What is the radius of this circumsphere? Very simple!

The "sphere" is 1-dimensional. The center of the "sphere" is $\frac{1+999}{2} \Longrightarrow 500$ . The point $500$ is $499$ from both $1$ and $999$ .

The definition of a sphere in(\mathbb{R}^n is the set of points at a radius from a center, both specified in that dimensionality. Usually the distance is in terms of Euclidean distance, which in the 1-dimensional case is absolute value. This question is what number is equidistant from both 1 and 999.