gistfile1.js

    // grab the initial top offset of the navigation 
    var $sticky_navigation = $('#site-nav');
    var sticky_navigation_offset_top = 102;

    // our function that decides weather the navigation bar should have "fixed" css position or not.
    var sticky_navigation = function(){
        var scroll_top = $(window).scrollTop(); // our current vertical position from the top

        // if we've scrolled more than the navigation, change its position to fixed to stick to top, otherwise change it back to relative
        if ($(window).width() > 600) {
            if (scroll_top > sticky_navigation_offset_top) {
                $sticky_navigation.css({ 'position': 'fixed', 'top': 0, 'left': 0 }).addClass('site-nav-fixed');
            } else {
                $sticky_navigation.css({ 'position': 'absolute' }).removeClass('site-nav-fixed');
            }
        } else {
            $sticky_navigation.css({ 'position': 'static', 'top': 'auto', 'left': 'auto' });
        }
    };

    // run our function on load
    sticky_navigation();

    // and run it again every time you scroll
    $(window).scroll(function() {
        sticky_navigation();
    });