Astronomy Engine

Astronomy Engine is a suite of open source libraries designed to calculate the positions of celestial bodies and predict astronomical events. It is optimized to be small, fast, and accurate to within $\pm$1 arcminute.

Core Capabilities:

• Celestial Body Positions: Calculates positions for the Sun, Moon, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
• Coordinate Systems: Provides heliocentric and geocentric Cartesian vectors. Supports conversions between Equatorial J2000, Equatorial equator-of-date, Ecliptic J2000, Topocentric Horizontal, and Galactic (IAU 1958) orientations.
• Observer-based Calculations: Determines apparent horizon-based positions for any observer on Earth given latitude, longitude, and elevation (in meters), with optional atmospheric refraction correction.
• Event Prediction:
  • Rise, set, and culmination times for the Sun, Moon, and planets.
  • Civil, nautical, and astronomical twilight (dawn/dusk).
  • Moon phases (new, first quarter, full, third quarter).
  • Lunar and solar eclipses.
  • Transits of Mercury and Venus.
  • Equinoxes and solstices.
  • Planetary conjunctions, oppositions, and apsides.
  • Maximum elongation for Mercury and Venus.
• Advanced Features:
  • Visual magnitude determination.
  • Jupiter's moon positions (Io, Europa, Ganymede, Callisto).
  • Custom simulation of user-defined small bodies (asteroids, comets).
  • Constellation identification for sky points.
  • Lunar libration and axis orientation/rotation angles.

The Astronomy Engine Python API provides a comprehensive suite of tools for astronomical calculations, categorized into several functional areas:

• Celestial Body Positions: Calculate vectors and coordinates (Right Ascension, Declination, Ecliptic, Horizon) for the Sun, Moon, and planets.
• Geographic Helpers: Convert between geocentric vectors and observer-specific geographic coordinates.
• Time-based Events: Search for rise, set, and culmination times, as well as specific altitudes (e.g., for twilight).
• Lunar & Planetary Cycles: Calculate moon phases, eclipses, transits, lunar apsides (perigee/apogee), and planetary apsides (perihelion/aphelion).
• Visual Properties: Calculate visual magnitude, elongation, and illumination.
• Coordinate Transformations: Convert vectors between various orientation systems (EQJ, EQD, ECL, ECT, HOR, GAL) using rotation matrices.
• Gravitational Simulation: Use the GravitySimulator class to model the trajectories of small bodies (asteroids, comets, spacecraft) under the gravitational influence of the Sun and planets.

The demo/c/ directory contains several example programs demonstrating various astronomical calculations using the Astronomy Engine C API. These examples cover topics such as coordinate transformations, celestial body positions, and time calculations.

Key capabilities demonstrated in the examples include:

• Celestial Body Positions: Calculating equatorial and horizontal coordinates for the Sun, Moon, and planets (positions.c).
• Rise/Set Times: Calculating sunrise, sunset, moonrise, and moonset times (riseset.c).
• Lunar Phenomena: Calculating Moon phases (moonphase.c) and predicting lunar eclipses (lunar_eclipse.c).
• Coordinate Conversions: Converting galactic coordinates to horizontal coordinates (galactic.c) or finding where the ecliptic intersects the horizon (horizon.c).
• Time & Seasons: Calculating true solar time using the Sun's hour angle (solar_time.c) and determining equinoxes and solstices (seasons.c).
• Advanced Geometry: Using geoid functions like VectorObserver and ObserverVector to triangulate geographic coordinates from angular directions (triangulate.c).
• Imaging: Generating a PNG world map showing sunlight and moonlight intensity on a Mercator projection (worldmap.cpp).

The Kotlin demo is organized into separate source files, each representing a specific astronomical topic. You can run any of these by passing their name (lowercase) to the ./rundemo script.

• constellation: Finds which constellation the Moon is in at a given time and demonstrates binary searching for constellation border crossings.
• jupitermoons: Calculates coordinates for Jupiter and its four major moons (Io, Europa, Ganymede, Callisto) from Earth, including light-delay corrections.
• lunareclipse: Searches for the first 10 partial or total lunar eclipses after a specified time.
• moonphase: Determines the Moon's current phase and predicts the next 10 quarter phases.
• positions: Displays equatorial and horizontal coordinates for the Sun, Moon, and planets based on observer latitude, longitude, and time.
• risesetculm: Calculates rise, set, and culmination times for the Sun and Moon.
• seasons: Calculates equinoxes and solstices for a given calendar year.