Lecture 20: Tides

Why are there two high tides a day? This lecture examines another of the consequences of gravity, the twice-daily tides raised on the Earth by the Moon. Tides are a consequence of differences in the gravity force of the Moon from one side to the other of the Earth (stronger on the side nearest the Moon, weaker on the side farthest from the Moon). The Sun raises tides on the Earth as well, about half as strong as Moon tides, giving rise to the effect of Spring and Neap tides that strongly correlate with Lunar Phase. We also look at body tides raised on the Moon by the Earth, and how that has led to Tidal Locking of the Moon's rotation, which is why the Moon always keeps the same face towards the Earth. We then explore the combined effects of tidal braking of the Earth, which slows the Earth's rotation and increases the length of the day by about 23 milliseconds per century, and causes the steady Recession of the Moon, which moves 3.8cm away from Earth every year. Tidal effects are extremely important to understanding the Dynamical Evolution of many bodies in the Solar System, as we'll see time and again in the second half of the class. Recorded 2006 Oct 18 in 100 Stillman Hall on the Columbus campus of The Ohio State University.