heliacal adjective. [hɪ'lʌɪǝk(ǝ)l] M16.
[from medieval Latin heliacus (from Greek hēliakos, from hēlios sun) + -AL¹.]
Relating to the sun. Chiefly, in ancient astronomy: heliacal rising, the first rising of a star after a period of invisibility due to conjunction with the sun (in a given year); heliacal setting, the last setting of a star before a period of invisibility due to conjunction with the sun.

■ heliacally adverb M16.

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