@spec ecliptic_to_horizontal(number(), number(), number(), number()) ::
  {float(), float()}

Convert an ecliptic longitude (on the ecliptic plane, lat = 0) to local horizontal coordinates (altitude, azimuth).

Uses standard spherical trigonometry:

1. ecliptic → equatorial via obliquity rotation
2. equatorial → hour-angle via local sidereal time
3. hour-angle + declination → altitude / azimuth

All inputs and outputs are in degrees. Azimuth convention: 0° = North, 90° = East, increasing clockwise (standard astronomical/navigational convention).

The result is the geometric direction. Ecliptic latitude is assumed to be zero, and no correction is applied for atmospheric refraction, parallax, or observer elevation.

Parameters

• ecl_lon_deg — ecliptic longitude in degrees (0–360; other values are handled by the trigonometry and need not be normalized)
• obliquity_deg — obliquity of the ecliptic in degrees (~23.44 currently)
• lst_deg — local sidereal time in degrees (0–360); multiply sidereal hours by 15 to get degrees
• lat_deg — observer geographic latitude in degrees (−90 to 90, north positive)

Returns

{altitude_deg, azimuth_deg} where altitude ∈ [-90, 90] and azimuth ∈ [0, 360). Azimuth is not meaningful at the zenith and nadir, where it degenerates to 0.0.

Examples

Seen from the equator with the vernal equinox on the meridian, the summer-solstice point (ecliptic longitude 90°) sits on the eastern horizon, north of due east by the obliquity:

iex> {alt, az} = AstroUtils.Coordinates.ecliptic_to_horizontal(90.0, 23.44, 0.0, 0.0)
iex> {Float.round(alt, 6), Float.round(az, 6)}
{0.0, 66.56}