**Precession of Perihelion of Mercury, and Einstein**
Newton's laws of motion and universal gravitation stood relatively unchallenged as the best theory of gravity for over two centuries. However, during the latter part of this period, a problem was noticed.

Due to interactions between the planets, their orbits can precess over time (rotation of orientation of the orbit). Newton's laws described this very well, for the most part. However, in 1859 Urbain Le Verrier discovered that the rate of precession for the planet Mercury disagreed with the predictions from Newton's theory. This anomalous precession lacked a good explanation for decades. Though many ideas were suggested, they all failed to hold up to scrutiny.

Part of the problem is that Newton's theory depended on the assumption that mass, distance, and time are constant regardless of where you observe them.

Albert Einstein published his theories of Special Relativity in 1905 and General Relativity in 1915, which proposed a deeper underlying reality in which time, space, and mass are much more fluid, and depend on the frame of reference in which you measure them. Newton's theory is still mostly true within a single frame of reference, and on small scales and at low speeds where the differences from Einstein's system are negligible. However objects in orbit have independent reference frames, and Mercury is in a particularly fast orbit, deep within the sun's gravity well.

This new theory described a system in which space and time (together referred to as spacetime) can be distorted by the presence of matter and energy. Time is flexible, and runs slower in places that are deeper within gravitational fields. With the curvature of spacetime mediating the gravitation between the sun and Mercury, whose orbit is very close, the remaining anomalous precession could now be easily explained.

Einstein was aware of this problem in astronomy, and in his paper he proposed three separate tests that could be performed to prove his theory, and included Mercury's precession as one of them. In fact, it was soon demonstrated that his theory's predictions matched very closely with observed measurements, which cemented General Relativity as something to be taken seriously, and it remains the currently accepted theory of gravity.

It should be noted that only at extreme speeds or scales will Einstein's relativistic effects be noticed. Even with Mercury's orbit, the effects were subtle, and required precise measurements to be detectable. Newton's and Kepler's mathematical models work so well within the scale of the solar system that they are still used to launch satellites and other spacecraft to destinations throughout, and predict their motion. For this reason Einstein's theories often are not seen as a replacement, but rather a more complete realization of the same physical laws.

And this brings us full circle, to why I wanted to use a Mercury-like planet in a tight, fast orbit for my circumnavigation!

This of course was a very superficial historical look at the development of an understanding of gravity and planetary orbits. If you're interested in more detail, there's certainly a wealth of information online about these subjects. We've only just scratched the surface, of course.