# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Equation of Time**

**From:**Andr�s Ruiz

**Date:**2003 Aug 21, 16:46 +0200

The Sun, approximate coordinates: (RA, Dec) Sun's right ascension and declination // D: the number of days and fraction (+ or -) from the epoch referred to as J2000.0, // which is 2000 January 1.5, Julian date 2451545.0: // JD: the Julian date day = day + (hour+min/60.0+sec/3600.0)/24.0; JD = JulianDate( day, month, year ); D = JD-2451545.0; // all the constants (therefore g, q, and L) are in degrees. g = 357.529+0.98560028*D; q = 280.459+0.98564736*D; // L: approximation to the Sun's geocentric apparent ecliptic longitude (adjusted for aberration). L = q+1.915*SIN( g )+ 0.020* SIN( 2.0*g ); // Reduce g, q, and L to the range 0? to 360? g = ang_0_360( g ); q = ang_0_360( q ); L = ang_0_360( L ); // Sun's ecliptic latitude, b, can be approximated by b = 0.0; // The distance of the Sun from the Earth, R, in astronomical units (AU) R = 1.00014-0.01671*COS( g )-0.00014*COS( 2.0*g ); // mean obliquity of the ecliptic, in degrees: e = 23.439-0.00000036*D; // right ascension in degrees // RA is always in the same quadrant as L. // RA = ATAN( COS( e )*SIN( L )/COS( L ) ); // the proper quadrant will be obtained. RA = ATAN2( COS( e )*SIN( L ), COS( L ) ); // right ascension in hours RA = RA/15.0; // Reduced to the range 0h to 24h RA = time_0_24( RA ); // declination Dec = ASIN( SIN( e )*SIN( L ) ); // The Equation of Time, apparent solar time minus mean solar time // Eqt and RA are in hours and q is in degrees. EqT = q/15.0 - RA; // angular semidiameter of the Sun in degrees SD = 0.2666/R; See at: http://www.geocities.com/andresruizgonzalez/ More accurate algorithm: Astronomical Algorithms. by Jean Meeus, 2 Ed edition (December 1998). ISBN: 0943396638