You're right to be confused! This is a bit of a tricky concept. While it's true that a planet's speed may not change significantly over the course of an orbit, its velocity definitely does, and this is what causes the acceleration. Here's why:

* Speed vs. Velocity:

* Speed is how fast something is moving. It's a scalar quantity, meaning it only has magnitude (like 30 km/h).

* Velocity is how fast something is moving *and* in what direction. It's a vector quantity, meaning it has both magnitude and direction (like 30 km/h to the east).

* Circular Motion: A planet orbiting the sun follows a roughly circular path. In circular motion, the planet is constantly changing direction. Even if the speed remains constant, the change in direction means the velocity is changing.

* Acceleration: Acceleration is the rate of change of velocity. Since the planet's velocity is changing due to the changing direction, it is constantly accelerating.

* Centripetal Acceleration: The acceleration experienced by an object moving in a circle is called centripetal acceleration. It's always directed towards the center of the circle (in this case, the sun). This acceleration is what keeps the planet in its orbit, preventing it from flying off in a straight line.

To summarize:

* A planet's speed might be relatively constant, but its velocity is constantly changing due to the changing direction of motion.

* This change in velocity means the planet is constantly accelerating towards the center of its orbit (the sun).

* This acceleration is called centripetal acceleration, and it's what keeps the planet in its orbit.