twenty times larger in the sky than it 

 does today. The image gives fresh 

 meaning to a perennial question, 

 "Why does the full Moon look so large 

 on the horizon?" 



In case you're wondering about that 



Moon was having a simultaneous and 

 similar effect on Earth. When you do 

 the math, you find that all that friction 

 and bulge-making slowed Earth's rota- 

 tion, slowed the Moon's rotation, and 

 slung the Moon into ever-higher or- 



The Earth-Moon system will then 

 have achieved a "double tidal lock." 

 This never-invented wresting hold 

 may sound rare, but it's actually com- 

 mon, particularly among double-star 

 systems in our galaxy. Right here in 



12, 000-mile boundary, it's known as 

 the Roche limit. Inside that limit. 

 Earth's tidal force exceeds an object's 

 ability to hold itself together solely 

 through the strength of its own gravi- 

 ty. So if you were a pile of rubble — 

 rather than a living organism, held to- 

 gether by molecular bonds — and you 

 had the bad luck to wander into that 

 zone, you would swiftly disassemble in- 

 to your component rocks. 



To most people, tides are just the 

 daily back-and-forth sloshing 

 that takes place where the ocean meets 

 the shore. But that's just the most visi- 

 ble sign of what happens when one side 

 of a rotating cosmic object finds itself 

 closer than the other side to a strong 

 source of gravity. As the object rotates, 

 gravity pulls more powerfully on the 

 side facing that source than it pulls on 

 the far side, raising tides even in solid 

 matter. "Solid body" tides stretch and 

 pull solid matter, rhythmically deform- 

 ing the object and thus causing friction 

 deep within. 



The deformations show up in the 

 object as bulges, which would nor- 

 mally align with the offending source 

 of gravity. But because the newborn 

 Moon rotated quickly, its tidal bulge 

 ended up a bit ahead of, rather than 

 ■limed right at, Earth. Meanwhile, the 



bits. And as the orbits grew progres- 

 sively larger, the strength of the tidal 

 forces precipitously dropped. 



Since its youth, then, the Moon has 

 been rotating so slowly that it takes ex- 

 actly the same amount of time to com- 

 plete one full rotation as it does to ex- 

 ecute one full orbit of Earth. In fact, 

 Earth has locked the Moon into that 

 arrangement — a natural culmination of 

 their tidal pas de deux — just as Jupiter 

 has locked its inner satellites and Pluto 

 has locked Charon, its largest moon. 

 Whenever two bodies reach this stage, 

 you get a "far side" dilemma: observers 

 on one object (say, Earth) never get to 

 see more than one side of the object 

 that orbits them (the Moon). 



The Moon's tidal forces on our 

 planet continue to slow Earth's rota- 

 tion. Every century, the length of our 

 day increases by about one and a halt 

 milliseconds. (To keep up, earthlings 

 invented leap seconds, but that's an- 

 other story for another afternoon.) 

 Meanwhile, the Moon's orbit is con- 

 tinuing to grow, by about one and a 

 half inches per year, and so the lunar 

 month is getting longer. What's going 

 on is that the Moon is trying to get 

 even and give Earth its own far side. 

 That will happen when Earth's rota- 

 tion rate has slowed enough to be 

 equal to the Moon's orbital period. 



our own backyard, Charon has man- 

 aged to lock Pluto just as Pluto has 

 locked Charon. 



By the time the Moon tidally locks 

 Earth, the system will have slowed down 

 so much that the Earth day and the 

 lunar month will both last almost fifty 

 present Earth days, greatly simplifying 

 the calendar. Long before that, though, 

 the Sun will become a red giant and 

 vaporize the Earth-Moon system. But 

 let's ignore that complication. 



Consider instead the Sun's tidal in- 

 fluence on the Earth-Moon duo. The 

 Sun, too, is busy doing the tidal lock — 

 perpetually slowing Earth's rotation so 

 that, if there were no Moon, our plan- 

 et would eventually show the Sun on- 

 ly one face. Meanwhile, the Moon will 

 reverse its earlier trend and begin to spi- 

 ral back toward Earth. Eventually the 

 Moon will drift within the Roche lim- 

 it, break apart, and end up, once again, 

 briefly resembling the rings of Saturn. 



The ever-changing lunar orbit is, 

 by happy coincidence, just the 

 right size to give sky watchers a thrill. 

 The Sun is roughly 400 times larger 

 than the Moon, but for the moment, 

 it's also about 400 times farther away. 

 To an observer on Earth, then, they 

 both appear about the same size on the 

 sky. So dumb luck makes for striking 



36 



NATURAL HISTORY Apnl2006 



