be on the wane

To be decreasing or lessening. Thank goodness this storm is finally on the wane—I can't tolerate another day of rain. The group's power in this region is on the wane, but that doesn't mean another faction won't step in. Though his career is on the wane these days, the director was once a juggernaut of the film industry.

be on the ˈwane

(written) be becoming smaller or less strong: Their political power is on the wane.

When the moon is on the wane it appears smaller in the sky.