An <a href="https://en.wikipedia.org/wiki/Annulus" class="extiw" title="en:Annulus">Annulus</a>.
<a href="//commons.wikimedia.org/wiki/User:Nandhp" title="User:Nandhp">User:Nandhp</a>
2006-09-07
Own work, based on <a href="//commons.wikimedia.org/wiki/File:Coronageometria.jpg" title="File:Coronageometria.jpg">Image:Coronageometria.jpg</a>.