The distance from surface to surface through the center of a planet would be that planet's "diameter".
Aphelion
The diameter of a circle is a straight line passing from one side to the other and through the center. It is the distance across the circle through its widest point.
The distance from the center of a circle to its perimeter
This is called perihelion.
Earth. It looks blue from a distance
apothem
The distance of perigee is the point in an object's orbit that is closest to the center of the body it is orbiting around, such as a planet or a star. The distance of perigee varies depending on the specific orbit and the object in question.
it mean the long 0 degrees through the center... it mean the long 0 degrees through the center...
The distance from the fixed point at the center of a circle to any point on the curve is called the radius.
The word 'planet' is Greek for 'wander'. Ancient people watched 5 "stars" move through the sky - kind of like the moon did - only slower and smaller.
I'm not familiar with the word urans. If you mean the planet Uranus, it's diameter is 31518 miles and the distance on the surface from one side to the other is 49508 miles
The Welsh word for planet is 'planed' (plural - planedau)