Equivalence of Definitions of Small Circle
Theorem
The following definitions of the concept of Small Circle are equivalent:
Definition $1$
A small circle on a sphere $S$ is defined as the intersection of $S$ with a plane which does not pass through the center of $S$.
Definition $2$
A small circle on a sphere $S$ is defined as a circle on the surface of $S$ whose center is not at the center of $S$.
Proof
Let $S$ be a sphere.
Definition $(1)$ implies Definition $(2)$
Let $C$ be a small circle on $S$ by definition $1$.
Recall definition $1$ of Small Circle:
A small circle on a sphere $S$ is defined as the intersection of $S$ with a plane which does not pass through the center of $S$.
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Thus $C$ is a small circle on $S$ by definition $2$.
$\Box$
Definition $(2)$ implies Definition $(1)$
Let $C$ be a small circle on $S$ by definition $2$.
Recall definition $2$ of Small Circle:
A small circle on a sphere $S$ is defined as a circle on the surface of $S$ whose center is not at the center of $S$.
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Thus $C$ is a small circle on $S$ by definition $1$.
$\blacksquare$