Transversing Equirectangular

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

path {
  fill: none;
  stroke-linejoin: round;
}

.sphere,
.graticule {
  stroke: #aaa;
}

.equator {
  stroke: red;
  stroke-width: 2px;
}

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

var width = 960,
    height = 580,
    scale1 = 145,
    scale2 = 85;

var projection = d3.geo.equirectangular()
    .scale(scale1)
    .translate([width / 2, height / 2])
    .precision(.3);

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)
  .append("g");

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

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

svg.append("path")
    .datum({type: "LineString", coordinates: [[-180, 0], [-90, 0], [0, 0], [90, 0], [180, 0]]})
    .attr("class", "equator")
    .attr("d", path);

var feature = svg.selectAll("path");

animation();

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

</script>

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