Heliacal Events

A heliacal event marks the moment a planet becomes visible or invisible near the horizon at dawn or dusk. These events have been observed and recorded for thousands of years — the heliacal rising of Sirius signaled the Nile flood in ancient Egypt, and Babylonian astronomers tracked planetary visibility cycles as a foundation of their omen literature.

What makes a planet visible?

A planet is always "there" in the sky, but whether you can actually see it depends on the interplay between the planet's brightness, the sky brightness, and the observer's visual acuity. Near the horizon at twilight, the sky is bright enough to wash out faint objects. A heliacal event is the threshold crossing — the first or last moment conditions allow visibility.

The Morphemeris API uses the Schaefer sky brightness model, which accounts for:

• Atmospheric conditions — pressure, temperature, humidity, and visibility range all affect how much light the atmosphere scatters and absorbs
• Observer characteristics — age, visual acuity (Snellen ratio), whether using binoculars or a telescope
• Geometry — the angular separation between the Sun and the planet, their altitudes above the horizon, and the planet's apparent magnitude

Event types

Why are evening first and morning last restricted to inner planets? These events correspond to a planet emerging from behind the Sun (superior conjunction) into the evening sky, or disappearing behind the Sun from the morning sky. Only Mercury and Venus, which orbit closer to the Sun than Earth, exhibit this pattern. Outer planets transition between morning and evening visibility differently — through opposition rather than conjunction.

The visibility cycle

A planet's visibility follows a repeating cycle as it moves relative to the Sun:

Outer planets (Mars through Pluto):

1. Morning first (heliacal rising) — planet emerges from the Sun's glare at dawn
2. Acronychal rising — planet rises at sunset, visible all night (near opposition)
3. Evening last (heliacal setting) — planet disappears into the Sun's glare at dusk
4. Period of invisibility (near conjunction with the Sun)
5. Cycle repeats

Inner planets (Mercury, Venus):

1. Morning first — planet appears in the pre-dawn sky
2. Morning last — planet disappears into the Sun's glare (approaching superior conjunction)
3. Evening first — planet appears in the post-sunset sky
4. Evening last — planet disappears into the Sun's glare (approaching inferior conjunction)
5. Cycle repeats

Customizing the calculation

All atmospheric and observer parameters are optional — the API uses sensible defaults (standard atmosphere, normal vision, naked eye). Override them when modeling specific conditions:

• High altitude or dry climate? Lower pressure_mbar and humidity_pct
• Ancient observer study? Adjust age_years
• Telescope observation? Set telescope_aperture_cm and telescope_magnification

See the /v1/heliacal reference for the full parameter list.

Limitations

The Sun and Moon are excluded — heliacal events are defined relative to the Sun's position, so the Sun itself cannot have one. The Moon's rapid motion and varying brightness make the heliacal model inapplicable.