question about planets?

I assume you mean 'Is the radius vector perpendicular to the line from planet to earth?'

In general, no for 2 reasons:

1) The planet's direction of motion is not necessarily perpendicular to its radius vector. In an elliptical orbit, the only times when this is true is at perihelion and aphelion.

2) The earth's motion closer to or away from the sun needs to be taken into account. For example, it the earth is moving towards from the sun, this will tend to delay the time of greatest elongation.

If all orbits were circular, your statement would be true.

The radius vector does not just depend on the lateral elongation but also is a factor of the density sorrounding the planet that is if the planet has any other planets in the vicinity.