Tidal circularization

Effect of tidal forces on an orbiting body Tidal circularization or orbital circularization is an effect of the tidal forces between a body in orbit around a central celestial object, whereby the eccentricity of the orbit is reduced over time so that it becomes less and less elliptical. Typical situation Figure 1: Torque applied to orbit caused by tidal bulge and lag angle In Figure 1, consider two stars, denoted Body 1 and Body 2. Initially think of Body 2 as a point mass. The gravity from Body 2 applied to Body 1 produces tidal bulges (see Tidal Force). Let's assume the orbital period is slower than the rotation of Body 1 (ω < Ω) as shown in figure 1. One might expect a lag angle as shown. If Body 1 is 100% elastic (e.g. gas bodies are usually very elastic but a bag of sand is not very elastic) then the bulge would not have a lag angle. The more inelastic, the larger the lag angle. The larger the difference in angular velocities (ω/Ω), the larger the lag angle. If ω > Ω, the lag angle will be in the other direction. For a star we can think of inelasticity as viscosity. The main cause of inelasticity in a star seems to be convection forces inside the star. When the lag angle