A sphere, one of its small circles, and a right triangle to determine the radius of the small circle.
<a href="//commons.wikimedia.org/wiki/User:Vinicius.314" title="User:Vinicius.314">Vinícius Machado Vogt</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>