Tidal Locking
Tidal locking is a phenomenon that I, when starting this post, didn't really understand. I knew that it's a term used for (astronomical) bodies that rotate on their own axis at the same rate as they orbit their parent body - or, seen another way, the day and year have the same length.
An example that is close to home is the Moon, which has a period of approximately 28 days. It is this property that causes the Moon's surface to always look the same - the famous valleys and mountains in the night's sky
But, the Moon is not the only body in the solar system with this property - there is also Mercury1. Making the base assumption that our solar system is not too far from the norm, this behaviour seems to be rather common - which strikes me as odd, as I can't immediately see a physical reason for it.
Thus, I went off and started reading. I quickly found out two things: Mercury is not tidally locked (although, it took until 1965 for this to be confirmed - it's doing something slightly more interesting which you can read about yourselves). And that tidal locking is even more common than I had previous thought. The Wikipedia article has both a list of tidally locked bodies in the solar system2 and a list of those only suspected to be3
There are plenty of explanations for why on the web and, in retrospect, I probably should have been able to work out why - you can't treat planets as point particles. Consider for a moment, your own body in Earth's gravitational field. As I suspect you may already know, the force exerted on a body is inversely proportional to the distance between the bodies squared, and proportional to the mass of each object. Less accurately, but more usefully, the classical acceleration felt by a body due to the gravitational pull of another body is [latex] \frac{m G}{d^2} [/latex] where m is the mass of the other body, d is distance between the too bodies, and G is the gravitational constant.
Taking the meaning of body to be a human body, we can firstly find out why people like hugs - as a separation of approximately 30cm between centres of mass, there is an acceleration (using my mass at the other body) of 74 nano meters per second squared. Assuming both bodies are accelerating towards each other at this rate, the enter of masses should reach each other 24 minutes of hugging.
Anyway, I digress - although, hopefully helpfully, as anyone who has had a 24 minutes hug should know that this is not what happens. This can be attributed to the fact that humans are not points of mass, which leads us to another interesting diversion - as you feet (when standing) are probably between one and two meters closer to the centre of mass of the Earth than your head is, and is thus they will be experiencing different levels of force - roughly $$6.14\mu m / s^2$$, which is notably more than caused by hugging. Similarly, walking up stairs all the time will do great things for your measured weight
With these silly things in mind, back to the realm of celestial bodies. When the Moon was not tidally locked, and rotating, it was still in Earth's gravitational field and, as it rotated, different parts were being pulled towards the Earth at different rates. Now, the moon is quite rigid - unlike the ocean tides on Earth that are a noticeable bulge that follows the moon around, the effect would be very small. However, as the slightly bluging face of the moon begins to rotate past the Earth, slightly more mass will be on one side than the other.
This mass is then pulled back towards the Earth, against the relative rotation of the planet. At the same time, the original process of pulling mass from everywhere is on going. The tidal bulges are dragged creating torque, is how it seems to be put more formally. There is one more mechanism at work - the conservation of angular momentum, which causes the change in rotation to create and opposite change in orbital frequency, contributing to the process of tidal locking
This is, as with all things involving gravity, a symmetric process. Because the difference in mass of the objects is normally large, this process seems to only happen to the smaller body; that said, the Earth day is changing in length ever so slowly.
- 1 ↑ This is wrong. Bare with me
- 2 ↑ http://en.wikipedia.org/wiki/Tidal_locking#Solar_System
- 3 ↑ http://en.wikipedia.org/wiki/Tidal_locking#Solar_System_2