How do planets move?

We often say that the planets circle the Sun. However, Johannes Kepler discovered that their orbits are not circular but slightly elliptical.

When Nicolaus Copernicus introduced his heliocentric world view, with the planets orbiting a central Sun, he believed those orbits would be perfect circles. Johannes Kepler was the first to realise that planetary orbits are in fact elliptical. His three laws of planetary motion describe the relationship between a planet’s changing distance to the Sun, its changing orbital velocity, and its period of revolution around the Sun. Later, Isaac Newton’s universal law of gravity provided the mathematical basis for Kepler’s laws, which hold throughout the Universe.

Long ago, people thought that the Sun moved around the Earth. Now we know that the planets, including the Earth, move around the Sun. Do you know how long it takes the Earth to complete one orbit?