According to Kepler's Second Law, what does a line segment joining a planet and the Sun do?

Kepler's Second Law, also known as the Law of Equal Areas, states that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. This means that as a planet travels along its elliptical orbit, it moves faster when it is closer to the Sun and slower when it is farther away. However, the area swept out by this line segment over equal time periods remains constant, illustrating the conservation of angular momentum.

This law highlights the relationship between a planet's speed in orbit and its distance from the Sun, showing that the planet covers more distance in its orbit when it is nearer to the Sun, compensating for the slower movement when further away. Kepler's laws were groundbreaking in that they described the motion of celestial bodies without the need for complex forces, revealing the elegant geometry of orbits and how they are influenced by gravity.

In contrast, the other statements do not accurately summarize Kepler's Second Law. The line segment does not remain stationary, change in length based on distance, or deviate from a straight path. Instead, it maintains a consistent relationship with the areas it sweeps out over time, which is central to understanding planetary motion in our solar system.