Sun position and elevation

CesiumJS
1

Hello everybody! :slight_smile:

For my Cesium App (paragliding simulator online) I am searching a way to know the precise sun position in the sky, in terms of orientation and elevation on the horizon, knowing a position on the earth and an UTC time.

I know that there are very complicated formulas to do that… but since Cesium Engine have the capability to draw the sun in the sky… i thought that it had just done the “dirty job” :D… there is a way to retrieve those informations?

Thank you!

2

some time ago somebody suggested to me to use this functions :
"to get the sun position use Simon1994PlanetaryPositions.computeSunPositionInEarthInertialFrame and Transforms.computeTemeToPseudoFixedMatrix"

but i can’t understand how exactly… can somebody provide me an example?

serching in the source code i found also this : “viewer.scene.context.uniformState.sunPositionWC”… but similar to the other function (Simon1994PlanetaryPositions.computeSunPositionInEarthInertialFrame) this returns 3 numbers than i am not able to understand…

how to get the sun position elevation in degrees?

3

Hello,

Cesium does indeed compute and draw the sun where it should be depending on the time. It looks like you found the function we use to do Simon1994PlanetaryPositions.computeSunPositionInEarthInertialFrame. That function takes a JulianDate as the input and returns a Cartesian3 for the sun’s position.

You will have to do your own calculations to figure out the orientation and elevation from a position on the earth surface.

Best,

Hannah

4

Has any function been added since 2016 to know azimuth and altitude of Sun and Moon?

I have this function to calculate elevation of a point w.r.t.another but it does not work:

function getElevation(startPoint, endPoint) {
// Return elevation in degree of endPoint w.r.t. startPoint
// https://community.cesium.com/t/calculate-elevation-angle-between-two-points/5629/5
// https://sandcastle.cesium.com/?src=Hello%20World.html&label=Showcases&gist=d917ac40dfb225cb9899a84403b0c719

// startPoint = new Cesium.Cartesian3.fromDegrees(12.636018, 41.995557, 167);  (lon, lat, alt)
// endPoint = new Cesium.Cartesian3.fromDegrees(12.801439, 42.058937, 1181); (lon, lat, alt)

	//Obtain vector by taking difference of the end points
	var scratch1= new Cesium.Cartesian3();
	var difference = Cesium.Cartesian3.subtract(endPoint, startPoint, scratch1);
	difference = Cesium.Cartesian3.normalize(difference, scratch1);
	//console.log("Difference: " + difference);

	//Obtain surface normal by normalizing the starting point position
	var scratch2 = new Cesium.Cartesian3();
	var surfaceNormal = Cesium.Cartesian3.normalize(startPoint, scratch2);
	//console.log("Surface normal: " + surfaceNormal);

	//Take the dot product of your given vector and the surface normal
	var dotProduct = Cesium.Cartesian3.dot(difference, surfaceNormal);
	//console.log("Dot product: " + dotProduct);

	//Arcos the result
	var angle = 90 - Math.acos(dotProduct) * Cesium.Math.DEGREES_PER_RADIAN ;
	//console.log("Result: " + angle);
	return angle;
}
5

@jumpjack

Thank you for sharing this function with the community! To the best of my knowledge, we haven’t yet added this feature. I will check in with the rest of the development team and get back to you if there are any updates :rocket: :grinning:

-Sam