
of “alternative” explanations of
ancient monuments (aliens,
UFOs, etc…).
 Besides the extraordinary nature
of these claims, these
explanations take an implicitly
condescending view of ancient
peoples.
 Even very early prehistoric
people achieved a high degree
of culture (evidenced from
Lascaux and early musical
instruments) -- often forgotten
today.
A 50,000-year old flute!
What Ancient Astronomy Was NOT
 Ancient astronomy (prior to the Greeks) was generally NOT
 Divorced from cultural and religious systems of thought.
 Therefore, scientific, as we would think of it today -- closer to
what we term astrology.
Constellation Virgo (from Flamsteed, 1729)
Birth of Western Philosophy and Science
 Modern science was born with Thales of Miletus (624 - 548 BC),
who is believed to be the first Westerner to clearly delineate a
separation between mystical beliefs and natural explanations.
This was the origin of both Western philosophy and “natural
philosophy”.
 The concept of the scientific method was still far off in the future.
Natural philosophy gave rise to scientific hypotheses, but these
were unchecked by experimental validity.
“Thales,” says Cicero, “assures that water is the arche [principle] of
all things; and that God is that Mind which shaped and created all
things from water.”
Nietzsche on Thales
 “Greek philosophy seems to begin with a preposterous idea, with
the proposition that water is the origin and mother-womb of all
things. Is it really necessary to stop there and become serious ?
Yes, and for three reasons: Firstly, because the proposition does
enunciate something about the origin of things; secondly,
because it does so without figure and fable; thirdly and lastly,
because in it is contained, although only in the chrysalis state, the
idea: Everything is one. The first mentioned reason leaves
Thales still in the company of religious and superstitious people,
the second however takes him out of this company and shows
him to us as a natural philosopher, but by virtue of the third,
Thales becomes the first Greek philosopher. If he had said: "Out
of water earth is evolved," we should only have a scientific
hypothesis; a false one, though nevertheless difficult to refute.”
Babylonian World Map
Ancient World Map due to Anaximander
(Reconstructed)
Fra Mauro Map of World (c. 1459)
What Led the Ancient Greeks to a Natural World
View?
 Several explanations have been offered :
 The Greeks were a practical-minded people interested in sea travel,
commerce, and trade. Unlike the ancient Egyptians, they lacked
“official” state religion headed by the head-of-state.
 Because of their travels, they were exposed to many (often
apparently contradictory) belief systems and mythos.
 The Greeks had developed a system of writing based on Phonencian
lettering around the 9th century BC.
 Probably a combination of all of these factors played a role.
Science and Astronomy in the Ancient World
 Philosophy and science flourished in the Ancient world, and
include numerous remarkable achievements :
 Democritus (c. 460 - ) formulated the atomic hypothesis
 Euclid (330 - 275 BC) of Alexandria formalized a system of geometry
in the 13 books of his Elements
 Aristarchus (310 - 230 BC) of Samos determined (somewhat
roughly) the distances and sizes of the moon and the sun, and
formulated a Sun-centered solar system
 Eratosthenes (276 - 194 BC) of Cyrene determined the size of the
Earth using nothing more than a well and a stick
Method of Eratosthenes to Measure Size of Earth
Using Only a Rod
Science after the Fall of Rome
 After the Fall of Rome, the Byzantine and Islamic cultures passed
down the torch of the ancients, and expanded the forefront of
knowledge.
 Beginning about two centuries after the Hagira, the Abassid
Caliphate in Baghdad founded the House of Wisdom, and began
an ecumenical scholarly program, including the translation of
major Greek works into Arabic.
 Later (primarily in Moorish Spain), European scholars
retranslated these Arabic works into Latin, retaining many Arabic
terms (zenith, azimuth, algorithm, zero…) and stars (Algol, Vega,
Altair, Deneb…)
Science in the Muslim World during Middle Ages
 Arab scientists advanced the state of science considerably
 al’ Khwarizmi (790 - 850 AD) combined Hindu and ancient Greek
methods and founded modern algebra in his book “Hisab al-jabr w’almuqabala” in 830 AD
 Mathematician Muhammad bin Ahmad introduced the concept of
zero in 967 AD
 Ibn al-Haitham (965 - 1040), one of the greatest scientists of any
time, discovered fundamental principles of motion and optics.
Passing the Torch Back to Europe
 Ahmad Al-Farghani’s Ptolemeic astronomical text “The Elements”
was translated by Gerard of Cremona in Toledo. This was the
source of Dante’s astronomical knowledge for The Divine
Comedy.
 Gerard’s translation was reworked into a university-level text in
the 13th century by John of Hollywood (Johannes de
Sacrobosco). It went through 200 editions over four centuries.
 A number of Islamic astronomers began to doubt the validity of
the ancient geocentric models of the solar system, possibly laying
the groundwork for Copernicus. Ibn Rushd (12th century) wrote
“…the astronomy of our time offers no truth, but only agrees with
calculations and not with what exists.”
Gustave Dore illustration of Dante’s Divinia
Comedia, Paradiso Canto
European Astronomy and Mechanics in Early
Renaissance
 The knowledge brought back through Moorish Spain reached its
full culmination in the work of early Renaissance scientists.
 Niklaus Copernicus (like Aristarchus) suggested a heliocentric, as
opposed to geocentric model of the solar system.
 A single generation of scientists -- Tycho Brahe, Johannes Kepler,
and Galileo Galilei -- laid the groundwork for the monumental
work of Isaac Newton.
 Newton created a unified description of physical laws which apply
equally to earthly and heavenly bodies.
From Newton’s Principia
Recent PBS Documentary
QuickTime™ and a
decompressor
are needed to see this picture.
Scientific Method
 In 16th and 17th centuries, Age of Reason thinkers Francis Bacon
and Rene Descartes laid the philosophical foundations for the
scientific method.
 Their work emphasized the primacy of observation and
experimentation both in motivating new hypotheses and in
checking their validity.
 Birth of modern science as we know it today.
Scientific Method
 The scientific method consists of several elements :
 Observation
 Hypothesis
 Prediction
 Experiment
Comments on the Scientific Method
 There is a subtle distinction between observation or experiment
and inference.
 ``Theory" in the scientific context has a specific meaning.
 Scientific theories are always provisional, rather than final and
immutable, and are always subject to continual refinement and
sharpening by successive approximation.
 Scientific inquiry has limitations. There exist questions that can
neither be asked nor answered under the scientific method.
Concept Question
 When flipping through the paper, you come across the weather
prediction. Your horoscope claims that “Today, there will be a 30%
chance of rain.” Is this a scientific prediction? If so, why? If not,
why not?
What is Astrophysics?
 Beginning in the mid-17th century, the ancient subject of
astronomy combined with the emergent science of physics to
create a new understanding of the cosmos : astrophysics.
 The actual term came into usage in the 19th century, when
breakthroughs in our understanding of light led to enormous
advances in our understanding of the composition of stars and
the gaseous medium between them.
“The evolution of the world can be compared to a display of
fireworks that just ended; some few red wisps, ashes, and smoke.
Standing on a cooled cinder, we see the slow fading of the suns,
and we try to recall the vanishing brilliance of the origin of the
worlds.”
-- Lemaitre
II. Scientific Overview
How Can We Know What the Universe Was Like
in the Past?
 Because light travels at a finite speed, the farther we look, the
longer the light takes to reach us.
 To appreciate this, imagine viewing a very bright flash of light
from above :
How Old is the Universe?
Light Travel Times
 Astronomers sometimes measure distance in terms of the
distance that light travels in a fixed amount of time.
 One light-second
Distance = velocity x time
= (speed of light) x (1 second) = (3 x 108 m/s) (1 s) = 3 x 108 m
Roughly the distance from the moon to the Earth.
Light Travel Times (cont.)
 One light-year
The number of seconds in a year is
1 year = (365 d) (24 hours/d) (60 min/hour) (60 s/min) = 3.15 x 107 s
approximately equal to 3 x 107 s
1 LY = (speed of light) x (1 year) = (3 x 108 m/s) (3 x 107 s ) =
= 9 x 1015 m
Roughly one-quarter the distance between the Earth and the nearest
star (other than the Sun).
Seeing the Night Sky
 The receptors in our eyes are tuned to only a tiny portion of the
entire electromagnetic spectrum
 Light is a vibration in the electromagnetic field, much as sound is
a vibration in air
 If our ears were sensitive to the same dynamic range of
frequencies as our eyes, we could only hear less than an
octave!!
Visible and Infrared Images
Infrared Images of the Orion Cluster
Chandra Images of the Crab Nebula
We are Star Stuff
 Stars generate energy by nuclear reactions in their cores.
 This process of nuclear “burning” converts lighter elements like
hydrogen and helium into heavier elements, like carbon, oxygen,
nickel, and iron.
 These heavier elements are eventually blown back into space by
stellar winds and explosions, and are incorporated into new
generations of stars and planets.
Hubble Space Telescope Image of Eagle Nebula
A Stellar Nursery
The Massive Doomed Star Eta Carinae
Galaxies are Stellar Factories
 At the point of their formation, galaxies contain enormous
quantities of gas
 Over time, this gas accumulates in spiral arms to form dense
clouds.
 These dense clouds themselves obscure the surrounding starlight
and cause the clouds to become unstable to gravitational
collapse -- leading to the birth of stars within them.
Spiral Disk Galaxy NGC4414
On Even Larger Scales, Clusters of Galaxies
The Cosmic Year
 Imagine that the entire history of the universe could be
condensed into one “cosmic year” (originally due to Carl Sagan).
 One “cosmic day” equates to the 13.7 billion year history of the
universe, divided by 365 -- roughly 38 million years.
 On January 1, the big bang occurs.
 In February, the Milky Way forms.
 At the beginning of September, the Earth forms.
The Cosmic Year (cont.)
 By the end of September, life on Earth has started.
 On December 26, the dinosaurs emerge.
 On December 30, the dinosaurs become extinct.
 On 9 PM, December 31, the early hominids develop.
 At 11:58 PM, December 31, modern humans evolve.
 11 seconds ago, the pyramids were built.
 1 second ago, Galileo discovers his laws of motion.
Ray and Charles Eames
La Chaise (Ray & Charles Eames, 1948)
Ray and Charles Eames with Model of
Mathematica: A World of Numbers Exhibit
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Next Week : An Introduction to the Night Sky