In this chapter, expressions are derived for the tide-generating force and the associated tide-generating potential. The Moon and Sun act as the tide-generating bodies. The declination is introduced followed by an alternative expression for the tide-generating potential in terms of terrestrial coordinates, which serves as a starting point for Chapter 4. The Moon and Sun act as tide-generating bodies; their combined effect is qualitatively shown to result in a spring-neap cycle.

This chapter provides a systematic qualitative overview of the periodicities involved in the motions of the Earth and Moon that are relevant for tides. Key features are the ellipticity of the orbits and the declination. The celestial origin of the different years (sidereal, tropical, anomalistic) is explained, and the same for months (sidereal, tropical, anomalistic, synodic) and days (sidereal, solar, lunar). The long-period variations (lunar apsidal precession and lunar nodal cycle) are also explained. The implications of the solar tide-generating force on the Earth–Moon system are outlined (evection and variation). The chapter ends with a convenient list of all the relevant periods.