Lecture 20: Tides

Why are there two high tides a day? This lecture examines tides caused by the 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 correlate with Lunar Phase. We will also discuss 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 end with a discussion of the combined effects of tidal braking of the Earth, which slows the Earth's rotation by about 23 milliseconds per day century, and causes the steady Recession of the Moon by 3.8cm away from Earth every year. Tidal effects are extremely important to understanding the dynamical evolution of the Solar System, as we'll see time and again in the second half of the class. Recorded 2007 Oct 17 in 1000 McPherson Lab on the Columbus campus of The Ohio State University.