What early astronomer discovered why planets stay in their orbits?

Question

A. Newton
B. Galilei
C. Copernicus
D. Kepler

Answer 1

Kepler (December 27, 1571 – November 15, 1630)

Johannes Kepler's primary contributions to astronomy/astrophysics were his three laws of planetary motion. Kepler, a nearly blind though brilliant German mathematician, derived these laws, in part, by studying the observations of the keen-sighted Danish astronomer Tycho Brahe.

Brahe invited Kepler to work with him in Prague and when Brahe died in 1600 Kepler succeeded him as Imperial Mathematician to the Hapsburg Emperors.

Sir Isaac Newton's later discovery of the laws of motion and universal gravitation depended strongly on Kepler's work. Although from the modern point of view, Kepler's laws can be seen as a consequence of Newton's laws, historically, it was the other way around: Kepler provided a kinematic mathematical model of the empirical observations, which Newton then interpreted using calculus and his new physics.

Kepler's first law
The orbit of a planet about a star is an ellipse with the star at one focus.

Kepler's second law
A line joining a planet and its star sweeps out equal areas during equal intervals of time.

Since the distance from a planet to its orbiting star varies, one can conclude that in order for the area being swept to remain constant, a planet must vary in speed. The physical meaning of the law is that the planet moves faster when it is closer to the sun.

Kepler's third law
The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits.

If T = orbital period of planet
and a = semimajor axis of orbit
The expression T^2a^–3 has the same value for all planets in the solar system as it has for Earth

Kepler did not understand why his laws were correct; it was Isaac Newton who discovered the answer to this more than fifty years later. The second law can also be seen as a statement of conservation of angular momentum, which is a logical consequence of Newton's laws in the special case of a force that acts along the line connecting two objects.

Newton's Law of Gravity said that "every object in the universe attracts every other object along a line of the centres of the objects, proportional to each object's mass, and inversely proportional to the square of the distance between the objects."

So it was Kepler who developed the laws of planetary motion and Newton who supplied the theory as to why these laws were the way they were.

Answer 2

Newton made a theory about why the planets stays in orbit(Gravity forces). That means that he made it possible to calculate the orbits and many other things, but since Einstein showed that Newtons theory wasn't correct, it is wrong to say that he discovered why the planets stays in orbit.

Answer 3

Newton - objecs in motion stay in motion unless acted on by an opposing force. The planets' orbits are a balance between the suns gravity and the plants' forward motion (momentum). All abiding by Newton's laws.

Answer 4

newton

Answer 5

newton

Answer 6

A