## ADDED Requirements

### Requirement: SolarProvider implementation

The system SHALL export `createSolarSystemProvider` returning a `SolarProvider` that computes real solar system body positions.

#### Scenario: Sun altitude at noon

- **WHEN** `getBody("Sun", observer, time)` is called for an observer at latitude `0` at local noon
- **THEN** the returned `altitude` is approximately `90` minus the absolute declination offset

### Requirement: Astronomy.js engine injection

The system SHALL accept an injectable `AstronomyEngine` in `createSolarSystemProvider`, defaulting to astronomy.js.

#### Scenario: Deterministic engine injection

- **WHEN** a deterministic mock `AstronomyEngine` is injected
- **THEN** `getBodies` returns exactly the mocked positions

### Requirement: Keplerian propagator for custom planets

The system SHALL propagate positions for planets with `OrbitalConfig` using a Keplerian solver from epoch.

#### Scenario: Custom planet position

- **WHEN** a `PlanetConfig` with valid `OrbitalConfig` is used as observer's planet or target body
- **THEN** `getBodies` includes the custom planet with a propagated heliocentric position

### Requirement: Angular diameter computation

The system SHALL compute `angularDiameterArcmin` as `2 * arctan(radius / distance) * (180/π * 60)`.

#### Scenario: Sun angular diameter

- **WHEN** `getBody("Sun", ...)` is called
- **THEN** `angularDiameterArcmin` is approximately `32` arcminutes

### Requirement: Apparent magnitude for real bodies

The system SHALL compute `apparentMagnitude` for real bodies using phase angle and distance via astronomy.js.

#### Scenario: Venus magnitude range

- **WHEN** `getBody("Venus", ...)` is called at multiple times
- **THEN** `apparentMagnitude` varies between approximately `-4.9` and `-3.8`

### Requirement: Sunrise and sunset computation

The system SHALL compute sunrise and sunset dates in UTC for a given observer and calendar date.

#### Scenario: Equator sunrise consistency

- **WHEN** `getSunrise` and `getSunset` are called for an equatorial observer on an equinox date
- **THEN** sunrise is approximately 12 hours before sunset

### Requirement: Season computation

The system SHALL export `getSeason` defaulting to Earth but accepting any `PlanetConfig` with `axialTilt` and `OrbitalConfig`.

#### Scenario: Northern hemisphere season

- **WHEN** `getSeason(new Date("2026-06-21"), EARTH)` is called
- **THEN** the result is `summer`

### Requirement: Equinox and solstice prediction

The system SHALL export `getNextEquinox` and `getNextSolstice` for any configured planet.

#### Scenario: Next equinox after a date

- **WHEN** `getNextEquinox(new Date("2026-01-01"), EARTH)` is called
- **THEN** the result is a `Date` in March 2026

### Requirement: Lunar phase

The system SHALL export `getLunarPhase` returning phase (0–1), illumination percentage, and name for Earth/Moon.

#### Scenario: Full moon phase

- **WHEN** `getLunarPhase(fullMoonDate)` is called
- **THEN** `phase` is approximately `0.5`
- **AND** `illumination` is approximately `100%`

### Requirement: Eclipse prediction

The system SHALL export `predictEclipse` for solar and lunar eclipses on Earth.

#### Scenario: Known eclipse

- **WHEN** `predictEclipse(new Date("2026-01-01"), "solar")` is called
- **THEN** the returned `EclipseEvent` has a date after the input and a `totality` value

### Requirement: Constellation lookup

The system SHALL export `getConstellation` mapping RA/Dec to IAU constellation name.

#### Scenario: Orion lookup

- **WHEN** `getConstellation(raOrion, decOrion)` is called with Orion's centre coordinates
- **THEN** the result equals `"Orion"`

### Requirement: Atmosphere extinction for surface observers

The system SHALL apply atmospheric extinction to apparent magnitudes when the observer has an atmosphere.

#### Scenario: Horizon extinction

- **WHEN** `getBody("Sun", surfaceObserver, time)` is called with the Sun near the horizon
- **THEN** `apparentMagnitude` is dimmer than the vacuum value