Interactive, adaptable celestial map done with the D3.js visualization library. So, GeoJSON for sky stuff. Which surprisingly nobody has done yet, it seems.

Features display of stars and deep sky objects (DSOs) with a selectable magnitude limit up to 6, or choose different GeoJSON data source for higher magnitudes. Also shows constellations with names, lines and/or boundaries, the Milky Way band and grid lines. Alternate coordinate spaces e.g. ecliptc, galactic or supergalactic are also possible. Full support for zoom and rotation with mouse or gestures.

Since it uses D3.js and HTML5 canvas, it needs a modern browser with canvas support, so any recent flavor of Chrome/Firefox/Safari/Opera or IE 9 and above should suffice. Check out the demo at armchairastronautics.blogspot.com or download the tarball celestial.tar.gz containing everything for local usage, which works with Firefox; Chrome needs to be started with command line parameter --allow-file-access-from-files to load local json files. Or use a local web server environment, quite easy to do with node.js.

See some examples:
Interactive form demo
Spinning sky globe
The Milky Way halo, globular clusters & satellite galaxies
The Local Group of galaxies
Asterisms with locations & time selection
Asterisms with zoom & pan
Zoom & pan animations
A different kind of Messier marathon

Include the necessary scripts d3.min.js, d3.geo.projection.min.js and celestial.js from the lib subfolder or the first two alternatively from the official d3.js server http://d3js.org/, and then simply edit the default options and display the map with Celestial.display(config).

var config = { 
  width: 0,           // Default width, 0 = full parent element width; 
                      // height is determined by projection
  projection: "aitoff",    // Map projection used: see below
  transform: "equatorial", // Coordinate transformation: equatorial (default),
                           // ecliptic, galactic, supergalactic
  center: null,       // Initial center coordinates in set transform
                      // [longitude, latitude, orientation] all in degrees 
                      // null = default center [0,0,0]
  orientationfixed: true,  // Keep orientation angle the same as center[2]
  adaptable: true,    // Sizes are increased with higher zoom-levels
  interactive: true,  // Enable zooming and rotation with mousewheel and dragging
  form: true,         // Display form for interactive settings
  location: false,    // Display location settings (no center setting on form)
  controls: true,     // Display zoom controls
  container: "map",   // ID of parent element, e.g. div, null = html-body
  datapath: "data/",  // Path/URL to data files, empty = subfolder 'data'
  stars: {
    show: true,    // Show stars
    limit: 6,      // Show only stars brighter than limit magnitude
    colors: true,  // Show stars in spectral colors, if not use default color
    style: { fill: "#ffffff", opacity: 1 }, // Style for stars
    names: true,   // Show star names (Bayer, Flamsteed, Variable star, Gliese, 
                      whichever applies first in that order)
    proper: false, // Show proper name (if present, above name otherwise)
    desig: false,  // Show all names, including Draper and Hipparcos
    namelimit: 2.5,  // Show only names/designations for stars brighter than namelimit
    namestyle: { fill: "#ddddbb", font: "11px Georgia, Times, 'Times Roman', serif", 
                 align: "left", baseline: "top" },  // Style for star names
    size: 7,       // Maximum size (radius) of star circle in pixels
    data: 'stars.6.json' // Data source for stellar data, 
                         // number indicates limit magnitude
  },
  dsos: {
    show: true,    // Show Deep Space Objects 
    limit: 6,      // Show only DSOs brighter than limit magnitude
    names: true,   // Show DSO names
    desig: true,   // Show short DSO names
    namestyle: { fill: "#cccccc", font: "11px Helvetica, Arial, serif", 
                 align: "left", baseline: "top" }, // Style for DSO names
    namelimit: 6,  // Show only names for DSOs brighter than namelimit
    data: 'dsos.bright.json', // Data source for DSOs, 
                              // opt. number indicates limit magnitude
    symbols: {  //DSO symbol styles, 'stroke'-parameter present = outline
      gg: {shape: "circle", fill: "#ff0000"},          // Galaxy cluster
      g:  {shape: "ellipse", fill: "#ff0000"},         // Generic galaxy
      s:  {shape: "ellipse", fill: "#ff0000"},         // Spiral galaxy
      s0: {shape: "ellipse", fill: "#ff0000"},         // Lenticular galaxy
      sd: {shape: "ellipse", fill: "#ff0000"},         // Dwarf galaxy
      e:  {shape: "ellipse", fill: "#ff0000"},         // Elliptical galaxy
      i:  {shape: "ellipse", fill: "#ff0000"},         // Irregular galaxy
      oc: {shape: "circle", fill: "#ffcc00", 
           stroke: "#ffcc00", width: 1.5},             // Open cluster
      gc: {shape: "circle", fill: "#ff9900"},          // Globular cluster
      en: {shape: "square", fill: "#ff00cc"},          // Emission nebula
      bn: {shape: "square", fill: "#ff00cc", 
           stroke: "#ff00cc", width: 2},               // Generic bright nebula
      sfr:{shape: "square", fill: "#cc00ff", 
           stroke: "#cc00ff", width: 2},               // Star forming region
      rn: {shape: "square", fill: "#00ooff"},          // Reflection nebula
      pn: {shape: "diamond", fill: "#00cccc"},         // Planetary nebula 
      snr:{shape: "diamond", fill: "#ff00cc"},         // Supernova remnant
      dn: {shape: "square", fill: "#999999", 
           stroke: "#999999", width: 2},               // Dark nebula grey
      pos:{shape: "marker", fill: "#cccccc", 
           stroke: "#cccccc", width: 1.5}              // Generic marker
    }
  },
  constellations: {
    show: true,    // Show constellations 
    names: true,   // Show constellation names 
    desig: true,   // Show short constellation names (3 letter designations)
    namestyle: { fill:"#cccc99", font: "12px Helvetica, Arial, sans-serif", 
                 align: "center", baseline: "middle" }, // Style for constellations
    lines: true,   // Show constellation lines, style below
    linestyle: { stroke: "#cccccc", width: 1, opacity: 0.6 }, 
    bounds: false, // Show constellation boundaries, style below
    boundstyle: { stroke: "#cccc00", width: 0.5, opacity: 0.8, dash: [2, 4] }
  },
  mw: {
    show: true     // Show Milky Way as filled multi-polygon outlines 
    style: { fill: "#ffffff", opacity: 0.15 }  // Style for MW
  },
  lines: {  // Display & styles for graticule & some planes
    graticule: { show: true, stroke: "#cccccc", width: 0.6, opacity: 0.8 },   
    equatorial: { show: true, stroke: "#aaaaaa", width: 1.3, opacity: 0.7 },  
    ecliptic: { show: true, stroke: "#66cc66", width: 1.3, opacity: 0.7 },     
    galactic: { show: false, stroke: "#cc6666", width: 1.3, opacity: 0.7 },    
    supergalactic: { show: false, stroke: "#cc66cc", width: 1.3, opacity: 0.7 }
  },
  background: {        // Background style
    fill: "#000000",   // Area fill
    opacity: 1, 
    stroke: "#000000", // Outline
    width: 1 
  }, 
  horizon: {  //Show horizon marker, if location is set and map projection is all-sky
    show: false, 
    stroke: null, // Line
    width: 1.0, 
    fill: "#000000", // Area below horizon
    opacity: 0.5
  }
};

// Display map with the configuration above or any subset therof
Celestial.display(config);

Supported projections: Airy, Aitoff, Armadillo, August, Azimuthal Equal Area, Azimuthal Equidistant, Baker, Berghaus, Boggs, Bonne, Bromley, Collignon, Craig, Craster, Cylindrical Equal Area, Cylindrical Stereographic, Eckert 1, Eckert 2, Eckert 3, Eckert 4, Eckert 5, Eckert 6, Eisenlohr, Equirectangular, Fahey, Foucaut, Ginzburg 4, Ginzburg 5, Ginzburg 6, Ginzburg 8, Ginzburg 9, Hammer, Hatano, HEALPix, Hill, Homolosine, Kavrayskiy 7, Lagrange, l'Arrivee, Laskowski, Loximuthal, Mercator, Miller, Mollweide, Flat Polar Parabolic, Flat Polar Quartic, Flat Polar Sinusoidal, Natural Earth, Nell Hammer, Orthographic, Patterson, Polyconic, Rectangular Polyconic, Robinson, Sinusoidal, Stereographic, Times, 2 Point Equidistant, van der Grinten, van der Grinten 2, van der Grinten 3, van der Grinten 4, Wagner 4, Wagner 6, Wagner 7, Wiechel and Winkel Tripel. Most of these need the extension d3.geo.projections

Style settings
fill: fill color (css color value)
opacity: opacity (number 0..1)
Line styles
stroke: outline color (css color value)
width: line width in pixels (number 0..)
dash: line dash ([line length, gap length])
Text styles
font: well, the font (css font property)
align: horizontal align (left|center|right|start|end)
baseline: vertical align (alphabetic|top|hanging|middle|ideographic|bottom)
Symbol style
shape: symbol shape (circle|square|diamond|ellipse|marker or whatever else is defined in canvas.js)

Exposed functions & objects

• Celestial.add({file:string, type:dso|line, callback:function, redraw:function)
  Function to add data in json-format (dso) or directly (line) to the display
  file: complete url/path to json data file (type:dso)
  type: type of data being added
  callback: callback function to call when json is loaded (dso)
  or to directly add elements to the path (line)
  redraw: for interactive display, call when view changes (optional)

• Celestial.getData(geojson, transform)
  Function to convert geojson coordinates to transformation
  (equatorial, ecliptic, galactic, supergalactic)
  Returns geojson-object with transformed coordinates

• Celestial.getPoint(coordinates, transform)
  Function to convert single coordinate to transformation
  (equatorial, ecliptic, galactic, supergalactic)
  Returns transformed coordinates

• Celestial.container
  The object to add data to in the callback. See D3.js documentation

• Celestial.context The HTML5-canvas context object to draw on in the callback. Also see D3.js documentation

• Celestial.map
  The d3.geo.path object to apply projection to data. Also see D3.js documentation

• Celestial.mapProjection
  The projection object for access to its properties and functions. Also D3.js documentation

• Celestial.clip(coordinates)
  Function to check if the object is visible, and set its visiblility
  coordinates: object coordinates in radians, normally supplied by D3 as geometry.coordinates array

• Celestial.setStyle(<style object>)

• Celestial.setTextStyle(<style object>)
  Set the canvas styles as documented above under style settings. Seperate functions for graphic/text
  <style object>: object literal listing all styles to set

• Celestial.Canvas.symbol()
  Draw symbol shapes directly on canvas context: circle, square, diamond, triangle, ellipse, marker,
  stroke-circle, cross-circle

Exposed functions

• Celestial.rotate({center:[long,lat,orient]}) Turn the map to the given center coordinates, without parameter returns the current center

• Celestial.zoomBy(factor) Zoom the map by the given factor - < 1 zooms out, > 1 zooms in, without parameter returns the current zoom level

• Celestial.apply(config) Apply configuration changes without reloading the complete map. Any parameter of the above config-object can be set except width, projection, transform, and *.data, which need a reload and interactive, form, controls, container, which control page structure & behaviour and shouls only be set on the initial load

• Celestial.resize({width:px|0|null}) Change the overall size of the map, canvas object needs a complete reload Optional width: new size in pixels, null or 0 = full parent width

• Celestial.redraw({transform:equatorial|ecliptic|galactic|supergalactic}) Load all the data and redraw the whole map. Optional transform: change the coordinate space transformation

• Celestial.reproject({projection:<see above>}) Change the map projection. projection: new projection to set

Exposed functions

• Celestial.animate(anims, dorepeat)
  Set the anmation sequence and start it.
  anims: sequence data (see below)
  dorepeat: repeat sequence in endless loop

• Celestial.stop(wipe)
  Stop the animation after the current step finishes.
  wipe: if true, delete the list of animation steps as well

• Celestial.go(index)
  Continue the animation, if animation steps set.
  index: if given, continue at step #index in the anims arrray,
  if not, continue where the animation was stopped

Animation sequence format:
[
{param: Animated parameter projection|center|zoom
value: Adequate value for each parameter
duration: in milliseconds, 0 = exact length of transition
callback: optional callback function called at the end of the transition
}, ...]

GeoJSON data files
(See format specification in readme.md for the data folder)

• stars.6.json Stars down to 6th magnitude [1]
• stars.8.json Stars down to 8.5th magnitude [1]
• dsos.6.json Deep space objects down to 6th magnitude [2]
• dsos.14.json Deep space objects down to 14th magnitude [2]
• dsos.bright.json Some of the brightest showpiece DSOs of my own choosing
• messier.json Messier objects [8]
• lg.json Local group and Milky Way halo galaxies/globiular clusters. My own compilation [6]
• constellations.json Constellation data [3]
• constellations.bounds.json Constellation boundaries [4]
• constellations.lines.json Constellation lines [3]
• asterisms.json Asterism data [7]
• mw.json Milky Way outlines in 5 brightness steps [5]
• templ.json GeoJSON templates for all the different object types used

Sources

• [1] XHIP: An Extended Hipparcos Compilation; Anderson E., Francis C. (2012) VizieR V/137D
  Star names & designations:
  HD-DM-GC-HR-HIP-Bayer-Flamsteed Cross Index (Kostjuk, 2002) VizieR IV/27A
  FK5-SAO-HD-Common Name Cross Index (Smith 1996) VizieR IV/22
  General Catalogue of Variable Stars (Samus et.al. 2007-2013) VizieR B/gcvs
  Preliminary Version of the Third Catalogue of Nearby Stars (Gliese+ 1991) VizieR V/70A
• [2] Saguaro Astronomy Club Database version 8.1
• [3] IAU Constellation page, name positions and some line modifications by me
• [4] Catalogue of Constellation Boundary Data; Davenhall A.C., Leggett S.K. (1989) VizieR VI/49
• [5] Milky Way Outline Catalog, Jose R. Vieira
• [6] Lots of sources, see blog pages for complete list
• [7] Saguaro Astronomy Club Asterisms (scroll down)
• [8] Messier Objects with Data, H.Frommert/seds.org
  All data converted to GeoJSON at J2000 epoch, positions converted from 0...24h right ascension to -180...180 degrees longitude as per GeoJSON requirements, 0...12h -> 0...180º; 12...24h -> -180...0º

Other files

• celestial.js main javascript object
• celestial.min.js minified javascript
• celestial.tar.gz data, minified script and viewer, all you need for local display
• LICENSE
• readme.md this file
• style.css stylesheet
• map.html simple map viewer with editable configuration
• viewer.html interactive map viewer/demo
• lib/d3.*.js necessary d3 libraries
• src/*.js source code for all modules

Thanks to Mike Bostock and Jason Davies for D3.js and d3.geo.projections. And also thanks to Jason Davies for d3.geo.zoom, which saved me some major headaches in figuring out how to rotate/zoom the map.

Released under BSD License