Yes, HA is a Scrabble word!

Letter values in Scrabble US: H 4, A 1.

Is it a valid word?

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

(astronomy) the angular distance of a celestial point measured westward along the celestial equator from the zenith crossing; the right ascension for an observer at a particular location and time of day

Also: hour angle.