The average synodic rotational period of the Sun is approximately 27.27 days. (synodic means 'relative to an observer on earth') The sidereal orbital period of the moon around the earth is known accurately and is 27.321661 days. (sidereal means 'relative to an observer fixed in space' i.e. 'relative to the stars'). These two figures are remarkably close, and suggest there might be some kind of tuning relationship between them. As pointed out by Janus in another thread, it is not easy to see why there should be a relationship between the sun's apparent rotational period relative to the Earth, and the time it takes the moon to orbit the earth relative to the fixed stars. However, there is another lunar period which is very close to the moon's synodic orbital period, and that is the moon's nodical period. The length of the nodical period is 27.2122 days, which is also very close to the solar synodic period of 27.27 days approximately. The nodical period is the period between two crossings of the ecliptic in the same direction. Here is a feasible method here for tuning to occur, as the sun's gravity tries to pull the orbital plane of the moon into the plane of the ecliptic. Solar asymmetries or tidal effects could possibly act over long periods to tune the moon's nodical period (and thus also its sidereal period) to the solar synodic period. (Since the sun is a fluid body, it's rotation rate varies depending on latitude, and the 'average' rate may not be directly measurable. I am not sure exactly what method is used to determine the above figure of 27.27 days, but it would seem logical to me to define it as follows: The synodic period of rotation of an equivalent rigid body of the same mass, radius, mass distribution and angular momentum as the sun. Since this 'average' period is preumably difficult to determine and conceptualise, it may only have become known relatively recently in relation to other simple orbital periods, and the co-incidence between these two periods may not have been noticed before) Since the sun's average synodic rotational period is clearly difficult to define and deduce, the small apparent variation between 27.21 and 27.27 may be due to the methods by which it is defined and derived. The effective period for the purpose of tuning the nodic period may in fact be 27.21 days. The following web page has a thorough discussion of the various periods of the lunar orbit: http://www.astronomy.org/astronomy-survival/eclipse.html [Broken] (This thread is a continuation of a sub-discussion that arose in the "Why doesn't the Moon spin? thread". I thought it deserved its own thread) Any comments?