Apsidal precession

Imagine you're on a playground spinning around and around on a merry-go-round. As you spin, the position of your feet and your head changes. This happens because your body is tilting or "precessing" around the central axis of the merry-go-round.

Now, let's compare this to the orbit of a planet around a star. The planet's orbit is also tilted, and instead of a merry-go-round, the planet is held in place by the gravitational pull of the star. The point where the planet is closest to the star is called the perihelion, and the point where the planet is farthest from the star is called the aphelion.

But, over time, something interesting happens. Just like how your body tilts while spinning on a merry-go-round, the planet's orbit also tilts or "precesses", causing the perihelion to shift relative to the stars. This motion is called apsidal precession.

One analogy for apsidal precession is to think of it like a spinning top. As the top spins, it also wobbles or "precesses" around its axis. Similarly, the planet's orbit precesses around its own axis, resulting in a shift in the position of the perihelion over time.

Apsidal precession is caused by the gravitational influence of other planets or objects on the orbit of the planet. This effect is particularly pronounced in the orbit of Mercury, which has been observed to precess at a rate that cannot be fully explained by the gravity of other planets in our solar system. This led to the development of Einstein's theory of General Relativity, which predicted that the curvature of spacetime around the massive sun could also contribute to the precession of Mercury's orbit.

In summary, apsidal precession is the tilting or wobbling of a planet's orbit, causing the perihelion to shift over time. It is caused by the gravitational influence of other planets or objects, and it plays an important role in our understanding of the dynamics of our solar system.