var active_banner = 0;

function changeBanner() {
    $('banner_'+active_banner).fade({ duration: 2.0, from: 1, to: 0 });
    active_banner = (active_banner+1) % num_banners;
    $('banner_'+active_banner).appear({ duration: 2.0, from: 0, to: 1 });
}

function startBannerCycle() {
    new PeriodicalExecuter(changeBanner, banner_cycle_time);
}

Event.observe(window, 'load', startBannerCycle);