““Newton's law of gravity says that the acceleration of any object as it orbits another is proportional to the mass of the body it is orbiting…Thus if you know the speed of a body in orbit around a star and its distance from the star, you can measure the mass of that star. The same holds for stars in orbit around the center of their galaxy; by measuring the orbital speeds of the stars, you can measure the distribution of mass in that galaxy.””
![“Newton supposed that the case of the planet was similar to that of [a ball spun around on the end of an elastic string]; that it was always pulled in the direction of the sun, and that this attraction or pulling of the sun produced the revolution of the planet, in the same way that the traction or…” quote by William Kingdon Clifford](https://sms4smile.com/quotes/img-quotes/860/william-kingdon-clifford-17148_cause_newton-supposed-that-the-case-of-the-pla_381195.jpg)
