Article - Solar system

When the Moon Becomes Earth's Nemesis
Jul 26, 2013 12:49 PM ET // by Mark Thompson
With the catchy name S/2004 N 1, a tiny 20 kilometer (12.4 mile) wide chunk of rock became the 14th
moon to be identified in orbit around the giant planet Neptune. A little closer to home, however, the debate
continues as to exactly how many moons Earth has, but there is one whose identity cannot be mistaken.
It has been seen by nearly every human being who has ever lived and its often mystical appearance has
inspired artists and musicians over the centuries. Without it, our planet and lives would be very different
If you live by the coast then you will be familiar with one of the more visual effects the moon has on our
lives, the tides. All objects exert a gravitational pull on all other objects and it is the force of gravity from
the Earth that pulls on the moon keeping it in orbit.
The moon too has a gravitational pull which tugs at the Earth and exerts a tidal force, which can be seen by
the regular rising and falling of the oceans. The highest tides seen on Earth have been recorded at the Bay
of Fundy in Canada with a whopping 16.2 meters (53 feet).
It’s not just the Earth's water which is affected by the tides -- the land too is pulled and distorted by a meter
or so.
It would be reasonable to think that the tidal bulge on the Earth produced by the moon sits directly on a line
between the two objects. But due to the rotation of the Earth and the reluctance of material to move, the
bulge is actually dragged slightly ahead of the moon.
The gravity from this extra little lump of material adds a little extra tug on the moon’s orbit, accelerating it.
As we know from Kepler’s laws of motion, when something is sped up in orbit, its orbital altitude will
increase. In the case of the moon, it is moving away from us at a rate of 3.78 centimeters (1.5 inches) per
year. In addition, the moon also exerts a tiny pull on the tidal bulge and because the moon is slightly behind
the bulge, it slows the rotation of the Earth by about 4 hours every billion years!
Don’t go getting too excited about days getting longer though because this Earth-moon tidal tug-o-war may
have devastating effects (in a few billion years).
The Earth is like a giant plate spinning on top of a performer’s pole and, like the plate, if it spins too
slowly, the wobble will get bigger until it falls over. OK, so that is a pretty abstract analogy but if the Earth
does slow down too much then its natural wobble will become more extreme.
This will lead to a more extreme tilt to the polar axis – that would bring some increasingly extreme seasons.
It is quite likely that humans would survive such an event because we are able to adapt quickly and even
use technology to help. The planet’s ecosystem, on the other hand, may have problems evolving fast
enough, succumbing to the extra hot summers and sub-arctic winters.
So, the next time you take a walk by the coast or look up at the moon, just remember that the Earth and
moon are enjoying a wonderfully symbiotic relationship at the moment. But in billions of years’ time, if it
happens before the sun dies or before we get zapped by a gamma ray burst or even destroy ourselves, then
that quiescent mystical object might just become our nemesis!
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