When an object moves in a circle at a constant speed, its direction constantly changes. A change in direction causes a change in velocity. This is because velocity is a vector quantity – it has an associated direction as well as a magnitude. A change in velocity results in acceleration, so an object moving in a circle is accelerating even though its speed may be constant.
Gravitational attraction provides the centripetal force needed to keep a planet in orbit around the Sun, and a satellite in orbit around a planet. For example, gravitational attraction between the Earth and the Moon keeps the Moon in orbit around the Earth. An object moving in a circular orbit at a constant speed has a changing velocity. This is because velocity is a vector quantity that depends on speed and direction. The object in orbit is accelerating, even though its speed remains constant, because its velocity is changing.