Robin Walker

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