affix-sidebar.js

 $(document).ready(function(){
   // Explicitly set the left positioning,
   // based on the current position calculated by the Browser.
   // This "locks" the sidebar to its current x-axis, even
   // when we change the positioning strategy in a moment
   $('#sidebar').css('left', $('#sidebar').offset().left + 'px');

   $(window).scroll(function(){
     // Has the visitor scrolled down past the headmast?
     // If the headmast is 100 pixels tall, and
     // the visitor has scrolled down 115 pixels, then this will return true
     //
     if ( $(window).scrollTop() > $('#headmast').height() )
     {
       $('#sidebar').css({ position: 'fixed', top: '100px' });
     }
     else { // We are still at the top of the page

       // Revert back from 'fixed' positioning
       $('#sidebar').css({ position: 'absolute', top: 'auto' });
     }
   });
 });