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The Earth in Time
The Earth completes its journey around the Sun (one year) in just over
365 days, a day being the time it takes to rotate once on its axis. The
length of a year has probably changed very little since the Sun and its
attendant planets formed from the collapse of a cloud of interstellar
material, but it has not always been 365 days. This is because the length
of a day is slowly increasing at a current rate of 1.4 thousandths of
a second per century, as the Earth's rotation rate slows due to the effects
of the solar and lunar tides. This same effect is causing the distance
to our Moon to increase at a rate of 3.7 centimetres per year -- known
quite accurately since laser beams have been bounced back from reflectors
on the Moon placed there by Apollo astronauts. 1.4 thousandths of a second
per century takes a very long time to have any significant effect -- after
all
it's only a change of 14 seconds in a million years -- but if the rate
has always been the same since the Earth was born (unlikely to be exactly
true!) then the change in this time will have been 18 hours, making a
year about 1500 early-solar-system Earth-days rather than the 365 current
Earth-days.
The age of the solar system used to make this estimate is 4,600 million
years and comes from radioactive dating of rock samples from the Allende
meteorite, the oldest known solar system material. Rocks from the lunar
highland regions are only slightly younger than this. The interstellar
cloud which collapsed at this time to form the Sun and planets may well
have been triggered to do so by the shock-wave from the explosion of an
earlier, more massive star, formed perhaps 10 million years before the
Sun, as calculations suggest that a cloud with a mass small enough just
to form the Sun should not have been unstable to collapse of its own accord.
The formation of the Earth and the other planets around the young Sun
will have taken place over a period of perhaps a few million years, as
dust and ice particles aggregated together to form small bodies, these
small bodies collided and stuck to form larger bodies, and then these
larger bodies aggregated to form planets. Some collisions in the latter
stages of this process may have been on a very large scale -- the currently-favoured
idea on the formation of the Earth-Moon system is of the collision of
a body of similar size to Mars with the proto-Earth, resulting in extensive
melting of the material and a final system in which one body (the Moon)
is much less dense than the other (the Earth) which contains all the dense
iron core material of both proto-objects (the Giant Impact theory). The
Moon, Mercury, Mars and some other smaller solar-system bodies still show
evidence of the final phases of planet formation in the craters on their
surfaces, but the Earth has such an active surface, with crustal plates
moving around on its thickly-fluid mantle, that here no such features
remain. Such craters as are visible are relatively recent -- for example
Meteor Crater in Arizona is only 50,000 years old. Following its molten
phase the Earth will rapidly have lost any primordial atmosphere of hydrogen
and helium and developed a secondary atmosphere from volcanic effects
and outgassing, with water vapour, carbon dioxide, nitrogen and argon
prominent. This secondary atmosphere will slowly have evolved into the
present nitrogen/oxygen mixture primarily through the formation of carbonate
rocks and the influence of plant life. The Earth occupies a privileged
position in the Solar System, where its surface temperature allows the
presence of liquid water. If it were much further out or much closer in
this would not be the case, so the length of its year -- directly related
to its distance from the Sun -- is closely-linked to the presence of life
on its surface. The basis of this exhibit -- the passing of one year --
is closely linked to the possibility of our presence here to contemplate
it.
RNF Walker, 17.5.01, earth.txt
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