Yes, DECLINATION is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, C 3, L 1, I 1, N 1, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a condition inferior to an earlier condition; a gradual falling off from a better state
a downward slope or bend
(astronomy) the angular distance of a celestial body north or to the south of the celestial equator; expressed in degrees; used with right ascension to specify positions on the celestial sphere
a polite refusal of an invitation

Also: celestial latitude.

Not playable in Scrabble US: DEC.