Question

[deanhystad]

When you call aphelion() here, you pass velocity_aphelion as the third argument. velocity_aphelion is a function.

print(aphelion(perihelion, velocity_perihelion, velocity_aphelion))

Did you mean to pass velocity_perihelion instead? Or did you intend to pass the value calculated here:

print(velocity_aphelion(perihelion, velocity_perihelion))

If the latter, you'll need to assign the return value of velocity_aphelion() to a variable and pass that variable as the third argument to aphelion().