信以传信，疑以传疑

中国的传统是抄书，除非有十分的必要，都是把以前的书拿来抄上；外国的传统是原创，除非有十分的必要，都是要用自己的话来说道。但不管是哪种方法，都不免有抄走样的时候，而且是几乎不可避免的，抄着抄着就走样了。

 还是拿洋人做例子吧。

前几天看了本书[1]，里面说的好像托勒密能够非常精确地观测到土星轨道似的：

The Greek astronomers understood that the planets did not move in simple circles about the earth, but had somewhat more complex motions. Figure 2.1 shows Ptolemy's observations of the motion of Saturn in AD 133 against the background of the fixed stars. Rather than move in a smooth path across the sky, the path of the planet doubles back upon itself.[2]

这里说的是,“托勒密在公元133年观测到的土星运动”（Ptolemy's observations of the motion of Saturn in AD 133）。

从他给出的图来看，轨道测量精度似乎优于1角分，而且好像是托勒密观测到的：

Figure2.1: The motion of Saturn from 5 December AD 132 to 20 December AD 133 as observed by Ptolemy against the background of the fixed stars. (From O. Pedersen and M. Pihl, 1974, Early Physics and Astronomy, p.71, London: McDonaldand Co.)[3]

这里说的有些模糊，似乎是说，“从132/12/05到133/12/20，当托勒密观测时的土星运动”（The motion of Saturn from 5 December AD 132 to 20 December AD 133 as observed by Ptolemy）。

可是几页之后就说了，1400多年以后，第谷把观测精度提高到1角分的程度，比此前的水平提高了大约10倍：

In other words, the random errors in Tycho’s final determination of the planetary orbits amounted to only about 1 to 2 minutes of arc, whereas the minimum discrepancy which Kepler could find was at least four times this observational error. Before the time of Tycho, the random errors were about ten times greater and therefore Kepler would have no problem in fitting these earlier observations to models involving circular orbits. [4]

这有些奇怪，我碰巧闲得没事，就去找来那本书[5]，发现那里是这么说的：

Fig.7.2  The motion of Saturn in A.D. 133(when it was observed by Ptolemy) showing its first visibility $\Gamma$, the first stationary point $\phi$, the opposition $\theta$, the second stationary point $\psi$, and the last visibility $\Omega$. The motions in longitude and latitude produce a loop whose appearance may vary greatly from one opposition to the next.[6]

这样就明白了，这位作者说的是，“土星在133年的运动（那时候托勒密观测了它）”（The motion of Saturn in A.D. 133 (when it was observed by Ptolemy)）。其实这张图只是后来人（很可能就是这位作者）画的、133年的托勒密可能观测过的、土星实际运行的轨道图。托勒密应该是不可能达到这样的观测水准的，甚至托勒密自己有没有观测过，也还两说呢。

如果不是我读着读着就迷糊了，就是最前面那位作者抄着抄着就走样了。

[1] Longair, M.S., Theoretical Concepts in Physics (revised andenlarged, second edition), Cambridge University Press, 2003.

[2] Theoretical Concepts in Physics, Chapter 2, section 2.1,page 15

[3] Theoretical Concepts in Physics, Chapter 2, section 2.1,page 16

[4] Theoretical Concepts in Physics, Chapter 2, section 2.4,page 27

[5] Olaf Pedersen and Mogens Pihl, Early physics and astronomy:A historical introduction, American Elsevier, 1974.  

[6] Early physics and astronomy, page 71