The distance between the Sun and Neptune is not constant because Neptune's orbit is elliptical.

Here's a breakdown:

* Average distance: 4.5 billion kilometers (2.8 billion miles)

* Closest point (perihelion): 4.45 billion kilometers (2.76 billion miles)

* Farthest point (aphelion): 4.55 billion kilometers (2.83 billion miles)

So, depending on where Neptune is in its orbit, the distance can vary slightly.