$(document).ready( function() {
  function center(element) {
    if ($(window).height() > $(element).height()) {
      $(element)
        .css('marginTop', ($(window).height() - $(element).height())/2);
    }
    $(window).resize( function() {
      if ($(window).height() > $(element).height()) {
        $(element)
          .css('marginTop', ($(window).height() - $(element).height())/2);
      }
      else {
        $(element)
          .css('marginTop','');
      }
    })
  }
  center('body');
});