History of Astronomy

“Progress in science is often slow and intermittent and may require a great

deal of patience before significant progress is made” (McMillan 18).

Aristotle Greek Philosopher

384-322 B.C.Geocentric: Earth

center of the universe

Taught the perfect form: Circle

Pros: Described the moon and sun’s orbit

Cons: variations in planetary brightness and planets orbits

phillwebb.net

Claudius PtolemaeusPtolemy 40 ADPtolemaic model

◦ Described 5 planet orbits well

Building off Aristotle's version and what observers had observed in the sky dealing with planetary motion

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Claudius Ptolemaeus

ptolemy.berkeley.edu

Aristarchus of SamosLived: 310-230 B.C.All planets, including Earth,

revolve around the sun.Earth rotates on its axis once

each dayWhy his ideas weren’t accepted:

Politics, Aristotle’s ideas won the popular vote

“Usually one generation of scientists can bring sufficient objectivity to bear on a problem, though some especially revolutionary concepts are so swamped by tradition, religion, and politics that more time is need” (McMillan 31).

Nicholas CopernicusPolish Cleric

◦ 16th centuryCopernican Revolution

◦ All planets orbit the sun◦ Explained planets orbits and variation in

planetary brightness◦ Wrote of his findings, but claimed them as

only a mathematical hypothesis Keep him out of trouble with the church

◦ Never excepted during his life time Went against the religious view points at the time Contradicted conventional wisdom: didn’t match

Aristotle thinkers.

Nicholas Copernicus

indiamart.com

http://galileo.rice.edu/sci/theories/copernican_system.html

Diagram of the Copernican system, from De Revolutions

Galileo GalieiItalian mathematician

and philosopherUsed the brand-new

technology to further humans understanding = telescope◦ Telescope was invented

in Holland in early 17th century

Performed experiments to test his ideas◦ ‘Father of experimental

science’

quotationsbook.com

marcdatabase.com

Galileo’s DiscoveriesMoon has mountains, valleys, and

cratersSun has imperfections-dark blemishes

now known as sunspotsSun rotates approximately once per

month around an axis roughly perpendicular (right angle) to the Earth’s orbit.

Jupiter’s moon (4 of them)Venus show a complete cycle of phases,

similar to the moon

Galileo’s Popularity His ideas were opposite to what science thought at that

time 1610 published his findings agreeing with Copernican

theory. 1616 his ideas were judged contrary to accepted belief of

that time Both his and Copernicus were banned by the ‘Church’ Told to abandon his astronomical pursuits

◦ Which he didn’t “These actions brought Galileo into direct conflict with

the Church. The Inquisition forced him, under threat of torture, to retract his claim that Earth orbits the Sun, and he was placed under house arrest in 1633” (McMillan 30).◦ It wasn’t until 1992, did the ‘Church’ publicly forgave Galileo

‘crimes’

Tycho Brahe1546-1601, DenmarkStudied astrology, alchemy, and medicineKept meticulous records of stars, planets,

and celestial events.He move to Prague, after a falling out with

the Danish courtsThere he hired Kepler to find a theory that

could explain Brahe’s planetary data◦Year later Brahe died and Kepler inherited

Brahe potion (Imperial Mathematician of the Holy Roman Empire) and possession

Johannes Kepler16th century

German mathematician and astronomer

Took him 29 years to try and find an unifying principle to explain the motions of the planets ◦ Data was

collected by Brahe

libwebspace.library.cmu.edu

Kepler’s LawsI. The orbital paths of the

planets are elliptical with the Sun at one focus

◦ Ellipse is a flattened circle, a circle is a special kind of ellipse

II. An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time.

◦ When the planet is closer to the Sun it must move faster then it is further away

◦ This applies to any orbiting object

III. The square of a planet’s orbital period is proportional to the cube of its semi-major axis

astro.psu.edu

hyperphysics.phy-astr.gsu.edu

Isaac Newton

17th century British physicist/mathematician

Newtonian Mechanics◦ 3 basic laws of

motion, the laws of universal gravitation, and a little calculus

en.wikipedia.org

Newton’s 1st LawInertia: the tendency for an

object to keep moving in the same direction and speed unless acted upon by an outside force.

More mass = greater inertia and the more force is needed to change its motion

Newton’s 2nd LawThe acceleration of an object is

directly proportional to the net applied force and inversely proportional to the object mass◦Greater force or smaller mass =

increase acceleration ◦F=ma

Newton’s 3rd LawTo every action there is an equal

and opposite reaction

Laws of Universal GravitationGravitational force: things with

mass exerts attraction on other masses

“ The mutual gravitational attraction of the Sun and the planets, as expressed by Newton’s law of gravity, is responsible for the observed planetary orbits” (McMillan 37).◦The Sun pulls the planets changing

the planets forward motion into a curved path

Newton’s revision to Kepler’s LawsI. The orbit of a planet around the Sun is

an ellipse having the center of mass of the planet-Sun system at one focus

◦ The center of mass of two objects about the same size is outside either object. However if one object is larger then the other the center of mass shifts towards the larger object.

III. Changed the math to use the combined mass of the two object, which made it more correct and also allowed it to be used outside our solar system