Early Views of the Solar System

(With thanks to Mark Ritter & John Bloom)

The Roots of Astronomy• Already in the stone and bronze ages,

human cultures realized the cyclic nature of motions in the sky.

• Monuments dating back to ~ 3000 B.C. show alignments with astronomical significance.

• Those monuments were probably used as calendars or even to predict eclipses.

Stonehenge

• Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices

• Probably used as calendar.

Summer solstice

Heelstone

Constructed: 3000 – 1800 B.C.

Examples All Over the World

Big Horn Medicine Wheel (Wyoming)

Examples All Over the World

Caracol (Maya culture, approx. A.D. 1000)

Ancient Astronomers

• 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 models.

Ancient Greek Astronomers

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, ethereal bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

General Greek Principles of Science

• Nature can be understood(Not just randomly occurring phenomena)

• The diverse behavior observed in nature is held together in patterns

• Tendency to “Save the Phenomenon”

Euclid ~ 600 BC• What he got right:

– Predicted solar eclipses– Moon shines due to reflected sunlight– Universe explainable by ordinary knowledge

and reason• What he got wrong:

– Thought Earth was a flat, rotating disk– Thought water was a fundamental element

from which all other matter derives

Pythagoras ~ 530 BC• First to suggest that Earth is

a sphere

• One of the earliest advocates of a geocentric solar system:– Earth at center, surrounded

by system of concentric, rotating, transparent spheres

– Bodies attached to spheres in this order:

Moon, Mercury, Venus, Sun, Jupiter, Saturn, stars

                                              

                                                                          

Philolaus ~ 410 BC(student of Pythagoras)

• Suggested Earth moves around a central fire

• (NOT the Sun)• What were his

new ideas???

Ancient Greek Astronomers

• Eudoxus (409 – 356 B.C.): - Model of 27 nested spheres

• Aristotle (384 – 322 B.C.), major authority of philosophy until the late middle ages: - Universe can be divided in 2 parts:1. Imperfect, changeable Earth2. Perfect Heavens (described by spheres)

• Aristotle expanded Eudoxus’ Model to use 55 spheres.

the Aristotelian universe ~ 340 BC

• In ancient times philosophers argued from first principles, things that were accepted as “obviously” true

• Several important ideas ruled the ancient world…

Aristotle ~ 340 BC

• first: the earth was round• gave 4 reasons for earth’s roundness:

– Symmetry: the sphere is a perfect shape– Elements have their natural places: Earth’s pieces fall naturally

to Earth’s center, pressing it into a spherical shape (circular reasoning!)

– Shadow: Lunar eclipse shadow (Earth’s shadow on Moon) is always circular (what if Earth were a circular disk?)

– North Star: Polaris gets higher in the sky the further north one goes

• second: the earth was located in the center of the cosmos!

• third: the heavens were perfect, the earth imperfect

• all the cosmos traveled in perfect circles around us

• Plato pushed this, his student, Aristotle adopted it

• he gave us the Aristotelian way of looking at the cosmos…

• geocentric universe with all the heavenly objects moving in uniform circular motion

• Aristotelian concepts dominated ancient ideas about the universe for thousands of years…

• Two major themes here:1) our place in the cosmos2) the character of planetary motion

Aristarchus ~ 240 BC

• Rotation of Earth on its own axis accounts for daily motion of stars

• Earth revolves around Sun in a yearly orbit

He had it right before 200 BC! But his ideas failed to catch on…

Eratosthenes ~ 235 BC• Calculated Earth’s radius to within about 5% accuracy!• Basic method:

– Measured shadow lengths at two different cities directly North-South of each other (Syene and Alexandria)

– Calculated Earth’s circumference and radius using geometry– Central angle = 7.5 degrees, 500 miles from A to S, times 48 = 24,000 miles; – Actual angle = 7.2 degrees, 500 mi x 50 = 25,000 miles.

• great ancient astronomer of the 2nd century

• wanted to put it all this orbital motion into a mathematical model

• final effort to “Save the Phenomenon”• no significant changes for 1300 years!• Ptolemaic view = geocentric view

Ptolemy ~ 120 AD

• why did they think we were in center anyway?

• popular worldview

• the Sun appears to rise, transit the sky, and set

• it doesn’t feel like we are moving!

• and there was the problem of…

• parallax, the apparent motion of an

object b/c of the motion of the observer

• if we were going around the sun, the background

stars should change position!

• (they do, but not a lot since they are so far

away)

• planetary motion was difficult for the ancients to explain

• planets (wanderers) would suddenly backtrack!

• called retrograde motion

• Ptolemy tried to explain it with epicycles

(little circles - blue) on deferents (big circles -

white) • all in an attempt to keep the Aristotelian

(geocentric) view of the cosmos

• all Ptolemy’s attempts to mathematically explain Aristotle's universe were put together in a book preserved by the Arabs, Al Magisti, then translated

into Latin, The Almagest • but unknown to them it was all wrong! enter…

Copernicus ~ 1500 AD

• a Polish monk, Mikolai Kopernik (Lt. Nicolaus Copernicus) rediscovered the

heliocentric model of the universe

• but being for heliocentrism was being against Aristotle, and against the Church!

De Revolutionibus• finished in 1530, Copernicus wrote De

Revolutionibus Orbium Coelestrium (“on therevolution ofthe celestialorbs”)

• he didn’t givepermission forits publicationuntil he wasdying

• most important idea?• he put the sun (Sol) in the center!

• with the sun at the center the whole universe was simplified and elegant, and could explain things like retrograde motion

• we, moving faster on the inside, see other planets “seem” to move backward

• but!…

• Copernicus insisted on circular orbits!• so he had to come up with his own

epicycles so it worked better…

• his model was incorrect• but his hypothesis with

the sun in center was right

• but without a telescope it couldn’t be substantiated with evidence

• AND his circle obsession meant it couldn’t predict things very well

• AND it upset people because their whole universe changed!

• so it wasn’t accepted for a long time

Tycho Brahe~ 1500s

• the greatest pre-telescopic observational astronomer

• in 1572, he saw a supernova which he called a new star

• it showed no parallax, so he deduced it to be a part of the starry sphere

• but the star sphere was “perfect and unchanging”!!!

• so the new star challenged Ptolemy and Aristotle

• Brahe wrote a book on it called De Stella Nova

• which got the notice of lots of people and made him famous…

• Danish king Frederick II gave him island of Hveen to build an observatory (Uraniburg)

• here Tycho shows off the latest mural quadrant

• with tools like these (notice no scopes) he observed the heavens and took over 20 years of data!

• Tycho had his own version of what was going on

• neither Ptolemaic nor Copernican

• Earth was at center, but all the other planets went around the sun

• Tychonic view

• Tycho ended up in Prague and hired some new assistants to help prove his system was correct

• one of the assistants was Johannes Kepler

Johannes Kepler (1571 – 1630)

• Kepler had a not-so-fun life

• born in 1571 in Germany, he was poor, had an absent father, was sickly, had a nasty mother, etc., etc.

• a Copernican, Kepler even wrote a book Mysterium Cosmographicum which tried to prove that all the planets were built with regular solids as spacers!

• eventually, after being persecuted for being a Protestant, he headed for Prague to work for Tycho

• then Tycho died…• so, Kepler took all

of Tycho’s work to finish the Rudolphine Tables himself

joining Tycho

• with all of Tycho’s data, Kepler tried to find out how the planets really moved

• he discovered that Mars moves in an ellipse, not a circle!

• and it traveled at different speeds!

• he wrote a lot after this and in 1619 published The Harmony of the Worlds in which he said that the radii of the planets’ orbits are related to their periods

• all this led to his three fundamental laws of planetary motion…

Kepler’s three laws of planetary motion

• first law relies on the ellipse – an oval drawn around 2 points

• points called foci (sing: focus)

• semimajor axis (a) = half the longest diameter

• eccentricity (e) is half the distance between foci divided by the semimajor axis

• a circle is an ellipse with e = 0• greater e more elliptical

• Kepler’s First Law (Law of Ellipses) states that all planets travel in ellipses with the sun at one focus

• Second Law (Equal Areas-Equal Times) planets sweep out equal areas in equal

times• translation: they go faster when closer to

the Sun

• Third Law T2 = R3

• states that the closer a planet is to the Sun the shorter its “year”• T = period in Earth years

• R = distance in AUs• given one, you should be able to find the other• Kepler never knew why his laws worked, just

that they did

• finally, in 1627, Kepler finished the R Tables, a precise model of planetary motion

• (notice Tycho as part of the pillars of astronomy)

• a contemporary of Kepler was…

the Rudolphine Tables

Galileo Galilei(1594 – 1642)

• helped push science to a new level…

• GG had an academic upbringing and was quietly Copernican

• it was the telescope that pushed him over the top

• he used this recent invention to examine the sky and gather evidence for the heliocentric view

telescopic observations

• he immediately published his findings in Siderius Nuncius (The Sidereal Messenger)

• here he reported several big discoveries…

• 1: The Moon was not perfect!!!

• remember Aristotle said everything in the heavens was perfect?

• but the Moon looked sort oflike Earth!!!

Major Discoveries of Galileo• Surface structures on the moon; first estimates of the height of mountains on

the moon

• 2: There were lots of stars up there (many more than were previously supposed)

• the Milky Way (and the rest of the heavens) is substantial, like the earth, not ethereal

• 3: He saw four little “stars” near Jupiter

• these points of light were going around Jupiter!

• but they weren’t supposed to!!!! they should have been left behind!!!

• So, not everything orbits the Earth…

• and, if Jupiter can move and keep its “moons” so could Earth

Major Discoveries of Galileo

• Moons of Jupiter (4 Galilean moons)

• Rings of Saturn

(What he really saw)

(What he really saw)

4: He saw that the sun had spots (proving that the sun is not

perfect!), and that it rotated

Major Discoveries of Galileo

5: He observed that Venus went through phases (including “full Venus”)So what? it can only have all phases if it goes around the sun, not the Earth!

• but even with all this evidence people were still upset

• some didn’t even look through the telescope!

• The Catholic Church, still in love with Aristotle, told Galileo to be quiet

• it was in 1629 that he wrote his great defense of the Copernican way…

Galileo’s Dialogue• it was an imaginary debate between three

friends:

Salviati (pro-Copernican)Sagredo (indifferent)Simplicio (pro-Ptolemy)

• problem: Simplicio presented the Pope’s view of things … bad move!

• Galileo was brought before the Inquisition

• charged with not shutting up

• condemned to life imprisonment

• note: what Copernicus and Kepler and Galileo taught and discovered didn’t contradict the Bible…

• it contradicted people’s interpretations, traditions, and worldviews

• in trying to understand the skies in the 16/17 century, modern science was born

• nothing more than a logical way of observing, studying, and thinking about nature

• scientific method (Roger Bacon, 13th century)• cause and effect (Francis Bacon, 17th century)• et al…

The Birth of Modern Science

Isaac Newtonand orbital

motion• by now we have a

new outlook on the universe and new ways to study it all

• perfect time for this guy to show up…

Isaac Newton

• Isaac was very smart, raised in England, educated at Trinity, but…

• during the Black Plague of 1665 went home to

Woolsthorpe• while there he

thought through his laws of motion…

• which he wrote about in his Principia

• from the work of all the great guys before him he gave us three laws of motion which we still use

• he realized the same force that brings an apple to the Earth keeps the Moon in orbit

• and those other things must be pulling on us, too

• he figured if something has mass it has gravity

• mass is how much stuff/matter something has

• weight is the force that something exerts because gravity pulls on it

• He also realized that the distance plays a huge role

• and is an inverse square relation

• double the distance and gravity decreases by 4x.

• know the relationships…• the force of gravity attracting two

objects is proportional to their

masses, but inversely proportional to the squares of their distances

orb

ita l

moti

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• here’s why satellites orbit celestial bodies

• throw something hard enough and the Earth will curve out of the way

• it will “fall” forever• that is being in orbit

• this animation shows how

• circular velocity is the speed you have to achieve to orbit in a circle

• less, you crash; more, you might even leave!

• the Shuttle needs to get to about 8 km/s to do it.

• less over smaller bodies