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Welcome to Astronomy 320 Instructor : Clay Bratton Office: TBD Office Hours: TBD E-mail: cgbii@mac.com This is the best way to contact me. Why should we be interested in Astronomy? Personal curiosity about the world. Predict the seasons.

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welcome to astronomy 320

Welcome to Astronomy 320

Instructor : Clay Bratton

Office: TBD

Office Hours: TBD

E-mail: cgbii@mac.com

This is the best way to

contact me.

why should we be interested in astronomy
Why should we be interested in Astronomy?
  • Personal curiosity about the world.
  • Predict the seasons.
  • Appreciate some of the “gee-whiz” technology we live with.
  • Understand our history better.

Picture courtesy of Clive Ruggles

ancient peoples and the stars
Ancient Peoples and the Stars
  • El Karnak
  • Example of Egyptian construction.
  • Oriented toward the sunrise of the winter solstice.
  • Why?

Photo courtesy of Lyndsay Kyker

Chichen Itza
  • Astronomically oriented sites found all around the world
  • Cosmology of the Mayan’s was a living religious philosophy

Picture courtesy of Clive Ruggles

fajada butte
Fajada Butte
  • Summer solstice.
  • Not all astronomical constructions were large.
  • These petroglyphs created by the Chacoans mark the motion of the Sun over the whole year.
  • They also mark the position of the Moon over an 18.6 year cycle.

Courtesy of Bryan C. Bates

Big Medicine Wheel - Wyoming
  • Even nomadic or semi-nomadic peoples used astronomical sites.
  • Aligned with sun and several stars for rise on the summer solstice.

Photo courtesy of Lyndsay Kyker

  • Built in stages over several hundred years.
  • Possibly built to predict summer and winter solstices, eclipses and possibly more.

Picture courtesy of Clive Ruggles

  • Aristotle, more than any other thinker, determined the orientation and the content of Western intellectual history. He was the author of a philosophical and scientific system that through the centuries became the support and vehicle for both medieval Christian and Islamic scholastic thought: until the end of the 17th century, Western culture was Aristotelian. And, even after the intellectual revolutions of centuries to follow, Aristotelian concepts and ideas remained embedded in Western thinking. -The Catholic Encyclopedia.
Born on the Chalcidic pennisula of northern Greece.

Student in Plato’s Academy in Athens.

Established his own school the Lyceum.

Endorsed and developed the idea of an axiomatic system for each science.

On the Heavens


  • Aristotle put forward his notion of an ordered universe or cosmos.
  • Everything had its natural place, a privileged location for bodies with a particular makeup, and that the laws of nature were not the same in the heavenly and the earthly regions.
  • Earth was the center of the Cosmos.
  • Heavenly bodies were part of spherical shells of aether.
  • The cosmos encompassed all existence.
  • Aristarchus of Samos
  • Aristotle’s cosmology didn’t work! The heavenly bodies didn’t all seem to move in perfect circles.
  • Developed a new and improved model for motion of the heavens based upon eccentric, epicycle, and equant.
  • This was considered a minor refinement to Aristotle’s cosmology.

Sun, Moon, and stars all have simple movements in the sky

  • Planets:
  • Move with respect to fixed stars
  • Change in brightness
  • Change speed
  • Undergo retrograde motion
  • Tables to determine timing of astronomical events not sufficiently accurate.
  • Sailing ships (Spanish & Portuguese) were sailing out of sight of land for weeks at a time and needed better astronomy to navigate.
  • Julian calendar was no longer accurate.
  • Placed the SUN at the center of the Universe.
  • Tried to downplay this as only a mathematical hypothesis.
  • De Revolutionibus Orbium Coelestium - published the year of his death.
  • Most sophisticated treatis since the Almagest
  • Placed on list of Forbidden Books!
  • Heliocentric view not accepted rapidly.
  • Required complete overturning of Aristotelian Physics!
  • Small parallax implied great distances.

Sun is at center of solar system. Only Moon orbits around Earth; planets orbit around Sun.

This figure shows retrograde motion of Mars.


2.4 The Birth of Modern Astronomy

Phases of Venus cannot be explained by geocentric model

  • First quiz will be given next Wednesday

January 30th

You should be reading Chapters 2 and 1.

Chapter 2 will be covered on the first quiz.

tycho brahe
Tycho Brahe
  • Born to Danish nobility. (Therefore, rich!)
  • Observatory - Uraniburg
  • Tycho designed and built new instruments, calibrated them, and instituted nightly observations.
  • Hired J. Kepler as an assistant to calculate planetary orbits.
  • Developed a hybrid system where Moon and Sun went around the Earth but the planets revolved around the Sun.
  • First to correct for atmospheric refraction.
  • Showed the Heavens were not immutable.
  • New star 1572, comet 1577
kepler s first law
Kepler’s First Law
  • All planets move about the sun in an elliptical orbit with the sun at one foci

2.5 The Laws of Planetary Motion

Planetary orbits are ellipses, Sun at one focus

more on ellipses
More on Ellipses
  • All planets move in elliptical orbits.
  • The Eccentricity of and ellipse = (distance between foci)/(major axis of the ellipse)
  • The eccentricity for the orbits of all the planets is small (i.e. less than 0.1) except for Mercury and Pluto.
kepler s second law
Kepler’s Second Law
  • The straight line joining the Sun and a planet sweeps out equal areas in equal intervals of time.
kepler s third law
Kepler’s Third Law
  • The squares of the planets' orbital periods are proportional to the cubes of the semimajor axes of their orbits.
kepler s third law43
Kepler’s Third Law

T2 = (4/GM)a3

Rearrange this a little and we get…

T2 / a3 = 4/GM

But 4 is a constant,

as is G (Universal gravitational constant),

and M (Mass of Sun).

  • Contemporary of Brahe and Kepler
  • Wanted to be a Camaldolese monk but his father insisted he train to be a physician at the University of Pisa.
  • Discovered he had a talent and interest in Mathematics. (G.E. Course?)
  • La Balancitta - 1586
  • Lectured on dimensions of HELL - 1588
  • Appointed to chair of mathematics at Pisa
  • Wrote (but did not publish) De Motu
  • Moved to University of Padua - 1592
  • Argues against Aristotle! (Kepler’s star - 1604)
  • Wrote in a personal letter to Kepler he believed in the Copernican cosmology.
  • By 1604 he had discovered correct descriptions of the motion of falling bodies and projectiles.
  • 1609 - received reports of a spyglass constructed by a Dutchman. Immediately built his own.

In about two months, December and January, he made more discoveries that changed the world than anyone has ever made before or since. - Swerdlow

  • The Starry Messenger (1610)
    • Mountains on the Moon.
    • Milky Way made of millions of tiny stars.
    • Four objects orbiting Jupiter.
galileo s observations of jupiter
Galileo’s Observations of Jupiter
  • Jupiter as the large circle.
  • Orbiting objects (moons) as * nearby.
  • Observed Saturn (as planet with two lobes)
  • Observed phases of Venus just like the phases of the moon.
  • Observed and described sunspots.
  • Argued the Bible must be interpreted in the light of what science had shown to be true.Oops!
  • 1616
  • I hold that the Sun is located at the centre of the revolutions of the heavenly orbs and does not change place, and that the Earth rotates on itself and moves around it. Moreover ... I confirm this view not only by refuting Ptolemy 's and Aristotle 's arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.
  • Philosophy is written in this grand book, the universe, which stands continually open to our gaze. But the book cannot be understood unless one first learns to comprehend the language and read the characters in which it is written. It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures without which it is humanly impossible to understand a single word of it; without these one is wandering in a dark labyrinth.
  • 1632 -Published Dialogue Concerning the Two Chief Systems of the World - Ptolemaic and Copernican
  • Found himself in front of the Inquisition AGAIN!
  • This time he is convicted of heresy!
  • 1992 - Pope John Paul II admits “errors were made”…case closed.
isaac newton
Isaac Newton
  • Mathematician and Physicist.
  • One of the foremost scientific intellects of all time!
  • His school described him as 'idle' and 'inattentive’.
  • NOTE: Idleness and inattentiveness are not generally considered hallmarks of a great intellect!!!
isaac newton62
Isaac Newton
  • 1661 - entered Trinity College as a sizar.
  • Worked toward a Law degree but studied a variety of other material.
    • Galileo’s version of Copernican astronomy.
    • Kepler’s Optics
  • Interest in mathematics began with an astrology book?
isaac newton63
Isaac Newton
  • Received Bachelor’s degree in 1665
  • Plague closed the University for two years.
  • Advances in mathematics, optics, astronomy, and physics.
isaac newton64
Isaac Newton
  • By 1669 Newton was elected to the Lucasian Chair at Trinity.
    • Expanded his work on optics.
    • Developed a reflecting telescope.
  • 1672 - elected a member of the Royal Society.
  • Among other books he published…
    • Opticks
    • Philosophiae naturalis principia mathematica
isaac newton key optics results
Isaac Newton - Key optics results
  • Light is made of corpuscles. (presaging quantum mechanics)
  • White light is composed of a spectrum of colors. (Spectroscopy)
  • Interference to measure (very accurately) the thickness of thin films.
  • Chromatic aberation in lenses.
isaac newton mechanics and gravitation results
Isaac Newton - Mechanics and Gravitation results
  • Gravitation is the force governing the motion of the moon and planets.
  • Calculated masses of various planets.
  • Described the motion of comets and their possible returns.
  • Tidal ebb and flow.
  • Precession of the equinoxes from the gravitational pulls of the Sun and Moon.


newton s 1st law the law of inertia
Newton’s 1st LawThe Law of Inertia
  • A body at rest (v = 0) or in motion with a particular velocity (v), will stay at rest or in motion with that same velocity UNLESS acted upon by a net external force (F0)
  • F = 0  v = 0
newton s 2nd law
Newton’s 2nd Law
  • The rate of change, with time, of the product of the mass of an object and its velocity is equal to the net external force exerted on the object.
        • F = (mv)/  t
  • More commonly : F = m a
newton s 3rd law
Newton’s 3rd Law
  • For every action there is and equal and opposite reaction.
  • FAon B = - F B on A
newton s universal law of gravitation
Newton’s Universal Law of Gravitation

F = G M1 M2 / r 2

G is the universal gravitational constant (= 6.67 x 10 -11 )

M1,2 are the masses of the two bodies.

r is the distance between the centers of the two bodies.



For two massive objects, gravitational force is proportional to the product of their masses divided by the square of the distance between them

modified version of kepler s law of periods 3rd law
Modified version of Kepler’s Law of Periods (3rd Law)
  • Bodies actually orbit around a common center of mass.
  • (M1+M2)T2 = (d1+d2)3
  • I know not what I appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell, whilest the great ocean of truth lay all undiscovered before me.
  • -Quoted in D Brewster, Memoirs of Newton
  • Read Chapters 1 and 2