Which of the following statements best describes Kepler's First Law of planetary motion?

• A. The orbit of a planet around the $\mathrm{Sun}$ is an ellipse with the $\mathrm{Sun}$ at one of the foci.
• B. The speed of a planet in its orbit is constant at all points.
• C. The force between a planet and the $\mathrm{Sun}$ is inversely proportional to the square of the distance between them: $F\propto \frac{1}{r^2}$.
• D. The period of a planet's orbit $T$ is equal to the square of its average distance $r$ from the $\mathrm{Sun}$: $T=r^2$.

Answer: (A)

Verified step by step guidance

1. Understand that Kepler's First Law describes the shape of a planet's orbit around the Sun.
2. Recall that Kepler's First Law states: The orbit of a planet is an ellipse, not a perfect circle.
3. Recognize that in this elliptical orbit, the Sun is located at one of the two foci of the ellipse.
4. Note that this law contrasts with the idea of circular orbits or constant orbital speed.
5. Therefore, the correct description is that the planet's orbit is an ellipse with the Sun at one focus.