AN EPHEMERIS FOR GRAVITY TIDE PREDICTIONS AT THE NANOGAL LEVEL

An ephemeris for precise gravity tide predictions must match the nominal sensitivity of the superconducting gravimeter, 1 ngal. It is shown that this requires over one thousand separate spectral lines in an ecliptic ephemeris, which implies several thousand spectral lines in an equatorial ephemeris, and a similar number in a tidal catalogue. Important tidal effects also arise from the direct tidal attraction of the planets, and indirectly from the planetary perturbations to the luni-solar ephemeris. Indirect planetary effects amount to nearly 100 ngal, and are larger than direct planetary effects, which are only about 10 ngal. Venus, Mars, Jupiter, and Saturn all influence the tide at or above the ngal level. The precision of the ephemeris has been tested by comparison with published values of the right ascension, declination, and sine horizontal parallax of the Sun and Moon over a 10 year period. This indicates that the tide raising potential generated from the ephemeris is good to better than 3 x 10(-5) m2 s-2, or 1 ngal in gravity.