How do I determine which focus of an elliptical orbit has the star?

[llandau]

Given an elliptical orbit, if I know all parameters of the ellipse, is it possible to determine in which of the two foci is the star? To be more precise, if I put into orbit a missile from earth, it will reach first the perihelion and then aphelion or vice versa? How can I know?

 

[olivermsun]

If you know the velocity of the projectile somewhere along the orbit, then you shoudl be able to tell.

 

[llandau]

You are right, it is straightforward, I should have given it more thought. Thanks, anyway.