title: What do 'a' and 'c' stand for in 'Haversine formula' to measure the distance between two points?
body: <pre><code>var R = 6371e3; // metres 
  var φ1 = lat1.toRadians(); 
  var φ2 = lat2.toRadians(); 
  var Δφ = (lat2-lat1).toRadians();
   var Δλ = (lon2-lon1).toRadians(); 
  
  var a = Math.sin(Δφ/2) * Math.sin(Δφ/2) + Math.cos(φ1) * Math.cos(φ2) * Math.sin(Δλ/2) * Math.sin(Δλ/2); 
  
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 
  
  var d = R * c;
  </code></pre>
  
  <p>Here in this code formula is used  in JavaScript. But anyway, there is a and c used in the original formula as well. What do they mean?</p>