Hopetoun Falls, Beech Forest, near Otway National Park, Victoria, Australia.
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The Godfrey Kneller's 1689 portrait: the Newton of the Principia, the man in his prime.
The hair's phony I think.
When an undergraduate he had to retire to his home in Lincolnshire to wait out the plague of 1665--1666 and that is where some of his earlier great discoveries in gravitation were made.
See the Bill of Morality 1665.
See Newton's own 1st edition Principia copy.
He wrote it in Latin under the impression that that would make it more accessible.
Latin can be translated.
But Newton also unfortunately did his proofs in a form of geometrical calculus that he invented and that never caught on.
Consequently, the Principia is really hard going for any modern person to read.
Lots of shortcuts and improvements in formalism make many of the results Newton labored to prove then much easier to prove now.
To illustrate how orbits arise Newton can up with Newton's Mountain as we now call it. Apparently, this doesn't appear in the Principia, in a later popularization The System of the World
He wasn't the first person to think of a reflecting telescope, but he was the first it seems to build a working model.
Scroll down on reflecting telescope to see the Newtonian reflector schematic diagram.
To illustrate how orbits arise Newton can up with Newton's Mountain as we now call it. Apparently, this doesn't appear in the Principia, in a later popularization The System of the World
The ideal case is two isolated bodies.
They orbit about their mutual center of mass. See orbit animation.
If one bodies is much more massive than the other, then it is effectively the center of mass: this is approximately true for the Sun-Earth system and more approximately true for the Earth-Moon system.
See the Orbit of the Moon and the Earth-Moon orbit animation
Inertial frames are fundamentally unaccelerated frames and somehow are determined by the nature of space or in modern physics spacetime.
Newtonian physics is still valid in non-inertial frames, but it's not referened to them.
In non-inertial frames, one can have accelerations without a force---without real forces---there are these fictitious or inertial forces which are just the effects of being in a non-inertial frame.
For example, the inertial forces, centrifugal force and the Coriolis force both of which appear in rotating frames.
The centrifugal force is just the ``force'' that tries to throw you out of the rotating frame and Coriolis force is an effect of moving in the rotating frame.
See the rotating parabolic disk animation which unfortunately is complicated by the parabolic shape of the disk for the bouncing particle---so four effects are going on at the same time centrifugal force, Coriolis force, gravitational force accelerating the particle, and collisions.
On the left, one sees the system from a rest frame.
On the right, one sees the system from the rotating frame.
The particle actually collides with the same point on the surrounding wall all the time actually.
