To test Copernicus’s model, Tycho took meticulous measurements of the positions of the planets, and he hired Kepler to interpret the data. Kepler found that the geocentric model did not accurately predict the planets’ positions, but neither did Copernicus’s heliocentric model. He found that Copernicus’s insistence that the planets’ orbits were perfectly circular was the problem. If the orbits were instead ellipses, then the heliocentric model would have been able to very accurately predict the positions of the planets. This is known as Kepler’s first law of planetary motion.
A. Elliptical orbits cause a planet to sometimes be closer to and sometimes farther away from the Sun.
B. The Sun is located at one focus of a planet’s elliptical orbit, and the planet is located at the other focus.
C. Halley’s Comet has a highly eccentric orbit.
D. The Sun is located at the center of a planet’s elliptical orbit.
E. The eccentricity of Mars’s orbit is zero.

The astronomer Johannes Kepler refined the Copernicus' heliocentric theory with the introduction of elliptical orbits with the formulation of his three laws of planetary motion, finally explaining why planets orbited the Sun at different speeds at different times, because prior to this, the heliocentric model used only circular orbits.

In this sense, the fact that planetary planetary orbits are elliptical was told by Kepler in the first Law of Planetary motion that states:

The orbit of a planet around the Sun, is in the form of an ellipse with the Sun at one of the two foci of that ellipse.