Yes, APEXES is a Scrabble word!

Letter values in Scrabble US: A 1, P 3, E 1, X 8, E 1, S 1.

Is it a valid word?

APEXES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the highest point (of something)
the point on the celestial sphere toward which the sun and solar system appear to be moving relative to the fixed stars

Also: solar apex, apex of the sun's way.