Definition of heliacal in English:

heliacal

adjective

Astronomy
  • Relating to or near the sun.

    • ‘It is possible that all the planetary orbs originate from an early attempt to record heliacal obscurity.’
    • ‘The Greeks and Romans, with fairly useless calendars, continued to depend upon observing the heliacal risings and settings of stars.’
    • ‘All ancient astrologers placed the greatest significance upon heliacal risings and settings and ancient Mesopotamian astrology was also heavily predisposed to observance of this kind of planetary phenomena.’

Origin

Mid 16th century: via late Latin from Greek hēliakos (from hēlios ‘sun’) + -al.

Pronunciation

heliacal

/həˈlīəkəl/