Code covered by the BSD License

# A MATLAB Script for Time and Coordinate Calculations

### David Eagle (view profile)

Interactive MATLAB script that can be used to perform time and coordinate calculations.

etilt1 (tjd)
function [oblm, oblt, eqeq, dpsi, deps] = etilt1 (tjd)

% this function computes quantities related to the orientation
% of the earth's rotation axis at tdb julian date tjd

% low-accuracy mode

% input

%  tjd = tdb julian date for orientation parameters

% output

%  oblm = mean obliquity of the ecliptic in degrees at date tjd

%  oblt = true obliquity of the ecliptic in degrees at date tjd

%  eqeq = equation of the equinoxes in time seconds at date tjd

%  dpsi = nutation in longitude in arcseconds at date tjd

%  deps = nutation in obliquity in arcseconds at date tjd

% note: the equation of the equinoxes includes the complementary terms

% ported from NOVAS 3.0

%%%%%%%%%%%%%%%%%%%%%%%

seccon = 180.0d0 * 3600.0d0 / pi;

% t0 = tdb julian date of epoch j2000.0 (tt)

t0 = 2451545.0d0;

t = (tjd - t0) / 36525.0d0;

% obtain nutation parameters in arcseconds

[delpsi, deleps] = nut2000b (t);

[el, elp, f, d, omega] = funarg (t);

% series from iers conventions (2003), chapter 5,
% table 5.2c, with some adjustments to coefficient values
% copied from iers function eect2000, which has a more
% complete series

cterms = 2640.96d-6 * sin (omega) ...
+   63.52d-6 * sin (2.0d0 * omega) ...
+   11.75d-6 * sin (2.0d0 * f - 2.d0 * d + 3.0d0 * omega) ...
+   11.21d-6 * sin (2.0d0 * f - 2.d0 * d +         omega) ...
-    4.55d-6 * sin (2.0d0 * f - 2.d0 * d + 2.0d0 * omega) ...
+    2.02d-6 * sin (2.0d0 * f            + 3.0d0 * omega) ...
+    1.98d-6 * sin (2.0d0 * f            +         omega) ...
-    1.72d-6 * sin (3.0d0 * omega) ...
-    0.87d-6 * t * sin (omega);

% compute mean obliquity of the ecliptic in arcseconds

obm = obliq(t);

% compute true obliquity of the ecliptic in arcseconds

obt = obm + deleps;

% compute equation of the equinoxes in arcseconds

ee = delpsi * cos(obm / seccon) + cterms;

% convert to output units

oblm = obm / 3600.0d0;

oblt = obt / 3600.0d0;

eqeq = ee / 15.0d0;

dpsi = delpsi * seccon;

deps = deleps * seccon;