As the Earth circles the Sun, every bright star spends a few weeks each year lost in the Sun's glare, too close to it in the sky to be seen. The heliacal rising is the first dawn on which the star becomes visible again, flashing briefly on the eastern horizon just before sunrise; the heliacal setting is the last dusk on which it can be glimpsed low in the west after sunset before vanishing. These first and last appearances were the original calendar of humankind: Egypt timed the Nile flood and its New Year by the heliacal rising of Sirius (Sothis), Babylonian astrologers tabulated star phases in their omen texts, Hesiod told Greek farmers when to plough and harvest by the Pleiades, and Indian tradition still honours the rising of Agastya (Canopus), which signals the retreat of the monsoon.

The exact dates depend strongly on the observer's latitude and the star's brightness — Sirius returns to Indian skies in early August, but several days differently in the far north or south of the country. This calculator uses the Swiss Ephemeris visibility model, which accounts for the star's magnitude, twilight brightness, atmospheric extinction, and the geometry of Sun and star at your chosen place, to predict the morning of first visibility and the evening of last visibility, the best minute to look, and the length of the star's annual disappearance.

Tips for catching the moment

• Find a spot with a clear, flat eastern horizon (for risings) or western horizon (for settings)
• Arrive ten minutes before the listed start time — the window is short, often under twenty minutes
• Haze, dust, or city glow can delay your first sighting by a day or two after the predicted date

Astrologers also read heliacal phases as a star being "reborn" from the underworld journey — a powerful moment in mundane and electional work. Discuss what a star's return means for your chart with a qualified astrologer.