.block

## .block
license: gpl-3.0

## README.md

      
        

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              README.md
            
          
        
      
    
  
    In honor of the vernal equinox, this animation shows the progression of the solar terminator at approximately 5.2 million times its normal rate. Each frame of the animation advances twenty-four hours ahead of real-time so that the seasonal changes of the solar terminator are visible. The blue region is night; the white region is day. Best accompanied with the theme song from Buck Rogers.

  

  

## index.html
<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  background: #fcfcfa;
}

.stroke {
  fill: none;
  stroke: #000;
  stroke-width: 3px;
}

.fill {
  fill: #fff;
}

.graticule {
  fill: none;
  stroke: #777;
  stroke-width: .5px;
  stroke-opacity: .5;
}

.land {
  fill: #222;
}

.boundary {
  fill: none;
  stroke: #fff;
  stroke-width: .5px;
}

.night {
  stroke: steelblue;
  fill: steelblue;
  fill-opacity: .3;
}

text {
  font-family: Menlo, monospace;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/d3.geo.projection.v0.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 570;

var π = Math.PI,
    radians = π / 180,
    degrees = 180 / π;

var formatDate = d3.time.format.utc("%b. %d, %Y"),
    formatTime = d3.time.format.utc("%X UTC");

var circle = d3.geo.circle()
    .angle(90);

var projection = d3.geo.kavrayskiy7()
    .scale(170)
    .translate([width / 2, height / 2])
    .precision(.1);

var path = d3.geo.path()
    .projection(projection);

var graticule = d3.geo.graticule();

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

svg.append("defs").append("path")
    .datum({type: "Sphere"})
    .attr("id", "sphere")
    .attr("d", path);

svg.append("use")
    .attr("class", "stroke")
    .attr("xlink:href", "#sphere");

svg.append("use")
    .attr("class", "fill")
    .attr("xlink:href", "#sphere");

var g = svg.append("g");

g.append("path")
    .datum(graticule)
    .attr("class", "graticule")
    .attr("d", path);

d3.json("/mbostock/raw/4090846/world-50m.json", function(error, world) {
  if (error) throw error;

  var dayOffset = 0;

  var date = svg.append("text")
      .attr("dy", "-1.35em");

  var time = svg.append("text");

  svg.selectAll("text")
      .attr("x", 16)
      .attr("y", height - 16);

  g.insert("path", ".graticule")
      .datum(topojson.feature(world, world.objects.land))
      .attr("class", "land")
      .attr("d", path);

  g.insert("path", ".graticule")
      .datum(topojson.mesh(world, world.objects.countries, function(a, b) { return a !== b; }))
      .attr("class", "boundary")
      .attr("d", path);

  var night = svg.append("path")
      .attr("class", "night");

  d3.timer(function() {
    var now = new Date(Date.now() + dayOffset);
    date.text(formatDate(now));
    time.text(formatTime(now));
    night.datum(circle.origin(antipode(solarPosition(now)))).attr("d", path);
    dayOffset += 864e5;
  });
});

d3.select(self.frameElement).style("height", height + "px");

function antipode(position) {
  return [position[0] + 180, -position[1]];
}

function solarPosition(time) {
  var centuries = (time - Date.UTC(2000, 0, 1, 12)) / 864e5 / 36525, // since J2000
      longitude = (d3.time.day.utc.floor(time) - time) / 864e5 * 360 - 180;
  return [
    longitude - equationOfTime(centuries) * degrees,
    solarDeclination(centuries) * degrees
  ];
}

// Equations based on NOAA’s Solar Calculator; all angles in radians.
// http://www.esrl.noaa.gov/gmd/grad/solcalc/

function equationOfTime(centuries) {
  var e = eccentricityEarthOrbit(centuries),
      m = solarGeometricMeanAnomaly(centuries),
      l = solarGeometricMeanLongitude(centuries),
      y = Math.tan(obliquityCorrection(centuries) / 2);
  y *= y;
  return y * Math.sin(2 * l)
      - 2 * e * Math.sin(m)
      + 4 * e * y * Math.sin(m) * Math.cos(2 * l)
      - 0.5 * y * y * Math.sin(4 * l)
      - 1.25 * e * e * Math.sin(2 * m);
}

function solarDeclination(centuries) {
  return Math.asin(Math.sin(obliquityCorrection(centuries)) * Math.sin(solarApparentLongitude(centuries)));
}

function solarApparentLongitude(centuries) {
  return solarTrueLongitude(centuries) - (0.00569 + 0.00478 * Math.sin((125.04 - 1934.136 * centuries) * radians)) * radians;
}

function solarTrueLongitude(centuries) {
  return solarGeometricMeanLongitude(centuries) + solarEquationOfCenter(centuries);
}

function solarGeometricMeanAnomaly(centuries) {
  return (357.52911 + centuries * (35999.05029 - 0.0001537 * centuries)) * radians;
}

function solarGeometricMeanLongitude(centuries) {
  var l = (280.46646 + centuries * (36000.76983 + centuries * 0.0003032)) % 360;
  return (l < 0 ? l + 360 : l) / 180 * π;
}

function solarEquationOfCenter(centuries) {
  var m = solarGeometricMeanAnomaly(centuries);
  return (Math.sin(m) * (1.914602 - centuries * (0.004817 + 0.000014 * centuries))
      + Math.sin(m + m) * (0.019993 - 0.000101 * centuries)
      + Math.sin(m + m + m) * 0.000289) * radians;
}

function obliquityCorrection(centuries) {
  return meanObliquityOfEcliptic(centuries) + 0.00256 * Math.cos((125.04 - 1934.136 * centuries) * radians) * radians;
}

function meanObliquityOfEcliptic(centuries) {
  return (23 + (26 + (21.448 - centuries * (46.8150 + centuries * (0.00059 - centuries * 0.001813))) / 60) / 60) * radians;
}

function eccentricityEarthOrbit(centuries) {
  return 0.016708634 - centuries * (0.000042037 + 0.0000001267 * centuries);
}

</script>

## thumbnail.png

      
        

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              thumbnail.png