Earth/Moon spin orbit resonance

[parallaxicality]

Wikipedia says that the Earth and the Moon will achieve spin orbit resonance in about 2 billion years, but this seems really soon; I've heard estimates of up to 50 billion years. If anyone could point me to a good source that gives an authoritative figure, I'd really appreciate it. Thanks.

[Tim Thompson]

Reference: Solar System Dynamics (http://www.amazon.com/Solar-System-Dynamics-Carl-Murray/dp/0521575974/ref=sr_1_1?ie=UTF8&s=books&qid=1212943527&sr=1-1); C.D. Murray & S.F. Dermott; Cambridge University Press, 1999 (mine is a 2001 paperback reprint).

Earth and the moon will reach double synchronous state, where the same side of the moon always faces the same side of Earth, in 50,000,000,000 years (page 184). The 2,000,000,000 year figure comes from assuming that the dissipation of tidal energy is dominated by weak solid body tides (page 181 and table 4.2 on page 173). See the general discussion in chapter 4, "Tides, Rotation and Shape", and specifically section 4.13, "Tidal Evolution", pages 178-183 and 4.14, "The Double Synchronous State", pages 183-186.

The 50,000,000 year figure has been known for a long time. My 1924 first edition of The Earth: Its Origin, History and Physical Constitution by Harold Jeffreys (http://www-history.mcs.st-andrews.ac.uk/Biographies/Jeffreys.html) also says 50,000,000,000 years. However, young Earth creationists typically quote the 2,000,000 year figure in attempts to prove "scientifically" that the Earth-moon system cannot be as old as 4,500,000,000 years (see, i.e. The Recession of the Moon and the Age of the Earth-Moon System (http://www.talkorigins.org/faqs/moonrec.html), written by me, from the Talk.Origins Archive (http://www.talkorigins.org/)).