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