Hē′li-ae or He-li′ae-al, a.

a. [Gr. ἡλιακός, belonging to the sun, from ἥλιος, the sun; N. Lat. heliacus, Fr. héliaque.] (Astron.) Emerging from the light of the sun, or passing into it; rising or setting at the same, or nearly the same, time as the sun.

The heliacal rising of a star is when, after being in conjunction with the sun, and invisible, it emerges from the light so as to be visible in the morning before sun-rising. On the contrary, the heliacal setting of a star is when the sun approaches so near as to render it invisible by its superior splendor.