Jump to content

  • Log In with Google      Sign In   
  • Create Account

Turbochist

Member Since 26 Oct 2013
Offline Last Active Yesterday, 05:59 AM

#5284834 How to calculate sun and planets positions around the Earth by real date and...

Posted by Turbochist on 03 April 2016 - 03:45 AM

Actually I've used this calculations you can watch results on http://www.openglobus.org:

It's not best like Simon's but good enough for me now and comparatively low calculations.

og.math.TWO_PI = 2.0 * Math.PI;
og.math.PI_TWO = Math.PI / 2.0;
og.math.RADIANS = Math.PI / 180.0;
og.math.DEGREES = 180.0 / Math.PI;

// Angle between J2000 mean equator and the ecliptic plane.
// 23 deg 26' 21".448 (Seidelmann, _Explanatory Supplement to the
// Astronomical Almanac_ (1992), eqn 3.222-1.
og.astro.J2000_OBLIQUITY = 23.4392911;

og.math.rev = function (x) {
    return x - Math.floor(x / 360.0) * 360.0;
};

og.math.Quaternion.yRotation = function (a) {
    a *= 0.5;
    return new og.math.Quaternion(0.0, Math.sin(a), 0.0, Math.cos(a));
};

/**
 * http://stjarnhimlen.se/comp/tutorial.html
 *  a  Mean distance, or semi-major axis
 *  e  Eccentricity
 *  T  Time at perihelion
 *
 *  q  Perihelion distance  = a * (1 - e)    
 *  Q  Aphelion distance    = a * (1 + e)
 *
 *  i  Inclination, i.e. the "tilt" of the orbit relative to the
 *     ecliptic.  The inclination varies from 0 to 180 degrees. If
 *     the inclination is larger than 90 degrees, the planet is in
 *     a retrogade orbit, i.e. it moves "backwards".  The most
 *     well-known celestial body with retrogade motion is Comet Halley.
 *
 *  N  (usually written as "Capital Omega") Longitude of Ascending
 *     Node. This is the angle, along the ecliptic, from the Vernal
 *     Point to the Ascending Node, which is the intersection between
 *     the orbit and the ecliptic, where the planet moves from south
 *     of to north of the ecliptic, i.e. from negative to positive
 *     latitudes.
 *
 *  w  (usually written as "small Omega") The angle from the Ascending
 *     node to the Perihelion, along the orbit.
 *
 *  P  Orbital period       = 365.256898326 * a**1.5/sqrt(1+m) days,
 *     where m = the mass of the planet in solar masses (0 for
 *     comets and asteroids). sqrt() is the square root function.
 *    
 *  n  Daily motion         = 360_deg / P    degrees/day
 *
 *  t  Some epoch as a day count, e.g. Julian Day Number. The Time
 *     at Perihelion, T, should then be expressed as the same day count.
 *
 *  t - T   Time since Perihelion, usually in days
 *
 *  M  Mean Anomaly         = n * (t - T)  =  (t - T) * 360_deg / P
 *     Mean Anomaly is 0 at perihelion and 180 degrees at aphelion
 *
 *  L  Mean Longitude       = M + w + N
 *
 *  E  Eccentric anomaly, defined by Kepler's equation:   M = E - e * sin(E)
 *     An auxiliary angle to compute the position in an elliptic orbit
 *
 *  v  True anomaly: the angle from perihelion to the planet, as seen
 *     from the Sun
 *
 *  r  Heliocentric distance: the planet's distance from the Sun.
 *
 *  x,y,z  Rectangular coordinates. Used e.g. when a heliocentric
 *         position (seen from the Sun) should be converted to a
 *         corresponding geocentric position (seen from the Earth).
 */
og.astro.earth.getSunPosition = function (jd) {

    var d = jd - og.jd.J2000;

    var w = 282.9404 + 4.70935E-5 * d;                  //longitude of perihelion
    var a = 1.000000;                                   //mean distance, a.u.
    var e = 0.016709 - 1.151E-9 * d;                    //eccentricity
    var M = og.math.rev(356.0470 + 0.9856002585 * d);   //mean anomaly

    var oblecl = og.astro.J2000_OBLIQUITY - 3.563E-7 * d; //obliquity of the ecliptic

    var L = og.math.rev(w + M); //Sun's mean longitude

    var E = M + og.math.DEGREES * e * Math.sin(M * og.math.RADIANS) * (1 + e * Math.cos(M * og.math.RADIANS)); //eccentric anomaly

    //Sun rectangular coordiantes, where the X axis points towards the perihelion
    var x = Math.cos(E * og.math.RADIANS) - e;
    var y = Math.sin(E * og.math.RADIANS) * Math.sqrt(1 - e * e);

    var r = Math.sqrt(x * x + y * y);           // distance
    var v = Math.atan2(y, x) * og.math.DEGREES; // true anomaly

    var lon = og.math.rev(v + w); //longitude of the Sun

    //the Sun's ecliptic rectangular coordinates
    x = r * Math.cos(lon * og.math.RADIANS);
    y = r * Math.sin(lon * og.math.RADIANS);

    //We use oblecl, and rotate these coordinates
    var xequat = x;
    var yequat = y * Math.cos(oblecl * og.math.RADIANS);
    var zequat = y * Math.sin(oblecl * og.math.RADIANS);

    var theta = og.math.TWO_PI * (d * 24.0 / 23.9344694 - 259.853 / 360.0); // Siderial spin time

    return og.math.Quaternion.yRotation(-theta).mulVec3(new og.math.Vector3(-yequat * og.astro.AU_TO_METERS,
        zequat * og.astro.AU_TO_METERS, -xequat * og.astro.AU_TO_METERS));

    //Convert to RA and Decl
    //var RA = Math.atan2(yequat, xequat) * og.math.DEGREES;
    //var Decl = Math.atan2(zequat, Math.sqrt(xequat * xequat + yequat * yequat)) * og.math.DEGREES;
};

Thanks guys!




#5283685 How to calculate sun and planets positions around the Earth by real date and...

Posted by Turbochist on 27 March 2016 - 04:22 AM

What do you think is a good idea to use VSOP87 model for real time earth moon etc. positioning? Or may be better to use some easier model? I'm working on "google earth like" program and wand to chose more effective and fast method for visualization. What do you think?




#5283327 How to calculate sun and planets positions around the Earth by real date and...

Posted by Turbochist on 25 March 2016 - 12:16 AM

Thanks! And another one question: I see that celestia app use in TAI time only seconds but what about milliseconds? Is it necessary if I want use sattellites for example?

 

And I see that celestia uses VSOP87 is it model for calculation ephemerides, do I understand right?




#5283257 How to calculate sun and planets positions around the Earth by real date and...

Posted by Turbochist on 24 March 2016 - 03:43 PM

Great sources so much help functions! Thank you for advice. But It's hard to understand whole algrorithm step by step.




#5185788 Terrain lighting artifacts

Posted by Turbochist on 08 October 2014 - 10:48 AM

But actually I doubt about my terrain grid tile is 33x33 points, so I have 33x33 calculated vertices normals, therefore my texture is 33x33 pixels. And next I have to apply lighting to the terrain tile with 33x33 normals texture. Is it right way?

 

P.S.

I have tried...but it s realy noticed on the snow.




PARTNERS