The full moon is shining brightly in the sky.

Literal

Full-moon [subject-が] sky [at-に] is-shining-clear.

さえる (usually 冴える) originally 'to be clear, sharp' — used of light, sound, or skill. Here 空にさえている describes the moon shining brightly and clearly in the sky. The ~ている marks the ongoing state of brilliance.