Question

Why do a planets speed change in orbit

Answer 1

According to Kepler's  second law of orbital motion, a plane's orbital speed changes , depending on how far it is from the sun. The closer a planet is to the sun, the stronger the sun's gravitational pull on it, and the faster the planet moves.  The farther away from the sun, the weaker the sun's gravitational pull and the slower it moves in its orbit.

The orbit of a planet around the sun is not  a perfect circle, but  an ellipse - a flattened circle.