In 1851 Foucault in his pendulum experiment found the incontrovertible proof of the rotation of the Earth. In the experiment a 28kg spherical mass was suspended on a long wire from the ceiling of the Pantheon in Paris. The pendulum was set in motion swinging in one particular direction. A magnetic device centered in the floor beneath it acts upon a cylinder inside the spherical mass, assuring continual motion (it compensates for friction and air resistance thus allowing perfect manifestation of the law of the pendulum).
If we imagine that the tip of the pendulum traced a path in a layer of damp sand on the floor of the Pantheon, we may fall victim to the illusion that the Pendulum’s plane of oscillation has moved full circle… in thirty two hours, describing an ellipse that rotates around its centre at a speed proportionate to the sine of its latitude (Umberto Eco, Foucault’s Pendulum) It is an illusion, of course, because it is not the pendulum’s plane of oscillation that changes but the position of the Earth beneath it.
This discovery, combined with Henderson’s observation of the stellar parallax of Alpha Centauri in 1838, prove not only that the Earth rotates but also that it is orbiting the Sun. Because the rotation of the Earth is a relatively recent discovery, there are no ancient records specifically monitoring such motion. Today, if we wish to discover the past rate of the Earth’s rotation, we must gather indirect evidence from ancient texts concerning equinoxes, meridian altitude data, observations of planetary conjunctions, occultation of stars by the moon or records of lunar and solar eclipses.
Such records will allow us to determine the length of the mean solar day at the time and “the length of the day is clearly a measure of the speed at which the Earth rotates” (Morrison, 1985). F. Richard Stephenson, in his work “Historical Eclipses and Earth’s Rotation”, comments that for such data to be of any use it must fulfill five criteria: – The observation must involve one of the brighter and more rapidly moving objects in the solar system (i.
e. the Moon, Sun or one of the inner planets Mercury, Venus and Mars). The exact Julian or Gregorian date of the observation must either be specified directly or be able to be determined unambiguously. – A reasonable accurate value of Universal Time must be reducible from the reported circumstances, or – as in the case of total solar eclipses – the rotational phase of the Earth must be able to be derived.
– The corresponding Terrestrial Time must be able to be precisely calculated from the dynamical equations of motion of the celestial body concerned. In most cases the geographical position of the observer must be known with some precision – preferably within a small fraction of a degree in both latitude and longitude (Stephenson, 43) Given these criteria, observations of solar and lunar eclipses emerge as the most useful data set.
Because of their importance to astrologers and due, also, to the fact that they are often impressive events; eclipses tend to be well documented. In fact observation of eclipses stretches back as far as the 7th Century BC and given that “the position at which an eclipse can be observed is a sensitive indicator of how far the Earth has turned between that time and today” (Morrison, 1985), such information is “the principle source of data for information on long-term variations in the Earth’s rate of rotation”.
Today we can monitor the rotation of the Earth much more closely using techniques such as; Occultation of stars, this is now a lot easier, more useful and more accurate since the invention of the telescope; Systematic monitoring relative to the atomic time scale, where an “atomic second” is derived from one of the frequencies at which a cesium atom vibrates and; Lunar laser ranging, whereby scientists fire a laser beam through an optical telescope pointed at reflectors placed on the Moon during landings, and the time for the beam’s return journey is recorded.
This tells us how far away the Moon is which is useful because of the fact that the Moon’s distance increases in proportion to the slowing rate at which the Earth spins. In fact ‘the distance to the farthest point of the lunar orbit is increasing by about 3. 8cm/year. Such detailed techniques have yielded some interesting observations. It has been shown that there are two types of fluctuation in the rate at which the Earth’s rotational energy dissipates: – Annual and inter-annual variations – Decade and centennial fluctuations Variations in (1) are “due to the pattern of winds in the atmosphere…
a periodic transfer of angular momentum from the atmosphere to the solid Earth, mainly through the force of the wind against mountain ranges” (Morrison, 1985). Fluctuations in (2) can be traced over the last 350 years or so, mainly using timings of occultation’s of stars by the moon. “The principle causal mechanism would appear to be angular momentum between the outer fluid core of the Earth and the solid mantle by the process of electromagnetic coupling. However, small alterations in global sea level owing to freezing or melting of polar ice caps may also partly responsible. ” (Stephenson, 27)
It is possible to assume that the early history of the moon is quite different and presents proc¬esses which are quite different from those that we observe on the earth at the present time. In fact, it presents processes that may have been charac¬teristic of the early history of the earth. The whole surface of the moon is so covered with collisions that the entire surface must have been thoroughly destroyed and converted to rubble and fine powder. If the moon was treated in this way, one would expect that the earth would also have been subjected to collisions of the same kind.
Since we have rocks now on the earth that are at least 3. 5 billion years old, the intense collisions on the moon must have occurred before that time. The experiments and chemical observations performed on the materials from the Apollo flights showed that a melting process must have occurred on the moon about 4. 6 billion years ago, and that this was followed by a solidification proc¬ess with the whole surface of the moon becoming quite rigid. From Apollo 12, scientists have found one small rock, the famous #13 rock, that itself has not been remelted in the course of 4.
0 billion years. It has a most remarkable chemical composition indicating an extreme chemi¬cal differentiation of original meteoritic material. The density of the moon is approximately 3. 36 g/cm3, which makes 3/5 the density of the Earth. Since this should indicate the composition of the moon as a whole, the moon may contain some 8 or 10 per cent of iron, the rest being that of the meteorites, whereas the earth has about 30 per cent of iron on the basis of the best estimates One of the assumptions can be that the interior of the moon contains about 2 per cent of water in its rocks.
However, we now know that the surface rocks of the moon are very dry, containing only a couple of hundred parts per million of water, and it is a little difficult to im¬agine that there are large amounts of water on the interior. The studying of the Moon represents large opportunities for scientists. What happened 4. 6 bil¬lion years ago on the earth is completely wiped out by volcanism, by sedimentation and so forth. We have no record of the earth older than 3. 5 billion years, but Moon has.
From the critical point of view, because there is no atmosphere and no surface water on the Moon due to its low gravity, traditional weathering processes do not occur there. But, as S. K. Noble reveals in her research the Moon experiences so called space weathering. Space weathering represents the sequence of processes that occur on Moon’s surface, in out case, as it does not have an atmosphere to protect it. Therefore, “weather” on the Moon occurs in the form of cosmic and solar rays, micrometeorite bombardment, vaporization and solar wind implantation (Noble, 2005). Bibliography
Eco U (1989), Foucault’s Pendulum, Ballantine Books, Random House NY Stephenson F R (1997), Historical Eclipses and Earth’s Rotation, Cambridge University Press Morrison L V (1985) The Day Time Stands Still, New Scientist, vol 106, 27 June 1985 NASA, The Moon Page < http://nssdc. gsfc. nasa. gov/planetary/planets/moonpage. html> It contains any possible statistical and scientific data on the Moon. S. K Noble MY Research < http://www. planetary. brown. edu/~noble/Myresearch. html> The web resource provides excellent information on the space weathering, particularly how space weathering impacts Moon’s soil.