8 letters · 9 points in Scrabble
adjective of or relating to the stars or constellations
adjective (of divisions of time) determined by daily motion of the stars
originFrom Latin sīdereus + -al (cf. Latin sīderālis), from sīdus (“star, constellation”), of unknown ultimate origin, likely a substrate language such as Pre-Greek.
Etymology from Wiktionary, CC BY-SA 3.0