Investigation of Change in the Computational Technique of the Sun's Physical Ephemeris in The Astronomical Almanac

In The Astronomical Almanac, the heliographic latitude and longitude of the sub-Earth point are printed for each day of the year at 0h TT. Prior to the 2009 edition, neither the light-time correction nor a correction for aberration were applied to the rotation. Having both the light-time and aberration compensated for in the zero point of the meridian is possible only for the Sun since the Sun is essentially always the same distance from the Earth and the aberration correction is nearly constant, therefore the differences in light-time and aberration between perigee and apogee are small. This brought about a situation where the computation technique of the Suns rotation has always been different from that of the other Solar System bodies. During the 2006 IAU General Assembly, the Working Group on Cartographic Coordinates and Rotational Elements hereafter known simply as the group decided that computing the Suns rotation differently has detrimental effects. First, it confuses many people which has led to erroneous computations. Second, the method used previously is only valid at the Earth. Third, the accuracy is compromised because neither the light-time nor aberration is constant since the Earth-Sun distance varies. To make the computation consistent and applicable anywhere, the group recommends that the Wo value for the Sun, the angle of the prime meridian at the standard epoch, be foredated by an amount which approximately corresponds to the light-travel time. Using this new value, the computation must take into account the light travel time that is, the rotation is antedated. The combination of foredating and antedating should have little effect on the values computed for locations on or near the Earth, but would make the computation technique more consistent with the other objects and be suitable anywhere in the Solar System. This paper documents the changes in the computational technique as suggested by the working group.