The program RIGTID together with its subroutines computes the tidal accelerations on a rigid earth, i. e. no response of the Earth is considered. The tidal accelerations are calculated directly from the positions of moon and sun separately, i. e. if wanted two groups of tides (solar or lunar) can be handled differently. The program does not use a potential development, it includes the second, third, and fourth degree potential of the moon, but only the second degree potential of the sun. It treats the earth as an elliptically shaped body but does not include the direct effect of the earth's ellipticity on the tidal accelerations (Wilhelm 1983 and Dahlen 1993). It also does not take into account the speed of light. No planets are included, this is discussed by Broucke, Zuern, and Slichter, (1972, DOI: 10.1029/GM016p0319). This program was developed at UCLA in 1971/72 by Walter Zuern with the help of subroutines provided by Roger Broucke from JPL describing the position of the moon much more accurately than is done by the program developed by Longman previously. The description of the celestial mechanics of the Earth-Moon-Sun system was still very accurate in 2017 when compared to ephemerides, which are the basis of the most modern tidal potential catalogues. The program was originally already intended for free distribution ("open source" in 2017 speak). It has happened before that modifications were made in the main program by some people (rigtid.f) which were later detected to be faulty. It is recommended that users track the path of their version either starting at UCLA (probably not possible any more) or at BFO in order to be sure that no unwanted modifications crept in. If the program is used and an author wants a reference, the above (short) publication is the only one available.

Source code of Fortran program

https://gitlab.kit.edu/kit/gpi/bfo/tides/rigtid

Broucke, R. A., Zuern, W. E., Slichter, L. B., 1972 The Lunar Tidal Acceleration on a Rigid Earth. in "Geophysical Monographs 16": Flow and Fracture of Rocks" (The Griggs Volume, Eds. Heard, H. C., Borg, I. Y., Carter, N. L., Raleigh, C. B.), Am. Geophys. Union, Washington, 319 - 324. DOI: 10.1029/GM016p0319