The eclipse bar is a new bar under your player frame on Blizzard standard UI. It takes 100 Solar/Lunar power to get from the middle to either side and you gain Solar power by casting Wrath and Lunar by casting Starfire.
Starsurge gives both Solar and Lunar power depending on which side you’re closest to.
When you reach Solar eclipse it buffs your nature spells until you hit 0 Eclipse power, and when you continue casting Solar spells you will reach Lunar eclipse which buffs your arcane spells until you hit 0 Eclipse power.
When you reach an Eclipse your spells will be highlighted on your actionbar.
Remember:
Lunar eclipse increases the damage of all your arcane spells which are Starfire, Moonfire, Starfall and Starsurge.
Solar eclipse increases the damage of all your nature spells which are Wrath, Insect Swarm, Sunfire, Typhoon, Starsurge and Hurricane.
