AstroLearn HK
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Heliacal Rising Simulator

HK default
declination
right ascension
latitude
day of year
lock time of day to:

Sun δ = 0.0° · altitude 0.0° at 6.0h

Before you start

A heliacal rising happens when a star reappears in the dawn sky just before sunrise, after being lost in the Sun's glare.

Key idea

As the Sun moves eastward along the ecliptic, background stars rise slightly earlier each night. Eventually a star becomes visible again in the pre-dawn sky.

What you see

  • Eastern horizon sky brightening with sunrise
  • A star becoming visible in the heliacal window
  • Playable timeline and quick time presets

Try this

  1. Press Play and watch the sky brighten
  2. Use the Pre-dawn preset to find the heliacal window
  3. Compare night vs sunrise presets

Why it matters in Hong Kong

Hong Kong's latitude (22.3°N) gives a clear eastern horizon over the South China Sea — ideal for watching dawn events.

Common mistakes

  • Thinking heliacal rising happens at the same calendar date every year
  • Confusing it with a normal sunrise

Glossary

Heliacal rising
First visible appearance of a star at dawn after a period of invisibility.

Related tools

dipper clocksun paths