1. In relation to or as determined by the position of the sun; especially, at or near the time when a star rises or sets with the sun.
Usage: astronomy; technical

Examples

• The star Sirius rises heliacally in late summer in the Northern Hemisphere.
• Ancient astronomers tracked the heliacal rising of Sirius to predict the flooding of the Nile.
• The planet's heliacally determined position was crucial for the calendar calculations.
• Heliacally speaking, the star becomes visible just before sunrise.
• The heliacal setting of Arcturus marked an important date in the agricultural calendar.