Kepler's First Law states that ...

• A. The orbit is a circle with the Sun at the center.
• B. The orbit is an ellipse with the Sun at one focus.
• C. The orbit is a parabola with the Sun at focus.
• D. Planets move in straight lines around the Sun.

Answer: (B)

Orbits under gravity are conic sections, and the path a planet follows around the Sun is an ellipse with the Sun located at one of the ellipse’s focal points. This arrangement lets the distance between the planet and the Sun vary as the planet travels, so its speed changes in just the right way to keep it bound to the Sun. The Sun being at a focus explains why the orbit isn’t simply a circle in general, even though a circle is a special case of an ellipse. A parabola describes an unbound, escape trajectory, not a closed orbit, and moving in a straight line would ignore the gravitational pull that shapes the orbit.