The imaginary line between a planet and the sun is called the line of apsides.

Here's a breakdown:

* Apsis: Refers to a point in a planet's orbit where it's either closest to the sun (perihelion) or farthest from the sun (aphelion).

* Line of apsides: The imaginary line connecting the perihelion and aphelion points of a planet's orbit. This line defines the major axis of the elliptical orbit.

So, while it's not always a straight line (due to the elliptical nature of orbits), it's the line that connects the two points of the planet's orbit that are most significant in terms of its distance from the sun.