The fact that planets move around the Sun in elliptical orbits rather than perfect circles was first clearly demonstrated by which astronomer?

Introduction / Context:
The motion of planets in the Solar System has been studied for centuries. Early models often assumed circular orbits, but precise observations eventually showed that planetary paths are actually ellipses with the Sun at one focus. The formulation of this fact as a clear law of planetary motion was a major milestone in astronomy. This question asks which astronomer first demonstrated that planetary orbits are elliptical rather than circular.

Given Data / Assumptions:

• We are considering historical contributions to the understanding of planetary orbits.
• Names given include Galileo Galilei, Martin Luther, Johannes Kepler, and Nicolaus Copernicus.
• The focus is on who formulated the law about elliptical orbits around the Sun.
• We assume standard historical accounts of the scientific revolution.

Concept / Approach:
Johannes Kepler, using the extremely accurate observational data collected by Tycho Brahe, studied the orbit of Mars and other planets. He found that circular orbits with epicycles could not fully explain the data. After years of analysis, he concluded that the planets move in ellipses with the Sun at one focus. This became Kepler first law of planetary motion. Copernicus introduced the heliocentric model but still used circular orbits. Galileo supported heliocentrism and made many telescopic discoveries but did not derive the elliptical nature of orbits. Martin Luther was a religious reformer, not an astronomer. Thus, the correct answer is Johannes Kepler.

Step-by-Step Solution:
Step 1: Recall that Copernicus proposed that the Sun, not the Earth, is at the centre of the planetary system, but he retained circular orbits and epicycles. Step 2: Galileo observations of the phases of Venus and moons of Jupiter provided evidence for heliocentrism but did not involve detailed orbital shapes. Step 3: Kepler inherited Brahe precise measurements of planetary positions and used them to model Mars orbit. Step 4: After testing and rejecting circular paths, Kepler discovered that an ellipse with the Sun at one focus described the motion accurately. Step 5: This result became Kepler first law: planets move in elliptical orbits with the Sun at one focus, clearly demonstrating the non circular nature of planetary orbits.

Verification / Alternative check:
Modern celestial mechanics confirms that, in Newtonian gravity, the natural closed orbit for a body bound to another by an inverse square force is an ellipse. Kepler empirical law was later derived from Newton gravitational theory. Historical records attribute this discovery of elliptical orbits to Kepler, whose three laws are named in his honour. Copernicus and Galileo are celebrated for other contributions, but not specifically for the elliptical orbit law, and Martin Luther is not associated with astronomy at all.

Why Other Options Are Wrong:
Galileo Galilei: He made important observations supporting heliocentrism but did not formulate the law of elliptical orbits. Martin Luther: A key figure in the Protestant Reformation, he was a theologian, not an astronomer. Nicolaus Copernicus: Proposed the Sun centred model but still used circular orbits and epicycles rather than ellipses.

Common Pitfalls:
Students often conflate the roles of Copernicus, Galileo, and Kepler, remembering only that all supported heliocentrism. To avoid confusion, remember the sequence: Copernicus proposed the heliocentric model, Galileo provided observational support, and Kepler refined the model by introducing elliptical orbits and quantitative laws of planetary motion. This clear division of contributions makes it easier to answer historical questions correctly.

Final Answer:
The demonstration that planets move around the Sun in elliptical orbits was first given by Johannes Kepler.