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Articles

1985: Vol. LXII, No. 2

Tide Predictions Using Satellite Constituents

Submitted
July 31, 2015
Published
2015-07-20

Abstract

Conventional harmonic tide predictions for the last century have used/factors to modify the amplitudes of lunar constituents and u's to correct the constituent equilibrium phases (V0) as a means of approximating for a given period (one year or less) the effect of the 18.61 year cycle of the revolution of the moon’s node. Historically, there was little choice; friction in geared mechanical tidepredicting machines imposed finite limits on the number of constituents used. Doodson (1921) clearly identified and evaluated satellite constituents; his study was updated using the latest astronomical constants by Cartwright and Tayler (1971) and by Cartwright and Edden (1973). Nevertheless, satellite constituents, now readily usable on modern computers, have not been used for tide predictions. As a result, predictions have really been quasi-harmonic, requiring modifying amplitudes and phases periodically, at present every year for U.S. predictions, every two months for Canadian, and every 30 days for U.K. predictions. With satellite constituents, nineteen years of hourly tide predictions for Seattle (1921-1939) were computed from initial settings for 1 January 1921. It was not to be expected that the accuracy of harmonic tide predictions would be improved significantly by the new procedure; comparisons of annual residual variances for predictions by U.S. and Canadian procedures indicate that any improvements are small. Nevertheless, this new method removes the need for rather contrived (however clever) procedures, in particular that of constituents modifying M1 and L2 by cycles per 8.85 years (revolution of lunar perigee) in the f and u corrections for these constituents.