a really basic kepler question

[stonecoldgen]

I know that kepler looked at Tacho Bradhe's work and knew the equations related to ellipses

but the question is as simple as:

how did kepler knew the sun was at one of the foci of the ellipse?

[phyzguy]

Kepler worked for decades studying the observations of Tycho Brahe, trying to come up with a set of rules that would describe the observations. After trying a huge number of things, he was able to distill it down to his three laws. I imagine he arrived at an ellipse with the sun at one focus through simple trial and error.