# How did Kepler derive his laws WITHOUT law of gravitation?

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1. Jun 13, 2015

### Hansa

How did Kepler derive his laws of Planetary Motion without knowing about Newton's law of gravitation? Specifically, the first law of planetary motion which says that planets follow elliptical paths - how did he figure that out without the knowledge of the gravitational pull of the sun? Was it purely based on observations?

2. Jun 13, 2015

### Staff: Mentor

I believe it was.

3. Jun 13, 2015

### Staff: Mentor

As far as I understand, Kepler gained access to all of Tycho Brahe's astronomical observational data when Tycho Brahe died. He used this data to model the solar system as heliocentric, with the planets orbiting the Sun in ellipses. He never developed an explanation for why they did this.

4. Jun 13, 2015

### Hansa

Thanks, that answers the question. :)

5. Jun 13, 2015

### Staff: Mentor

6. Jun 13, 2015

### theodoros.mihos

To use an movement low like $x(t) = x_0 + v_0t + \frac{1}{2}at^2$ I needn't any law of force or any other reason of motion. The motion is that I observe.

7. Jun 13, 2015

### andresB

Is there another way?

8. Jun 14, 2015

### jbriggs444

The alternative is to start with a force law and see what motion it predicts.

Kepler did not use a force law. He started with observations and determined a pattern of motion -- a traversal of an elliptical path at a rate determined by "equal areas in equal times". Newton was able to show after the fact that an inverse square central force law was consistent with this.

9. Jun 14, 2015

### tech99

According to a recent book, On the Shoulders of Giants, edited by Stephen Hawking, Kepler was hired by Tycho Brahe, who was an expert naked eye astronomer, to make sense of his observations of Mars. That was when Kepler found the planet was describing an ellipse.

10. Jun 18, 2015

### khaerulumam42

I think Kepler not just get data from Thycho, but Kepler continue Thycho's work

11. Jun 22, 2015

### CalcNerd

Kepler continued to work in Tycho's observatory, but he had poor vision (relatively poor, near sighted, could read w/o difficulty) and could not really use any of the instruments. And Galileo's telescope was starting to be distributed so that Tycho's tools were obsolete within a few years of his death as well. Everyone WANTED to believe orbits were perfectly circular or were composed of epicircles (circles within circles). This idea could replicate the orbit of Mars to a very close approximation. Kepler also did NOT want to accept Tycho's observations, but knew Tycho was too good of an observer to not use his data.

12. Jun 22, 2015

### PWiz

The funny thing is that Newton's law of gravitation is also based on observations.