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 hearof “alternative” explanations ofancient monuments (aliens,UFOs, etc…).

! Besides the extraordinary natureof these claims, theseexplanations take an implicitlycondescending view of ancientpeoples.

! Even very early prehistoricpeople achieved a high degreeof culture (evidenced fromLascaux and early musicalinstruments) -- often forgottentoday.

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 passeddown the torch of the ancients, and expanded the forefront ofknowledge.

! Beginning about two centuries after the Hagira, the AbassidCaliphate in Baghdad founded the House of Wisdom, and beganan ecumenical scholarly program, including the translation ofmajor Greek works into Arabic.

! Later (primarily in Moorish Spain), European scholarsretranslated these Arabic works into Latin, retaining many Arabicterms (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’al-

muqabala” 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

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 ofastronomy combined with the emergent science of physics tocreate a new understanding of the cosmos : astrophysics.

! The actual term came into usage in the 19th century, whenbreakthroughs in our understanding of light led to enormousadvances in our understanding of the composition of stars andthe gaseous medium between them.

“The evolution of the world can be compared to a display offireworks 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 theworlds.”

-- 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