Question

How is it that a planet doesn't move at a constant speed but is able to cover an equal distance over equal times?

Answer 1

Different radii

Explanation:

Be careful! The planet doesn't cover the same distance, it sweeps the same amount of area. If you're confused, look up Kepler's Second Law.

Planets move in an elliptical orbit, with another object (usually a star) at one of the ellipse's foci. This means that, as the planet moves in its orbit, the distance from the star is also changing. As the planet moves far away from the star, the radius is large. However, the attraction between the two objects is smaller, so the speed of the planet is relatively lower. The planet moves slower, but its "sweeping radius" is larger. When the planet moves closer to the star, the radius decreases. This means that the attraction between the two objects is larger. The planet moves faster, but its "sweeping radius" is smaller. In these two cases, the planet's speed and radius happen to "cancel" each other out, so after an equal amount of time, the planet has swept an equal area.

Like I mentioned above, if you are confused, look up Kepler's Second Law.

Answer 2

Planets don't move at constant speed and therefore don't cover equal distances. They do however cover equal areas. See below for more:

Explanation:

With this question, we move into an area of exploration that Johannes Kepler explored and derived three laws of motion.

To affirm the first part of your question - planets do not move at constant speed. Planets move along elliptical orbits (as do moons, etc) and so as a planet "falls towards" the Sun, orbital speed increases. As a planet "speeds away" from the Sun, orbital speed decreases.

To address the second part of your question, planets don't cover equal distances over equal times - with the speed of the planet changing, the distances covered are unequal.

What stays the same is the area described by a line drawn from the Sun to the planet's position at time 1, a line from the Sun to the planet at time 2, and the arc connecting those two positions. Those areas are equal. This is known as the Law of Equal Areas (Kepler's 2nd Law) and is one of the fundamental laws that Newton used to derive his laws of motion.

In the link below, there is more information on Kepler and all of his laws:

http://www.physicsclassroom.com/class/circles/Lesson-4/Kepler-s-Three-Laws