Foundations of Astronomy

The scientific method-winning or loosing?


statue of Geodorno Bruno

Geodorno Bruno--burned at the stake for maintaining his belief system (that the sun was the center of the solar system).


Reasons for early interest:

I) Stars as navigational aids. Mediterranean traders could sail to correct latitude….using Polaris as a guide… then sail east or west latitude and longitude guides


Note: the altitude of polaris above the horizon equals the observers latitude. Here is polaris as seen from San Fransisco, latitude 37 degrees.

Also note that two stars (Dubhe and Merak) shown here in the bowl of the big dipper point the direction of Polaris. Try it for yourself in the night sky!


II) Astrological and religious beliefs

image from:

III) Need for a Calendar, especially related to agriculture. The Nile Flood was an especially important, yearly event.

Early Western Cosmology

(models of the relation between the Earth and Celestial bodies)


I) Plato: Heavens were perfect and that the most perfect geometric object was the circle. Artists Rendering of Plato--stretched a bit and note quite true to form!

His beliefs were based on Socrates study of "forms".image of "Plato" a Plate, from:

II) Aristotle: Plato's student…demonstrated that the Earth was round:


a) Earth's shadow on the moon was always circular

b) the altitudes of stars change with latitude

Favored "Geo-Centric" model of the Universe. Why?


image of the Earth taken from apollo 13


Main difficulty with Geocentric Model: Retrograde motion.


Ptolemy's drawing of the orbit of a planet on a deferent revolving around an epycicle centered on the Earth.

Geocentric Model requires:

deference(large orbit) and epicycles (small circle to create the retrograde loops).


Ptolomey (pronounced Tolemy) more "accurate" model

relief of Ptolemy next to a model of the Earth and heavens

Requires 80 different circles,

Aristarchus of Samos (300 B.C.):

"Heliocentric" arguments relative sizes of the Earth and the Moon, and incorrectly for the Earth and Sun.

Erostathenes (250 B.C.):

measured the circumference of the Earth "the shadow in the well" method.



Dark Ages-Aristotelian beliefs Mandatory:

Copernicus (1473 - 1534)tried to update the Ptolemaic model and realized the "Heliocentric" model was simpler since distant stars were so far that no parallax could be seen.

Galileo(1564 -1642)

Turned his telescope on the moon, the Sun, and the Planets

Provided the proof of Copernicus theories.

He observed that:

Venus had phases like the moon,

photo from:

Conclusion: Venus orbited the Sun

--Jupiter has moons that Orbit Jupiter( not the Sun)

photo from:

--The Sun has spots

photo from:

--the Moon has craters (so celestial objects weren't perfect.)

Moon and Mars from NASA

--Saturn has "Ears"

Photo at left from the HST
at right:

Galileoattempted to prove that the Earth moves in several ways...

tides are due to a spinning, orbiting Earth-partically incorrect there. Galileo thought the Earth's motion about the sun would create an "inertial bulge" on the size of the Earth opposite the sun, but the bulge is not on the anti-solar side, and there are two "tidal bulges" each what do you think is the real reason for the tides?

image of Tides on Earth from:

Galileo also developed the physics needed to understand orbital motion:

Galileo also found that the acceleration (a) and speed (v) of a planet or moon in its orbit and its orbital radius (R) is related by:

a = V^2/R

(optional for experts: This equation shows for example, that planets closer to the sun will move more rapidly than planets farther away, assuming that the acceleration is greater when they are closer--Newton would soon prove this to be the case!)

Galileo published many of these results in a book entitled:

Dialogue Concerning the Two Chief World Systems.

which resulted in house arrest...there he continued is work on motion of bodies...all of which was quite useful, especially for newton!

Tycho Brahe(1546-1601)

image from :

Tyco wished to solve "the problem" (geocentric vs. heliocentric) but felt that better observations were needed. Tyco was most like interested in proving that the "tyconic Universe" was correct

Johannes Kepler ((1571 - 1630)

Mathematician...sought out Brahae for his famous data, and was hired by Brahe to fit data to the tyconic model universe. But...

Kepler was trying to fit data to the Kepler model Universe--the orbits of the six known planets fit into the largest spheres which could be inscribed into the six regular geometric solids--crazy by today's standards, but at least the orbits were centered on the sun!


Also, Brahae died and family wouldn't release data!


Eventually Kepler "acquired" Brahe's data and found that:

--the orbit of Mars just isn't a circle! Plato was wrong! This opened the door to a "true" understanding of the solar system.

Kepler's final Results:

  1. All Planets orbit the sun in Elliptical orbits with the sun at one focus

2. A line joining a planet and the sun sweeps out equal areas in equal times. See

3. The square of a Planets orbital period is proportional to the cube of its semi-major axis. (P^2 = constant x a^3)..see:

These three relations are now known as Kepler's three laws.

After Kepler Astronomers had:

Issac Newton(1642 -1727)

image of Issac Newton from:

Newton realized that

This is slightly complicated since both objects move..check out this quicktime movie:

He also discovered:

law of Universal gravitation :

F = G m1m2/d2

He realized that force is an interaction between two massive objects.

Also depends on the inverse square of the distance.

this law, together with Calcus Derives Kepler's laws.

-shows that the Orbits must be ellipses(Kepler's 1st law),

- planets must speed up as they approach perihelion (closest approach) (Kepler's 2nd law).

100 years later, Adams and Leverier Used basic laws to discover Neptune (with pencil and paper) (plus a new form of Calculus) so....

What does cause gravity?

--Einstein- gravity is due to the curvature of space time.

"marble in punch bowl"

predicts bending of light by massive objects, black holes

theory well verified

other interpretation: exchange of gravitons.

Also verified.

Will these pictures be unified one day?

Is this our mythology and superstition?