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 Subject: moon rotation Category: Science > Astronomy Asked by: alb-ga List Price: \$2.00 Posted: 04 Nov 2002 13:47 PST Expires: 04 Dec 2002 13:47 PST Question ID: 98625
 ```Why does our moon always face the earth while others rotate with respect to their planets? How exactly does ours face us; that is, is there ANY change over the centuries, however miniscule?```
 Subject: Re: moon rotation Answered By: alienintelligence-ga on 05 Nov 2002 00:19 PST
 ```Hi alb This is a really good question. The answer lies in the same reason we have high and low tides on Earth. The mass of the Earth tugs on the Moon in the same way the moon pulls on the Earth. Since the Earth is so much greater in size it is able to distort the sphere of the moon. The Earth draws mass from the Moon towards it. This elongation of the Moon reduces its rotational velocity and converts it to heat. Eventually the spin is nearly nullified. The elongation is drawn along an axis pointing to the Earth. It results in permanent tidal bulges which makes it dynamically most stable if one end of the bulge is always pointed towards the Earth. This process is called Tidal Locking. A very gradual process. The dynamics of it are explained in detail here: "[21.06] Tidal Despinning Timescales in the Solar System" C.F. Chyba (SETI Institute and Stanford University), P. J. Thomas (U. Wisconsin, Eau Claire) [ http://www.aas.org/publications/baas/v30n3/dps98/320.htm ] "Planets and satellites in the Solar System despin to a spin-evolved end-state due to tidal dissipation. The usual derivation for the despinning timescale sets the change in spin angular momentum equal to the gravitational torque acting on the object's tidal bulge (MacDonald 1964, Goldreich and Soter 1966, Peale 1974, 1977). The despinning timescale is found to be proportional to the difference between the initial and final spin angular velocities, and is finite. However, this approximate derivation ignores the orbital mean motion n of the despinning object, and is less and less satisfactory as the object's spin angular velocity w approaches n. We have instead calculated tidal despinning times by applying the formalism of Peale and Cassen (1978) to calculate tidal energy dissipation due to tides raised on a non-spin-locked object. Tidal heating in the latter case is larger than tidal heating in the spin locked case by a factor (1/7)[(w-n)/n](1/e2), where e is the orbital eccentricity. This factor is initially greater than 104 for many objects in the Solar System. Calculating despinning times from energy loss, we find that the despinning timescale includes a previously neglected term that goes to infinity logarithmically as w approaches n. In this sense all despinning timescales are in fact infinite. We therefore define an effective despinning timescale as the time required for despin tidal heating to fall below tidal heating due to orbital eccentricity. For many satellites in the Solar System, including such major moons as Io and Europa, the neglected term in the despinning timescale is in fact the dominant term. For some especially short-period satellites, such as Phobos or Amalthea, the resulting despinning timescales are one to two orders of magnitude longer than those previously accepted." -=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=--=o=- Since that's a big chunk of info here is a simpler explanation: "Tidal locking, or Orbit-spin resonance" by Anthony Lawson `Physics of the Solar system'. Feb 9, 1999 [ http://www.astro.soton.ac.uk/~ajl/course/mag/node9.html ] "Although the Moon changes phase throughout a month if we look carefully we can see that it actually keeps the same face directed towards us the whole time. This doesn't mean that it is not rotating, but that it has a rotation period the same length as its orbital period (this is the sidereal period, 27.3 days, not the synodic period). The Moon is in orbit-spin resonance, or tidally locked, with the Earth. How did this happen?" "As seen above the Earth creates a tidal bulge on the Moon which lies on the Earth-Moon line. However, if we imagine a time in past when the Moon was rotating faster, the rotation would tend to carry the bulge away from the Earth-Moon line. As it did so the Earth's gravity tried to hold it back slowing the rotation of the Moon ever so slightly. Over a long period of time this would slow the rotation of the Moon until eventually the spin period of the Moon matched that of its orbital period. When this point is reached the tidal bulge is always pointing towards the Earth and the rotation rate remains constant. This is the situation we see today. There are a number other examples of this tidal locking in the Solar system. Just like the Moon, the Galilean satellite Io has a spin period which is equal to its orbital period around Jupiter." This chart shows the orbital data of satellites in our Solar System. [ http://www.solarviews.com/eng/data1.htm ] Please ask for a clarification if there is a point you do not understand. I have provided a few search links to do additional research. "tidal locked" OR "tidally locked" moon [ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22tidal+locked%22+OR++%22tidally+locked%22+moon ] "tidal locked" OR "tidally locked" satellite [ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22tidal+locked%22+OR++%22tidally+locked%22+satellite ] "Tidal Despinning" [ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22Tidal+Despinning%22 ] thanks -AI``` Request for Answer Clarification by alb-ga on 07 Nov 2002 06:28 PST ```Thanks. I'm getting the idea but what about the second part -- how exact is the lock? From the explanation, it would seem that it would never be perfect, just getting better all the time. Also, what was your search strategy?``` Clarification of Answer by alienintelligence-ga on 11 Nov 2002 20:54 PST ```Hi again alb... For clarification you asked about the exactness of the moon's tidal locking. The term for this is libration. [ http://www.minervatech.u-net.com/moon/not_libr.htm ] "Although the Moon always presents the same face towards the Earth, due to its rotation and revolution being locked to the same period, the combined effect of these different librations allows us over time to see some 59% of its surface. " This is a great animation of it... [ http://www.minervatech.u-net.com/moon/not_libr_ac.htm ] The Inconstant Moon can provide some more data for you on our lunar friend [ http://www.inconstantmoon.com/inconstant.htm ] The search strategy for this question was at the end of my original answer, I will repost for clarity. Search terms first then URL. "tidal locked" OR "tidally locked" moon [ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22tidal+locked%22+OR++%22tidally+locked%22+moon ] "tidal locked" OR "tidally locked" satellite [ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22tidal+locked%22+OR++%22tidally+locked%22+satellite ] "Tidal Despinning" [ ://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22Tidal+Despinning%22 ] For your clarification I did these searches: moon faces earth percent [ ://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=moon+faces+earth+percent ] libration moon [ ://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=libration+moon ] thanks, -AI```
 ```Hello, The relationship between Pluto and its moon Charon may be of interest to you. Charon is in a synchronous orbit around Pluto. This means that they keep facing EACHOTHER the same way. Unlike Earth, which rotates if observed from the Moon, Charon and Pluto always look the same from eachother. Now, if we go back to the Earth-Moon case, the gravitational attraction of Moon on Earth causes tides and as Earth rotates there has to befriction between Earth's crust and the oceans. This friction is called the tidal friction and causes Work to be done against it. Therefore the angular momentum is NOT conserved and is lost. In other words, the rotation of Earth around its own axis is SLOWING DOWN due to friction mainly between the oceans and the crust. Eventually, this will cause the Earth and Moon to have a synchronous orbit just like Charon and Pluto. Actually, this will never happen because, by the time comes the Sun will have already become a red giant which threatens even the very existance of Earth and Moon.```
 ```Both answer and comment are good. There is a bit of wobble that allows us to see about 51% of the moon's surface from Earth over a period of months. I should think the bulge in the moon shifts steadily about one degree per billion years or so. Neil```
 ```I don't think Io can be "locked" to Jupiter because of Europa. My understanding is that Europa's interference keeps Io from having a perfectly circular orbit. The lack of a circular orbit will prevent synchronization (because orbital speed will change via Kepler's 2nd law, but axial rotational speed can't change), and the dragging of the tidal bulges back & forth across Io provide the heating necessary to power its volcanoes.```