Chapter III — Orbits and Gravity

In the last 400 years since the mathematical, physical, modular, and theoretical foundations of Modern Astronomy were laid by giants such as Nicolaus Copernicus, Galileo, Kepler, and Newton we have made extreme progress in our understanding of the cosmos.

However, our increased perceptiveness and analysis of what is actually going on in the heavens has led to new problems. Once we understood the shape of the solar system with the Heliocentric Model (how the Earth is a planet orbiting the Sun, which itself is moving through space), could see deeper into space with telescopes, and began to understand the mathematics and forces of gravity, more subtle problems emerged.

Mercury moved in such a way that varied slightly to what Gravity predicted. Outer and Inner planets, had perturbations in their orbit that also implied other forces — perhaps the existence of large gravitational bodies in our solar system that remain undetected.

How do you go about discovering planets on the edge of our solar system, well beyond what the eye can see without assistance? Which even modern telescopes have trouble discerning.

This is exactly the problem that astronomers of the 1800s were experiencing, as they explored our solar system, attempting to catalogue every significant cosmic object (whether planets, comets, or moons). [1]

If we could observe our solar systems from beyond it — looking inwards — this would be a significantly easier problem to solve. We could watch the Sun, and see the planets orbiting clearly, their motions plainly visible against a fixed perspective on our local star.

However, this is not the case.

Instead, we are trying to catalogue motions from a the surface of a planet that is itself in motion. Resulting in all movement appearing more complex, difficult to differentiate, and even more challenging to comprehend accurately, than it actually is. (As all from Hipparchus, to Ptolemy, and even Copernicus could attest to.)

As we saw in Chapter II — The Birth of Astronomy, astronomers from the Ancient World through to the Renaissance had to make their deduction while (unbeknownst to them for thousands of years) accounting for the motions of the Earth.

To solve these complex problems once and for all, better observations and better models of the planetary system around the Sun — the Solar System — were required.

In this chapter, we lay down the mathematical, geometrical shifts, along with the revolutions in our understanding of physics, that made all of this possible.

Chapter Contents

1. The Laws of Planetary Motion
2. Newton’s Great Synthesis
3. Newton’s Universal Law of Gravitation
4. Orbits in the Solar System
5. Motions of Satellites and Spacecraft
6. Gravity With More Than Two Bodies

• Astronomy Textbook
• Atlas
• Atlas Textbooks