A1 01 History and Concepts

Early Astronomers§2-1, 3-6, 4-1, 4-2, 4-3, 4-5

• Earth: Shape and Size?

• Solar System: Geocentric or Heliocentric?

• Galileo Galilei

Astronomy is the branch of science concerned with the nature of space, e.g. stars, planets, the universe.

Q: Who first discovered that the Earth is round? When?

In fact, the Earth is an Oblate Spheroid--it bulges slightly at the equator.

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Monday, August 16, 2010

The Earth is Round

On the Heavens By Aristotle

Written 350 B.C.E.Translated by J. L. Stocks

• ...the interposition of the earth that makes the eclipse, the form of this line will be caused by the form of the earth's surface, which is therefore spherical.

• Again, our observations of the stars make it evident, not only that the earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon. There is much change, I mean, in the stars which are overhead, and the stars seen are different, as one moves northward or southward.

http://classics.mit.edu/Aristotle/heavens.2.ii.html

Here are two ways

to show the earth is

round. Can you

think of more?

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Earth is 25,000 Miles Around

http://library.thinkquest.org/25672/earth.htm

...Eratosthenes. About 240 B.C., as librarian of Alexandria's already unsurpassed library of scrolls, he learned that Syene...stands almost exactly on the Tropic of Cancer. At noon the reflection of the midsummer sun was there visible in the water of a deep well. This showed that the sun was directly overhead and that its beams therefore pointed in a straight line toward the middle of the earth. On the same day, measurement of the noon shadow cast by a pillar at Alexandria shows that the sunbeam strikes the earth at an angle of 7.2°off the vertical. Sunbeams travel in parallel, so we may account for the difference only by the curve of the earth…. Eratosthenes thus knew that the angle between Alexandria … and Syene must be 7.2°, one fiftieth of the 360 degrees circle…. Syene lies nearly due south of Alexandria, and the road between them therefor lies almost exactly on a great circle passing through the North

http://www.juliantrubin.com/aboutfairs.html

and the South poles. Since it is almost exactly 480 miles long, the great circle is 50 times 480 miles in length: that is, the circumference of the earth is about 24,000 miles.

Actual value: 24,900 miles at the equator. 3




http://www.juliantrubin.com/a

http://www.juliantrubin.com/a

“Wandering Stars”

http://mars.jpl.nasa.gov/allaboutmars/nightsky/nightsky04/

Ancient astronomers noticed that five “stars” seemed to wander through the ecliptic--they didn’t stay fixed in a constellation like most stars. (The greeks called them “asterai planetai”.) In general, these “wandering stars” move west to east through the constellations of the zodiac. Occasionally however, they reverse direction, and move east to west; this is called retrograde motion. How to explain this behavior?

EastEast

West

West

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Geocentric ModelClaudius Ptolemaeus’s Almagest in c. 140 A.D.Because the planets seem to move backward some of the time...their observed motion cannot be explained by single circles. Ptolemy adopted a solution to this problem that he attributes to Apollonius (although earlier Greek writers, such as Hipparchus, also used this concept): Each planet moves on a small circle, called an epicycle.... Although these complex motions seem strange to those familiar with modern astronomy, they succeed in accounting for observed motions.

http://www.answers.com/topic/almagest?cat=technology

0:30http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/marsorbit.swf&movieid=marsorbit&width=825&height=550&version=6.0.0

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http://www.answers.com/topic/epicycle

http://www.answers.com/topic/epicycle





http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/marsorbit.swf&movieid=marsorbit&width=825&height=550&version=6.0.0











Heliocentric ModelNicolás Copérnic’s De Revolutionibus Orbium Coelestium in 1543.

De Revolutionibus famously proposed the heliocentric theory: the (now taken for granted) proposition that the Earth rotates around the Sun rather than vice versa. During Copernicus’ lifetime, orthodox opinion asserted the contrary view – that the Earth was fixed, unmoving at the centre of the Universe. This “geo-centric” myth was not easy to de-bunk: it was popularly held to be true by common sense perception supported by two millennia of philosophical tradition....

http://special.lib.gla.ac.uk/exhibns/month/apr2008.html

0:05http://mars.jpl.nasa.gov/allaboutmars/nightsky/nightsky04/

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Galileo first observed the moons of Jupiter on January 7, 1610 through a homemade telescope. He originally thought he saw three stars near Jupiter, strung out in a line through the planet. The next evening, these stars seemed to have moved the wrong way, which caught his attention. Galileo continued to observe the stars and Jupiter for the next week. On January 11, a fourth star (which would later turn out to be Ganymede) appeared. After a week, Galileo had observed that the four stars never left the vicinity of Jupiter and appeared to be carried along with the planet, and that they changed their position with respect to each other and Jupiter. Finally, Galileo determined that what he was observing were not stars, but planetary bodies that were in orbit around Jupiter. This discovery provided evidence in support of the Copernican system and showed that everything did not revolve around the Earth.

Galileo Discovers Four Moons Around Jupiter

http://www.solarviews.com/eng/galdisc.htmhttp://www.telescope1609.com/

Galileo.htm7


http://www.solarviews.com/eng/galdisc.htm


http://www.telescope1609.com/Galileo.htm




Galileo Discovers Four Moons Around Jupiter


Galileo Galilei’s Sidereus Nuncius, March 1610.

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http://www.solarviews.com/eng/people.htm#galileo

http://www.solarviews.com/eng/people.htm#galileo

Galileo: Phases of Venus

http://astronomy.fm/skylogs/skysafari/520/Galileo---Father-of-Modern-Astronomy.html

Possibly the most compelling argument Galileo made in favor of the Heliocentric Universe of Copernicus was based on the observations of Venus. Galileo observed the phases of Venus throughout the year. At times Venus presented a small but circular disk and at other times a large crescent. Based on these facts as illustrated in his drawings in Sidereus Nuncius, Galileo reasoned that Venus must orbit the Sun; proof of the Copernican Universe.


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Phases of Venus

http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/venusphases.swf&movieid=venusphases&width=870&height=600&version=6.0.0

http://astro.unl.edu/classaction/loader.html?filename=animations/renaissance/ptolemaic.swf&movieid=ptolemaic&width=900&height=660&version=6.0.0

According to Heliocentric Model

According to Geocentric Model

Since we do sometimes see Venus in a gibbous phase,which model can be ruled out?

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Elongation

http://www.eso.org/public/outreach/eduoff/vt-2004/Background/Infol2/EIS-D3.html

the angle between the planet and the sun

When are Mars, Jupiter, and Saturn brightest?What phase would they be in?

When are Mercury and Venus brightest?What phase would they be in?

(

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http://www.eso.org/public/outrea


Elongation

http://www.eso.org/public/outreach/eduoff/vt-2004/Background/Infol2/EIS-D3.html

the angle between the planet at then sun

When are Mars, Jupiter, and Saturn brightest?What phase would they be in?

When are Mercury and Venus brightest?What phase would they be in?

(

The maximum elongation of Venus is about 47°. Venus is a remarkable object in the night sky at its brightest, 35 days before or after inferior conjunction, when one third of the visible surface is illuminated. Under favourable conditions it is even possible to see the crescent shape of Venus with binoculars.

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• Earth: Shape and Size?

• Round--Aristotle, 350 BC

• About 12756.32 kilometers or 7926.41 miles--Eratosthenes, about 240 BC

• Solar System and the Motion of the Planets, esp. Retrograde:

• Geocentric--Ptolemy, 140 AD

• Heliocentric--Copernicus, 1543 AD

• Galileo Galilei: Moons of Jupiter, Phases of Venus

Early Astronomers§2-1, 3-6, 4-1, 4-2, 4-3, 4-5

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AstroTeam Classwork

• Class Action Questions

• Give four ways to demonstrate that the Earth is round.

All classwork due presently.

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Freedman, Kaufmann, Robert Geller; Universe, 9th ed.

• Ch 4: Online Quiz accessible from:http://bcs.whfreeman.com/universe9emust know: 1, 2, 3, 8, 9, 11, 12, 13, 14, 15, 16, 19, 20can’t hurt: 4, 6, 7, 10, 17, 18wish we could skip: 5

• Ch. 4, p. 92: 2, 3, 20 (name at least two observations).

Due at the beginning of the next class.

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http://bcs.whfreeman.com/universe9e


Physical Concepts§1.2, 1.4, 4.4, 4.5, 4.6, 4.7

• Some basic Astronomy terminology

• Kepler’s laws of planetary motion (1609)

• Newton’s three laws of motion, Newton’s universal law of gravity (1687)

Astronomy is the branch of science concerned with the nature of space, e.g. stars, planets, the universe.

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Astronomy Basics

• star

• planet

• moon (or satellite)

• asteroid

• comet

• solar system

• galaxy

• universe

• rotation (spin)

• revolution (orbit)

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Kepler’s Laws of Planetary Motion

1. The orbit of a planet about the Sun is an ellipse with the Sun at one focus.

2. A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.

3. The squares of the periods of the planets are proportional to the cubes of their semi-major axes (i.e. orbital radiuses):

paraphrased from Johannes Kepler's Astronomia Nova and Harmonices Mundi

published in 1609 and 1619.18


http://en.wikipedia.org/wiki/Johannes_Kepler

http://en.wikipedia.org/wiki/Johannes_Kepler

http://en.wikipedia.org/wiki/Harmonices_Mundi

http://en.wikipedia.org/wiki/Harmonices_Mundi

Kepler’s Laws of Planetary Motion

http://rst.gsfc.nasa.gov/Sect19/Sect19_2.html

http://astro.unl.edu/classaction/animations/renaissance/kepler.html 3

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http://astro.unl.edu/classaction/animations/renaissance/kepler.html













Newton’s Laws of Motion

1. An object at rest remains at rest, and an object in motion remains in motion, unless acted upon by on outside force. (aka: The Law of Inertia)

2. A force causes a mass to accelerate, aka F = ma

3. For every action, there is an equal and opposite reaction.

paraphrased from Isaac Newton’s Philosophiæ Naturalis Principia Mathematica published in 1687.

3http://www.nasa.gov/audience/foreducators/

diypodcast/nl-video-index.html: 11,14;19,21;25

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http://www.nasa.gov/audience/foreducators/diypodcast/nl-video-index.html













Newton's law of universal gravitation

F = Force

G = a constant: 6.67 x 10-11 m3/(kg•s2)

M’s = two masses

R = distance between the two masses

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http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html22


http://csep10.phys.utk.edu/astr161/lect/history/galileo.html




http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html23





Newton + Kepler

http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/circles/u6l3c.html

a

M

M = mass of central objecta = radius of orbitp = period of orbit, i.e. how

long it takes to orbit onceπ = pi, 3.14G = Gravitational constant,

6.67 x 10-11 m3/(kg•s2)

This equation is an approximation. It works when the mass of the

orbiting object is much less than the mass of the central

object.

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• Some basic Astronomy terminology

• rotation vs revolution

• solar system vs galaxy vs universe

• Kepler’s three laws of planetary motion (1609)

• Newton’s three laws of motion, Newton’s universal law of gravity (1687)

Physical Concepts§1.2, 1.4, 4.4, 4.5, 4.6, 4.7

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AstroTeam Classwork

• Class Action Questions

• The International Space Station orbits about 500 km above the surface of the earth. (The Earth’s radius is 6378 km). How does the force of gravity in the I.S.S. compare with that on the ground?

All classwork due presently.

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• Ch 2: Online Quiz accessible from:http://bcs.whfreeman.com/universe9emust know: 3, 5, 6, 7, 8, 9, 10, 11, 12, 16, 17, 18, 20can’t hurt: 1, 4, 13, 14, 15, 19wish we could skip: 2

• Ch. 4, p. 92: 16 (yes, one of these questions is a trick question), 26 (no need to explain), 27 (No more than three sentences).

Due at the beginning of the next class.

Freedman, Kaufmann, Robert Geller; Universe, 9th ed.

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A1 01 History and Concepts

Education

galileo galilei astronomy

vicinity of jupiter

otherand jupiter

compelling argument

moons of jupiter onjanuary

heliocentric modelhttp

observed motion

geocentric modelhttp