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Originally Posted by CraigS
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No arguments with that. However, we can roughly determine what the orbit of the planet was at any one particular time by looking at certain sections of the rock record, especially within the sedimentary rocks. Looking at varves or sediments from ancient salt lakes, for example, we can gauge an approximate value for how many days there were in the year at the time of deposition and knowing what the rotational period of the planet was at that particular time (by knowing the co-efficient of tidal drag on the crust/oceans etc etc) we can then give an approximation of the orbit. For example, glacial varves (lake deposits) from the Middle Devonian show that the Earth year was around 411 days in length. Knowing from other research that the day length at the time was around 22 hours, it can be calculated that the orbit of the planet at the time was more elliptical than it is now...most probably the planet swung out to 98-100 million miles and came into about 88-89 million. Meaning the eccentricity of the orbit was around 0.03 instead of being 0.017 as it is at present.
However, despite being able to do that in some instances, as Craig has said, we cannot predict what the orbit will be at any one stage over long periods of time with any certainty, due to those chaotic gravitational influences etc. We can only take approximate snapshots, as above, only if we have enough evidence and such to be able to do so.