Astronomy Practice Exam

What significant discrepancy led Kepler to develop a model of planetary orbits using ellipses?

1. 8 arcseconds

2. 8 degrees

3. 8 arcminutes

4. 16 arcminutes

The correct answer is: 8 arcminutes

Kepler developed his model of planetary orbits based on the significant observational discrepancies he noted while analyzing the data collected by Tycho Brahe. One key observation was that the observed positions of Mars did not match the predictions made using circular orbits, a previously accepted model for planetary motion. This mismatch was approximately 8 arcminutes, indicating that the circular model was insufficient to accurately describe the motion of Mars around the Sun. This realization prompted Kepler to explore a different geometric approach, leading him to conclude that planetary orbits are not circular but elliptical. The elliptical model accounts for variations in speed and distance as planets move in their orbits, providing a more accurate representation of their positions over time. Understanding this transition to elliptical orbits was crucial for the advancement of celestial mechanics and greatly improved the predictive power regarding planetary positions. The focus on the specific discrepancy of 8 arcminutes highlights how observational data serves as the foundation for theoretical development in astronomy.