Orthographic Shading

The above grid demonstrates an orthographic azimuthal projection. The lines are at uniform 10º increments of latitude and longitude.

Part 3 of 3.

Orthographic Grid
Orthographic Clipping
Orthographic Shading

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

circle,
path {
  fill: none;
  stroke: #123;
  stroke-opacity: .5;
}

circle {
  stroke-width: 2px;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var width = 960,
    height = 500;

var rotate = [-71.03, 42.37],
    velocity = [.018, .006];

var projection = d3.geo.orthographic()
    .scale(240)
    .clipAngle(90);

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);

var gradient = svg.append("defs").append("radialGradient")
    .attr("id", "gradient")
    .attr("cx", "75%")
    .attr("cy", "25%");

gradient.append("stop")
    .attr("offset", "5%")
    .attr("stop-color", "#ffd");

gradient.append("stop")
    .attr("offset", "100%")
    .attr("stop-color", "#ba9");

var fill = svg.append("circle")
    .attr("cx", width / 2)
    .attr("cy", height / 2)
    .attr("r", 240)
    .style("fill", "url(#gradient)");

var feature = svg.append("path")
    .datum(graticule);

d3.timer(function(elapsed) {
  projection.rotate([rotate[0] + elapsed * velocity[0], rotate[1] + elapsed * velocity[1]]);
  fill.attr("transform", "rotate(" + (rotate[0] + elapsed * velocity[0]) + " " + width / 2 + "," + height / 2 + ")");
  feature.attr("d", path);
});

</script>

https://d3js.org/d3.v3.min.js