633,634c633,661
< 		// use semi-empirical coefficient for GRS drift
< 		return remainder * 360. + re.offset - 0.2483 * qAbs(jd - 2456172);
---
> 		if( re.offset != 0.0 ) {
> 			// use semi-empirical coefficient for GRS drift
> 			qDebug() << "Jupiter: offset = " << re.offset << " --> rotation = " << (remainder * 360. + re.offset - 0.2483 * qAbs(jd - 2456172));
> 			return remainder * 360. + re.offset - 0.2483 * qAbs(jd - 2456172);
> 		} else {
> 			// http://www.projectpluto.com/grs_form.htm
> 			// CM( System II) =  181.62 + 870.1869147 * jd + correction [870d rotation every day]
> 			const double rad  = M_PI/180.;
> 			double jup_mean = (jd - 2455636.938) * 360. / 4332.89709;
> 			double eqn_center = 5.55 * sin( rad*jup_mean);
> 			double angle = (jd - 2451870.628) * 360. / 398.884 - eqn_center;
> 			double correction = 11 * sin( rad*angle) + 5 * cos( rad*angle) - 1.25 * cos( rad*jup_mean) - eqn_center;
> 			double cm2=181.62 + 870.1869147 * jd + correction;
> 			cm2=cm2 - 360.0*(int)(cm2/360.);
> 			// http://www.skyandtelescope.com/observing/transit-times-of-jupiters-great-red-spot/ writes:
> 			// The predictions assume the Red Spot was at Jovian System II longitude 216° in September 2014 and continues to drift 1.25° per month, based on historical trends noted by JUPOS.
> 			// GRS longitude was at 7/9 2014 216d with a drift of 1.25d every month
> 			double longitudeGRS=216+1.25*( jd - 2456543)/30; 
> 			qDebug() << "Jupiter: CM2 = " << cm2 << " longitudeGRS = " << longitudeGRS << " --> rotation = " << (cm2 - longitudeGRS);
> 			return cm2 - longitudeGRS;
> 			// To verify:
> 			// GRS at 2015-02-26 23:07 UT on picture at https://maximusphotography.files.wordpress.com/2015/03/jupiter-febr-26-2015.jpg
> 			//        2014-02-25 19:03 UT    http://www.damianpeach.com/jup1314/2014_02_25rgb0305.jpg
> 			//	  2013-05-01 10:29 UT    http://astro.christone.net/jupiter/jupiter2012/jupiter20130501.jpg
> 			//        2012-10-26 00:12 UT at http://www.lunar-captures.com//jupiter2012_files/121026_JupiterGRS_Tar.jpg
> 			//	  2011-08-28 02:29 UT at http://www.damianpeach.com/jup1112/2011_08_28rgb.jpg
> 			// stellarium 2h too early: 2010-09-21 23:37 UT http://www.digitalsky.org.uk/Jupiter/2010-09-21_23-37-30_R-G-B_800.jpg      
> 			
> 		}