title: Check if point is inside a circle
body: <p>I have a point expressed in lat/long</p>
  
  <pre><code>Position louvreMuseum = new Position( 48.861622, 2.337474 );
  </code></pre>
  
  <p>and I have a radius value expressed in meters. I need to check if another point, also expressed in lat/long, is inside the circle.</p>
  
  <p>If I were on a flat surface I can simply use the formula</p>
  
  <pre><code>(x - center_x)^2 + (y - center_y)^2 &lt;= radius^2
  </code></pre>
  
  <p>as deeply explained in these <a href="https://stackoverflow.com/a/481150/431537">SO answer</a>.</p>
  
  <p>However as per the latitude/longitude usage I can not use that formula because of the spherical nature of the planet.</p>
  
  <p>How can I calculate a distance from any given point to the center to be compared with the radius?</p>