Why planets move back and forth in the sky

We don’t really perceive depth when we look up at the stars and planets. They all appear to be moving across a flat plane. In the below picture, we have an inner planet and an outer planet. We can pretend they are the Earth and Mars, if we want. The inner planet moves around its orbit at a faster rate than the outer planet does. So as the inner planet gets near the outer planet, it is going to produce an interesting apparent motion.

We can see that at time 1, the apparent location of Mars On the flat plane of the sky is on the far right. Time two is on the left. And then Time 3 is center. Time 4 is to the right. And then Time 5 is on the far left. Below I’ve plotted the same points on a real photo. We can see that the object appears to move left then right then left again. In reality, the object is only moving left, but it appears to change as we approach and depart from our closest points to it.