The smaller the planets are, they are, other things being equal, of so much the greater density; for so the powers of gravity on their several surfaces come nearer to equality. They are likewise, other things being equal, of the greater density, as they are nearer to the sun.

In this quote, Isaac Newton is describing a relationship between the size, density, and gravity of planets. He argues that, other things being equal, smaller planets tend to have a greater density compared to larger ones. This is because, on smaller planets, the force of gravity is stronger, which causes their materials to be more tightly packed together. Newton suggests that this higher density allows the gravitational forces on their surfaces to be more equal across different planetary bodies.

Newton also makes the point that planets which are nearer to the sun tend to have greater density, assuming all other factors remain constant. This is due to the stronger gravitational forces exerted by the sun, which affect the composition and structure of the planet. Essentially, as planets are closer to the sun, their density increases because the gravitational pull from the sun impacts their development, possibly leading to a more compact and denser form.

The origin of this quote lies in Newton's laws of motion and his work on universal gravitation, which laid the foundation for much of modern physics and astronomy. These observations are part of Newton's broader theory about how gravity influences the behavior of celestial bodies. His pioneering work, notably in Principia Mathematica, established fundamental principles that still guide scientific understanding of planetary motion and structure.

In essence, Newton is explaining that the size, density, and gravitational forces of planets are interconnected, and these relationships help explain why smaller planets have a greater density and why proximity to the sun can influence a planet's physical characteristics. His observations provide a key insight into how gravity shapes the structure of planets and other celestial objects.