constructor() {

			const shader = Sky.SkyShader;
			const material = new THREE.ShaderMaterial( {
				name: 'SkyShader',
				fragmentShader: shader.fragmentShader,
				vertexShader: shader.vertexShader,
				uniforms: THREE.UniformsUtils.clone( shader.uniforms ),
				side: THREE.BackSide,
				depthWrite: false
			} );
			super( new THREE.BoxGeometry( 1, 1, 1 ), material );

		}
Based on "A Practical Analytic Model for Daylight"
aka The Preetham Model, the de facto standard analytic skydome model
https://www.researchgate.net/publication/220720443_A_Practical_Analytic_Model_for_Daylight

First implemented by Simon Wallner
http://www.simonwallner.at/projects/atmospheric-scattering

Improved by Martin Upitis
http://blenderartists.org/forum/showthread.php?245954-preethams-sky-impementation-HDR

Three.js integration by zz85 http://twitter.com/blurspline