As you probably know, the planets have pretty predictable orbits around the sun but what exactly causes the complex movement of celestial bodies in the sky from Earth's surface? Mars is an excellent example of this.
The apparent motion of planets is complex but predictable.
The apparent motion is due to the combination of three different motions:
- The rotation of the Earth,
- the motion of the Earth around the Sun,
- and the motion of the planet around the Sun.
The Planets move in ellipses, slightly perturbed by other planets.
These motions are predictable, and their combination is straightforward trigonometry. The resultant motion is complex, the planets appear to loop relative to the stars, but entirely predictable. We know exactly where the planets will be in the sky for any time for many thousands of years in the future or the past.