function getWindowHeight() {
    var windowHeight = 0;
    if (typeof(window.innerHeight) == 'number') {
        windowHeight = window.innerHeight;
    }
    else {
        if (document.documentElement && document.documentElement.clientHeight) {
            windowHeight = document.documentElement.clientHeight;
            }
        else {
            if (document.body && document.body.clientHeight) {
                windowHeight = document.body.clientHeight;
            }
        }
    }
    return windowHeight;
}

$(document).ready(function() {
    if (getWindowHeight() > $("#prehome").height())
    {
        var margin_top = (getWindowHeight() - $("#prehome").height()) / 2;
        $("#prehome").css("margin-top", margin_top); 
    }
});