Today is

EARLY COSMOLOGY TUTORIAL TOPICS Dr. Brian Monson has created this group of pages as a supplement for the unit on early astronomy that is taught in most college astronomy courses and has graciously granted ScienceMaster permission to republish them. It is by no means a complete discussion of this topic. To explore more work by Dr. Monson please visit his Planetary Conjunctions page.

CONTENTS Motions of the Celestial Bodies Geocentric Models First Iteration Second Iteration Heliocentric Models First Iteration Second Iteration Third Iteration Fourth Iteration

The distances to the planets can be calculated from Copernicus' model by using some simple trigonometry. The diagram above shows Venus at its maximum elongation from the sun. At this time it is 46° from the sun in the sky. Our line of sight is tangential to Venus' orbit at this time so the earth-sun, earth-Venus, and Venus-sun lines form a right triangle as shown. The sine of an angle in a right triangle is defined to be the ratio of the opposite side to the longest side or in our diagram a/c. So we can solve for a (Venus' radius) in terms of c (our radius) like this:

This means that Venus orbits the sun at 72% of the earth-sun distance. This is very close to the modern value for the semi-major axis of Venus' orbit. It's not exactly accurate because the orbits are not circles as Copernicus assumed but instead are ellipses as determined by Kepler. A similar calculation can be done to find the distances to the outer planets.

Cool   NASA   Websites

	Today is

EARLY COSMOLOGY TUTORIAL TOPICS Dr. Brian Monson has created this group of pages as a supplement for the unit on early astronomy that is taught in most college astronomy courses and has graciously granted ScienceMaster permission to republish them. It is by no means a complete discussion of this topic. To explore more work by Dr. Monson please visit his Planetary Conjunctions page.

CONTENTS Motions of the Celestial Bodies Geocentric Models First Iteration Second Iteration Heliocentric Models First Iteration Second Iteration Third Iteration Fourth Iteration

Aristarchus (310 - 230 BC)

Little is known about this man except that he suggested a simpler heliocentric model of the universe to explain retrograde motion. He suggested that retrograde motion was due to the differences in orbital speed of the planets around the sun. As the earth passes a slower moving outer planet, our line of sight shifts and makes the planet appear as if it is moving backwards relative to the stars. The same thing happens when you pass a slower moving car on the highway. The image below shows this happening for the earth and Mars.

Notice how the lines of sight shift between points 7 and 8. During this time interval, Mars would be in retrograde motion. A similar effect would occur as the earth was passed by Mercury or Venus. The geocentric model required that the center of Mercury's and Venus' epicycles always lie on the earth-sun line so that they would retrograde near the sun. The heliocentric model treats inner and outer planets in exactly the same way.

This model never caught on due to the objections discussed previously.

Cool   NASA   Websites