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Ancient AstronomyProbably the study of night sky in ancient times was:• neither religious nor scientific

• practical and down to earth • for human survival

The artifacts, records and structures etc. of ancient age that are still alive give us a glimpse of their knowledge and accuracy.

Stonehenge:• Britain, Construction period (2800 B.C - 1100 B.C.)• Largest stone weigh up to 50 tons.• Three-dimensional calendar or almanac.

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Observator ies in America

The Big Horn Medicine Wheel (Wyoming)

(Similar to Stone Stonehenge in structure.) Caracol temple (Mexico)

(around 1000 A.D. by Mayans)

Chinese Astronomy

Oldest Record of a Nova:The two central columns of characters of the inscription on this ancient Chinese oracle-bone, dating from about 1300 B.C.E., reads, " on the 7th day of the month, a chi-ssu day, a great new star appeared in company with Antares.”

• impor tance to “omens” such as comets and “guest stars”.

• they kept the careful and extensive records.

• still the scientists turn to the Chinese records to obtain data recorded dur ing Dark ages (~ 5th -10th century A.D.).

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Astronomy by Islamic world•Islamic Astronomy flourished and grew, for 6 centuries, from the depth of the Dark Ages to the beginning of the new Age.

•many mathematical techniques (e.g. trignometry) -in response to practical problems-date of holy days-direction of Mecca from given location on Earth-” zenith” and “azimuth”

-name of the stars e.g. Rigel, Betelgeuse and Vega etc.

Astronomy in India•A stone observatory, built in 18th century by Maharaja SawaiJai Singh in Jaipur, India. •Its complex instruments , whose settings and shapes are scientifically designed , represent the high points of medieval Indian astronomy .•The huge masonry instruments were used to study the movement of constellations and stars in the sky. •Enormous sun-dials still provide accurate time, .

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The Geocentr ic View:(Greek Astronomy)

• The Greeks tr ied to EXPLAIN and UNDERSTAND, not j ust PREDICT.• To them, the universe was the basically solar system -the Sun, Ear th, Moon and 5 planets known at that t ime.They observed that:

- over the course of a night, the stars slid smoothly across the sky.-over the course of a month, the moon moved smoothly in the sky relative to the stars.

- over a course of a year , the Sun progressed along the ecliptic.- 5 other bodies known to them were Mercury, Venus, Mars, Jupiter and Saturn.

Planetes- “ wanderer” ----> do not behave in as regular and predictable manner. They seem to speed up and slow down and appear to loop back and for th relative to stars. Sometimes they stop. Sometimes their br ightness change.ProgradePrograde motion: Eastward (forward)RetrogradeRetrograde motion: Westward (backward)

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Greek philosopher Ar istotle - (384-322 B.C.) :He gave PROOFS that the Earth was

SPHERICAL:

• objects all fell towards its center perpendicular to ground sphere. • noted shadows cast on moon during eclipse were always round.

Earth - at the CENTER of Universe

- is heaviest thing around therefore it shouldn't move.

The Sun and the Moon revolve with uniform motion around a circle with Earth at its center.

This simple model could not account for the variation in planetary brightness and their retrograde motion.

.Each planet was taken to move uniformlyaround a small circle, called an epicycle,whose center moved uniformly around Earth on a second and larger circle, known

as the deferent.

Greek Astronomer Ptolemy’s Model :• He worked in Alexandr ia a city in Egypt, from 127--151A.D. As

a geographer, he is the first known to have used latitudeand longitude on ear th.

• His astronomy book, (mu epsilon gamma iota sigma tau nu) -- megiste—or “ The Greatest” is usually known by its Arabic name Almagest.

• Ptolemy' s influence on astronomy was immense for he established a detailed GEOCENTRIC MODEL.

• To account for the paths of Sun, Moon and the five planets known at that t ime his model required a series of near ly 80 distinct circles.

While complex, IT WORKED While complex, IT WORKED ------ used used for 1500 years (~13 centur ies)for 1500 years (~13 centur ies)

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The Heliocentr ic ModelAr istarchus (310--230 BCE) of Samos:applied Euclid's geometry to get the distance to the Moon. The angular diameter is measured directly; the linear diameter comes from seeing how much of the Ear th's shadow the moon occupies dur ing a lunar eclipse (about 3/8). I f the Ear th's diameter is known, this allows the Moon's to be found.

used geometry to estimate that the Sun was 19 times fur ther than the Moon (therefore 19 times larger, since angular sizes are the same).

also estimated that Sun was about 7 times the diameter of the Ear th.

THIS LED HIM TO PROPOSE A HELIOCENTRIC COSMOLOGY --- with the BIG SUN at REST and the SMALL EARTH MOVING AROUND IT.

OBJECTIONS TO HELIOCENTRIC PICTURE:

• A moving ear th should yield a powerful wind that would blow us off .

• Stars didn't show measurable parallax (Greeks couldn't think of them be so much fur ther away than planets).

THE COPERNICANREVOLUTION

Nicholas COPERNICUS (1473--1543), a Polish cleric,argued in favor of :1. HELIOCENTRIC COSMOLOGY --- THE EARTH IS NOT

THE CENTER OF THE UNIVERSE .2. The center of the Earth is the center of gravity and for the

MOON.3. The other PLANETS REVOLVE around the SUN.4. The STARS are MUCH FARTHER from the EARTH than is the

SUN.5. STELLAR (and SOLAR) “ MOTIONS” ARE APPARENT: all

are due to motion of the ear th.6. PLANETARY “ MOTIONS” are also substantially due to the

ear th's motions.7. RETROGRADE PLANETARY MOTIONS are also a

consequence of Ear th’s motion.

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Retrograde Motion(The Copernican model)

• Copernicus' model still assumed perfectly circular orbits and did not dispense with epicycles --- but now the main orbits went around the Sun and they could be smaller .

• While this model fit the data available then, it was only slightly better and certainly not proven.

• SO WHY WAS IT ``BETTER'' ?HELIOCENTRIC MODEL was SIMPLER and more

BEAUTIFUL than the Ptolemaic geocentric model.

• Still , it gained few adherents: --- went against common sense--- and written in Latin

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The Birth of Modern AstronomyTwo scientists Galileo Galilei and Jonnes Kepler :--- made ineradicable impr ints in astronomy--- popular ized the Copernican viewpoint

GALILEO Galil ei (1564--1642)

• He taught mathematics and astronomy at Padua, I taly.

• He performed EXPERIMENTS: e.g., balls rolling down inclined planes and learned about INERTIA.

• He is now widely regarded as “ father of exper imental science” .

• He built a TELESCOPE in 1609 but not for commercial or mili tary use.

Galil eo was the first man who POINTED the TELE SCOPE at THE SKY and DISCOVERED:

The Lunar surface was irregular : it had mountains, valleys and crater etc. Moon wasn't a perfect sphere.

Planets looked like disks, stars still l ooked likepoints.Milky way compr ised of many stars --- many fainter stars could be seen between those visibleto the naked eye.Venus went through phases (including a full phase, not possible in Ptolemaic model) -- reflecting light from the sun.

The Sun had spots --- the most per fect of all heavenly bodies had blemishes that moved across the face of Sun-Æ

�Sun rotates.

Jupiter had four moons --- something other than earth had things going around it. (Strongest support in the favor of Copernican model and against Aristotles model)

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Galileo’s discoveries

All of Galil eo’s discover ies, published in 1610 in “ Sidereus Nuncius” (The starry Messenger) , challenged the Ar istotlean Catholic picture of the Universe.

While he had already thought Copernicus’ modelwas best Galil eo felt this DATA had PROVEN it.

While others in Germany, England, etc., confirmed these discoveries and accepted the HELIOCENTRIC PICTURE, the response in Rome wasn' t very good: although Vatican astronomers confirmed his observations, they didn' t accept his logic and demanded that Copernicus' model not be taught.

Galil eo was ordered to stop supporting this view, and hebecame more circumspect, finally publishing a “ Dialog Concerning the Two Chief World Systems” in I talian in 1632.

He was placed under house arrest in 1633.

Johannes KEPLER (1571--1630)• He was a German mathematician and an accomplished amateur

observer, who had already accepted the basic Copernican picture.• In contrast to Galil eo, he was a pure theor ist. His groundbreaking

work about planetary motions was pr incipally a collection of hisemployer, Tycho Brahe.

TYCHO Brahe (1546--1601), a Danish noble,buil t his own observatory at Denmark named Uraniborgand designed HUGE instruments including:

sextants (for measur ing angles)astrolabes (for locating positions on the sky)

These allowed for PRECISION MEASUREMENTS,accuracies of about 1 arc min in planetary andstellar positions were achieved.Tycho was the first to quote errors along with hismeasurements.

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KEPLER'S LAWS of PLANETARY MOTION

1. All planets follow elliptical orbits,

with the Sun located at one focus.

2. Every planet sweeps out equal areas in equal times as it orbits the Sun.

[In other words, planets move fastest when closest to the Sun (near perihelion) and slowest when furthest away (aphelion).]

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KEPLER'S First Law Second Law

Credit: Bill Drennon (Central Valley Christian High School, Visalia, CA USA)

3. The square of a planet’s orbital period is

proportional to the cube of its semi-major axis

(distance from the Sun).

P = a2 3

(in ear th years) (in astronomical units)

1 Astronomical unit (AU) = 1.5 x 10 Km (150 milli on Km)8

Keplers laws are called empir ical laws: not based on any physics or mathematics but j ust on observations.

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MATHEMATICS AND UNDERSTANDING

Isaac NEWTON (1642--1727) of England,is arguably the most impor tant scientist ever .

• Effectively, Newton was the founder of PHYSICS as well as THEORETICAL ASTRONOMY.

• CO-INVENTED THE CALCULUS (Leibnitz had better notation).

• WORKED IN OPTICS: PRISM, REFLECTING TELESCOPE

• MECHANICS AND GRAVITY: his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), published in 1684.

• Newton was knighted, and became first president of the Royal Society, later Director of the Mint.

NEWTON'S LAWS OF MOTIONFirst law:

An object at rest remains at rest and an moving object will move forever at the same speed and in the same direction unless some external force acts on it to stop or change its direction. This tendency is called Inertia.

More massive is the body, the more is the inertia.

(Ar istotle's view was that the natural state of an object was to be at rest and forces were needed merely to keep something moving at a constant speed, but Newton realized fr iction or air resistance were forces that slowed things down; Galil eo had already understood and realized this.)

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The rate of change of velocity of an object ---- speeding up, slowing down or simply changing direction is called its acceleration.

Second law:Acceleration of an object is directly proportional to the applied force and inversely proportional to its mass.

m

Fa =

1 Newton is the force required to cause a mass of 1 kilogram to accelerate at a rate of 1 meter per second.

Third law:Every action (force) has an equal and opposite reaction.

Acceleration

Force

Mass

GRAVITY• Newton hypothesized that any object having mass

always exerts an attractive force on all other massive objects. This attractive force is called gravitational force or gravity.

• This is directly proportional to the product of masses of objects and inversely proportional to the square of the distance between them.

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r

mmGF

×=

Gravitational forceDistance between two objects

Mass of object 1

Mass of object 2

22111067.6 KgmNG −×=

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Planetary Motion

Theunending competition between Sun’s gravity and planet’ s inertia results in a stable orbit, despite planet’ s continuous rapid motion through space.

Kepler’s laws reconsidered

First law:

The orbit of a planet around the Sun is an ell ipse, with the center of mass of the planet-Sun system at one focus.

Third law:

totalM

aP

32 =

In ear th yearsCombined mass of two objects in solar units

In AU

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Unbound System

Speed needed to escape from the Earth forever is 11.2 km/sec. It is called escape speed.