What's the difference between heliac and heliacal?

Heliac

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Definition:

• (a.) Heliacal.

Example Sentences:

• (1) Presenting their work at the Royal Astronomical Society’s National Astronomy Meeting at the University of Nottingham, the team propose that the corridors boast a number of features that would have allowed someone within them to glimpse an early view of Aldebaran’s so-called heliacal rising through the small window provided by the tunnel’s opening.

Heliacal

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Definition:

• (a.) 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.

Example Sentences:

• (1) Presenting their work at the Royal Astronomical Society’s National Astronomy Meeting at the University of Nottingham, the team propose that the corridors boast a number of features that would have allowed someone within them to glimpse an early view of Aldebaran’s so-called heliacal rising through the small window provided by the tunnel’s opening.