Question

How is the semi-major axis found if perihelion and aphelion distance are known?

Answer 1

The semi major axis distance is half the sum of the perihelion and aphelion distances.
Explanation :
Given that the orbit is an ellipse with semi major axis a and eccentricity e, then the perihelion distance is $a(1-e)$ and the aphelion distance is $a(1+e)$. Adding the two together gives $2a$.
Therefor the semi major axis distance is half the sum of the perihelion and aphelion distances.