Galactic

Celestial coordinate conversions and utilities

Installation

bower install galactic

or

npm install galactic

Usage

The following examples use some basic transforms:

var rad = function(deg) { return deg/180 * Math.PI; };
var deg = function(rad) { return rad*180 / Math.PI; };

Equatorial to Horizontal: Where is alpha centauri in the sky above Toronto right now?

// Create coordinate in equatorial coordinate system
var centauri = galactic({rightAscension: rad(219.9), declination: rad(-60.8339)});

// Observer (Toronto, now)
var observer = {latitude: rad(43.7001100), longitude: rad(-79.4163000), utc: Date.now()};

// Convert to horizontal given observer place/time
var horizontal = centauri.horizontal( observer );

// Where is it?
console.log('Altitude: ' + deg(horizontal.altitude()) + ', Azimuth: ' + deg(horizontal.azimuth()));

Ecliptic to Horizontal: Where is the sun right now in the skies of Toronto?

// Create coordinate in equatorial coordinate system
// Calculate sun's mean longitude (approx)
var n = galactic.dates.unixDateToJulian(Date.now()) - galactic.dates.J2000
var meanLongitude = 280.46 + 0.9856474*n
var sun = galactic({longitude: rad(meanLongitude), latitude: 0});

// Observer (Toronto, now)
var observer = {latitude: rad(43.7001100), longitude: rad(-79.4163000), utc: Date.now()};

// Convert to horizontal given observer place/time
var horizontal = sun.horizontal( observer );

// Where is it?
console.log('Sun - Altitude: ' + deg(horizontal.altitude()) + ', Azimuth: ' + deg(horizontal.azimuth()));

If a star is directly overhead in Toronto, where in the sky is it in Portland?

// Observer 1 (Toronto)
var observer1 = {latitude: rad(43.7001100), longitude: rad(-79.4163000), utc: Date.now()};
// Observer 2 (Portland)
var observer2 = {latitude: rad(45.5200), longitude: rad(-122.6819), utc: Date.now()};

// Up in Toronto
var up = galactic({azimuth: rad(0), altitude: rad(90)}, observer1);

// Convert to observer in Portland
var portlandSky = up.observer( observer2 );

// Where is it?
console.log('In Portland - Altitude: ' + deg(portlandSky.altitude()) + ', Azimuth: ' + deg(portlandSky.azimuth()));

Notes

• All angle values must be in radians
• Azimuth is referred to the south point of the horizon, the common astronomical reckoning
• The obliquity of the ecliptic is fixed at 23.439 degrees which is roughly accurate for Earth-based calculations at times near J2000. An alternate value of the obliquity can be given via obliquity: ### as part of the coordinate hash during instantiation.
• The equations don't account for atmospheric refraction, diurnal parallax, or other complicating factors

Building

Galactic uses ES6 modules and CoffeeScript. Grunt is used to transpile the code, convert to JS, and produce various forms of the library. To build:

npm install
grunt

AMD, CJS, and standalone (using Browserify) formats will be created in dist/

To Do

• Document API
• Observer accessor
• Mutator API
• Complete unit tests
• More Example usage
• Extract celestial body definitions to generalize
• Add handy additional functions, e.g. sun and moon positions given date
• NPM prepublish compile step