Why bridges don't sink into the rivers they span, or why bicycles are different sizes and shapes? Why are some buildings made of wood and others are made of steel and glass? Because they are made that way right?
Earth—Moon system[ edit ] Discovery history of the secular acceleration[ edit ] Edmond Halley was the first to suggest, in that the mean motion of the Moon was apparently getting faster, by comparison with ancient eclipse observations, but he gave no data.
When measured as a function of mean solar time rather than uniform time, the effect appears as a positive acceleration. Delaunaywas eventually accepted.
A part of the answer was suggested independently in the s by Delaunay and by William Ferrel: But eventually it became clear that three effects are involved, when measured in terms of mean solar time.
The mass of the Moon is sufficiently large, and it is sufficiently close, to raise tides in the matter of Earth. In particular, the water of the oceans bulges out towards and away from the Moon. As a consequence, there exists a substantial amount of mass in the bulge that is offset from the line through the centers of Earth and the Moon.
Since the bulge nearer the moon pulls more strongly on it than the bulge further away, this torque boosts the Moon in its orbit and slows the rotation of Earth.
As a result of this process, the mean solar day, which is nominally 86, seconds long, is actually getting longer when measured in SI seconds with stable atomic clocks.
The SI second, when adopted, was already a little shorter than the current value of the second of mean solar time. This led to the introduction of the leap second in  to compensate for differences in the bases for time standardization.
At that time, the Moon would always be overhead of a single fixed place on Earth. Such a situation already exists in the Pluto — Charon system. About 1 to 1. Even without this, the slowdown to a month-long day would still not have been completed by 4.
Up to a high order of approximation, mutual gravitational perturbations between major or minor planets only cause periodic variations in their orbits, that is, parameters oscillate between maximum and minimum values. The tidal effect gives rise to a quadratic term in the equations, which leads to unbounded growth.
In the mathematical theories of the planetary orbits that form the basis of ephemeridesquadratic and higher order secular terms do occur, but these are mostly Taylor expansions of very long time periodic terms.
The reason that tidal effects are different is that unlike distant gravitational perturbations, friction is an essential part of tidal acceleration, and leads to permanent loss of energy from the dynamic system in the form of heat. In other words, we do not have a Hamiltonian system here.
As in any physical process within an isolated system, total energy and angular momentum are conserved. The actual speed of the Moon also decreases. Although its kinetic energy decreases, its potential energy increases by a larger amount. The rotational angular momentum of Earth decreases and consequently the length of the day increases.
Tidal friction is required to drag and maintain the bulge ahead of the Moon, and it dissipates the excess energy of the exchange of rotational and orbital energy between Earth and the Moon as heat.
Most of the dissipation occurs in a turbulent bottom boundary layer in shallow seas such as the European Shelf around the British Islesthe Patagonian Shelf off Argentinaand the Bering Sea.
Oceanic tides actually rotate around the ocean basins as vast gyres around several amphidromic points where no tide exists. The Moon pulls on each individual undulation as Earth rotates—some undulations are ahead of the Moon, others are behind it, whereas still others are on either side.
Historical evidence[ edit ] This mechanism has been working for 4. There is geological and paleontological evidence that Earth rotated faster and that the Moon was closer to Earth in the remote past.
Tidal rhythmites are alternating layers of sand and silt laid down offshore from estuaries having great tidal flows.
Daily, monthly and seasonal cycles can be found in the deposits. This geological record is consistent with these conditions million years ago: The average recession rate of the Moon between then and now has been 2. The present high rate may be due to near resonance between natural ocean frequencies and tidal frequencies.
Laser pulses are bounced off mirrors on the surface of the moon, emplaced during the Apollo missions of to and by Lunokhod 2 in These measurements are fitted to the equations of motion.
From the period —, the results are: The model accurately predicts the changes in the motion of the Moon. Finally, ancient observations of solar eclipses give fairly accurate positions for the Moon at those moments. Studies of these observations give results consistent with the value quoted above.The PowerPoint PPT presentation: "FORCES AND THEIR EFFECTS" is the property of its rightful owner.
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Any conflict lasting this long would cause changes, and the aftermath of the wars affected both nations. The Japanese were not depending solely upon the volume of their air strength in these initial engagements, although they believed they possessed sufficient superiority in numbers over Allied air forces in the Pacific.
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