Sorting don't work on my navigation

Any ideas why my navigation don’t sort pages in any other order? I would like them to sort by the order number. Now the order is always the same. Sort oder desc displays the same order than asc.

Here’s the code:

<ul id="secondaryNavi">
<?php
    $args=array(
    'post_parent' => 8,
    'post_type' => 'page',
    'sort_column' => 'menu_order',
    'sort_order' => 'desc',
          );
     $my_query = null;
     $my_query = new WP_Query($args);
     if( $my_query->have_posts() ) {

while ($my_query->have_posts()) : $my_query->the_post(); ?>

<li><a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a>

<?php $meta_one = get_post_meta($post->ID,'description', true ); ?>

<div class="description"><?php echo $meta_one; ?></div>

</li>

<?php endwhile; } ?>

<?php wp_reset_query();
// Restore global post data stomped by the_post().?>

</ul>