heliacal

Heliacal is an adjective used in astronomy to describe phenomena related to the Sun. It applies to the apparent visibility of a celestial body in relation to the Sun’s glare, most often in the form of heliacal rising or heliacal setting. A heliacal rising occurs when a body first becomes visible above the eastern horizon just before sunrise after a period of solar conjunction; a heliacal setting is the last visible appearance in the western sky after sunset before the Sun’s glare makes it disappear.

Visibility depends on many factors: the object's intrinsic brightness, its angular separation from the Sun, the observer's latitude, season, atmospheric conditions, and horizon altitude. The exact moment of first visibility varies with these conditions, and in practice it may occur when the Sun is several degrees below the horizon; different astronomers use varying practical criteria for "visibility".

Historically, heliacal events were used to keep calendars and to signal seasonal changes. The heliacal rising of Sirius was reputedly linked to the Egyptian New Year and the annual Nile flood. Other cultures linked heliacal appearances of stars or planets to agricultural or ceremonial calendars.

Today the term remains in historical and observational contexts, particularly in the study of ancient calendars and astronomical archaeology. It emphasizes how the Sun’s glare governs when faint objects can be seen and, thus, how early astronomy connected celestial movements with daily life. The word derives from Greek helios, "sun."