# PSYC 2500 Lecture Notes - Lecture 3: Semi-Major And Semi-Minor Axes, Orbital Period, Geocentric Model

## Document Summary

Tycho used instruments to determine angular size and location of celestial objects. Observations of stars and planets accurate to 1" (1 arc minute) Believed sun orbited earth but planets orbited sun = hybrid theory. Believed planetary orbits related to perfect geometry of nested solids. 1st law: planetary orbits are elliptical with sun at 1 focus of the eclipse. 2nd law: an imaginary line connecting sun to any planet sweeps out equal areas in equals intervals of time, planets move faster when closer to sun slower when farther away. 3rd law relates the semimajor axis a to the orbital period p: p2 (in earth years) = a3 (in au) Predictions: any new object orbiting sun obeys 3 laws, orbit e close to 0, p= 1. In 3rd law semimajor axis a is measured in astronomical units (au) =avg earth sun distance. Determine size of orbits relative to earths orbits (used earths orbit as baseline)