why the trajectory of a panet around the sun is an ellipse?

[gearss]

Hi;
If someone can explain the principle with detailed
mathematical calculation procedure?

[jamrb]

I cannot include URLs yet (too newbie so far!), so search "Elliptic orbit" and "Kepler's laws of planetary motion" in Wikipedia.
I also have a very mathematical theoretical document about, but I am afraid it is in spanish language.