31:[] 2f:{"title":"View from Himawari 8","cells":[{"id":0,"value":"md`# View from Himawari 8`","pinned":false,"hidden":false,"mode":"ojs"},{"id":131,"value":"md`\n[Himawari 8](https://en.wikipedia.org/wiki/Himawari_8) is a Japanese geostationary weather satellite. Since it launched in late 2014, it has been taking stunning photos of the Earth, just like this one, every 10 minutes.\n\nHimawari is in geostationary orbit, so it always sees exactly the same view of Earth. I've used its orbit parameters (found on Wikipedia) to construct a D3 projection that precisely matches the perspective that the satellite sees. The outlines of land masses that you see on the image below are drawn by D3, using that projection.\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":4,"value":"svg`\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":157,"value":"viewof url = select({\n options: urls,\n value: urls[1].value\n})","pinned":false,"hidden":false,"mode":"ojs"},{"id":142,"value":"md`\nSince any image from Himawari shows the same view, you can change the URL to a different image (I've provided a few accessible from the select menu above) and the projected map will still line up.\n\nThe images I've used here are 1920 pixels square, but the original true color images from Himawari are a staggering 11,000 × 11,000 pixels (or 121 megapixels). [Here's an example](https://upload.wikimedia.org/wikipedia/commons/6/68/Himawari-8_true-color_2015-01-25_0230Z.png) — be warned, the file is 164MB. If you're on mobile, you can still check out this 100% crop showing eastern Japan:\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":184,"value":"html`\n\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":186,"value":"md`_Himawari_ is Japanese for sunflower.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":189,"value":"html`

`","pinned":false,"hidden":false,"mode":"ojs"},{"id":216,"value":"md`Code that produces the above visualization can be found below.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":6,"value":"projection = d3.geoSatellite()\n // distance affects how \"fisheyed\" the projection appears. it represents the distance in Earth radii\n // that the observer is from the center of the Earth. See below for how I've calculated it.\n .distance(distance)\n // Himawari is geostationary (not merely geosynchronous) which is only possible when latitude is zero.\n // Additionally, the image appears to have been taken (or post-processed) to show the north pole at the top.\n // Therefore, we only need to apply longitudinal rotation.\n .rotate([longitude, 0, 0])\n // this formula clips the projection so we only see the front face of the Earth\n .clipAngle(Math.acos(1 / distance) * 180 / Math.PI - 1e-6)\n // be sure to clip before using fitExtent, as it will change the result\n .fitExtent([[margin, margin], [width - margin, width - margin]], { type: \"Sphere\" })\n .precision(0.1)","pinned":false,"hidden":false,"mode":"ojs"},{"id":8,"value":"// d3's geoSatellite takes a `distance` parameter measured in Earth radii from Earth's center.\ndistance = (altitude + earth_radius) / earth_radius","pinned":false,"hidden":false,"mode":"ojs"},{"id":10,"value":"earth_radius = 6371 // kilometers (Wikipedia)","pinned":false,"hidden":false,"mode":"ojs"},{"id":15,"value":"altitude = 35793 // kilometers above Earth, measured relative to mean sea level (Wikipedia)\n","pinned":false,"hidden":false,"mode":"ojs"},{"id":17,"value":"longitude = -140.7 // 140.7° East (Wikipedia)\n","pinned":false,"hidden":false,"mode":"ojs"},{"id":116,"value":"path = d3.geoPath(projection)","pinned":false,"hidden":false,"mode":"ojs"},{"id":154,"value":"urls = [\n {\n label: \"2014-12-18 (true color)\",\n value: \"https://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/Himawari-8_true-color_earth.jpg/1920px-Himawari-8_true-color_earth.jpg\",\n },\n {\n label: \"2015-01-25 (true color)\",\n value: \"https://upload.wikimedia.org/wikipedia/commons/thumb/6/68/Himawari-8_true-color_2015-01-25_0230Z.png/1920px-Himawari-8_true-color_2015-01-25_0230Z.png\",\n },\n {\n label: \"2015-08-21 (enhanced color)\",\n value: \"https://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/Himawari_8_Full_Disk_Aug_21_2015_0210Z.jpg/1920px-Himawari_8_Full_Disk_Aug_21_2015_0210Z.jpg\",\n }\n]","pinned":false,"hidden":false,"mode":"ojs"},{"id":27,"value":"// apparent margin between the edge of the Earth and the edge of the image\nmargin = 0.006 * width","pinned":false,"hidden":false,"mode":"ojs"},{"id":120,"value":"land = topojson.feature(world, world.objects.land)","pinned":false,"hidden":false,"mode":"ojs"},{"id":122,"value":"sphere = ({ type: \"Sphere\" })","pinned":false,"hidden":false,"mode":"ojs"},{"id":30,"value":"world = d3.json(\"https://unpkg.com/world-atlas@1/world/50m.json\")\n","pinned":false,"hidden":false,"mode":"ojs"},{"id":32,"value":"topojson = require(\"topojson-client@3\")","pinned":false,"hidden":false,"mode":"ojs"},{"id":34,"value":"d3 = require(\"d3@5\", \"d3-geo-projection@2\")","pinned":false,"hidden":false,"mode":"ojs"},{"id":158,"value":"import {select} from \"@jashkenas/inputs\"","pinned":false,"hidden":false,"mode":"ojs"}],"files":"$W31","stdlib":"1","titling":"auto"}