How did Kepler's discoveries contribute to the field of astronomy?

• A. He established that planets move in $\mathrm{elliptical}$ orbits with the $\mathrm{Sun}$ at one focus, fundamentally changing our understanding of planetary motion.
• B. He showed that the brightness of stars is directly proportional to their distance from $\mathrm{Earth}$.
• C. He discovered that planetary orbits are $\mathrm{perfect} \mathrm{circles}$ and their speeds are $\mathrm{constant}$.
• D. He proved that the $\mathrm{Earth}$ is the center of the universe and all planets revolve around it.

Answer: (A)

Verified step by step guidance

1. Understand that Kepler's main contribution was formulating laws that describe planetary motion more accurately than before.
2. Recognize that before Kepler, the common belief was that planets moved in perfect circles at constant speeds, often centered around the Earth.
3. Kepler analyzed detailed astronomical data and discovered that planets actually move in elliptical orbits, not perfect circles.
4. He established that the Sun is located at one focus of these elliptical orbits, which was a fundamental shift from the Earth-centered models.
5. Kepler's laws also describe how the speed of a planet varies depending on its position in the orbit, which changed the understanding of planetary motion and laid the groundwork for Newton's theory of gravitation.