# Problem 4.9

In this section we saw that orbits are unstable for any value of $\beta$ that is not precisely 2 in (4.12). A related question, which we did not address (until now), is

howunstable an orbit might be. That is, how long will it take for an unstable orbit to become obvious. The answer to this question depends on the nature of the orbit. If the initial velocity is chosen so as to make the orbit precisely circular, then the value of $\beta$ in (4.12) will make absolutely no difference. Of course, in practice it is impossible to construct an orbit that is exactly circular, so the instabilities when $\beta\neq2$ will always be apparent given enough time. Even so, orbits that start out as nearly circular will remain almost stable for a longer period than those that are highly elliptical. Investigate this by studying orbits with the same value of $\beta$ (say, $\beta=2.05$) and comparing the hebavior with different values of the ellipticity of the orbit. You should find that the orientation of orbits that are more nearly circular will rotate more slowly than those that are highly elliptical. (尝试利用不是2的 $\beta$ 值构造圆形轨道，并讨论各种不同轨道情况下的稳定性。)