Riddle 1 - The Answer Is Always Predetermined

The brightest light you know shines brighter on me. I am first to your third. I am quicksilver on earth.

What am I?

A a B b C c D d E e F f G g H h I i J j K k L l M m N n O o P p Q q R r S s T t U u V v W w X x Y y Z z