Whether a terrestrial mass planet orbiting a low mass (solar mass or lower) star is habitable depends on a number of factors, including the rotation rate of the planet. For example, tidally locked planets, which always show the same face to their parent stars, are likely to have harsh climates. In this talk I will discuss the spin history of Earth, including Lunar and Solar ocean tides, and the Solar thermal tide. I will describe the physics of the thermal tide. Then, using geologic and paleontological data (some from workers at Yale), I will show that the thermal tide on Earth has had a significant and clearly detected effect on the length of day, a possibility first discussed by Kelvin. I will then describe how thermal tides can prevent planets from becoming tidally locked to their host stars; Venus may provide an example. I will also describe work showing that this is true even for planets with much less massive atmospheres than that of Venus, including planets similar to Earth.
