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For much of the past 70,000 years, the Sahara has closely resembled the desert it is today. Some 12,000 years ago, however, a wobble in the Earth's axis and other factors caused Africa's seasonal monsoons to shift slightly north, bringing new rains to an area nearly the size of the contiguous United States. Lush watersheds stretched across the Sahara, from Egypt to Mauritania, drawing animal life and eventually people.
Noobfun: its called precession its a wobble in the earths axis that has been known about for a long time
On the other hand, while the orbits of some particles are quite dispersed, it is still likely that the Taurid stream has a narrow and dense core consisting of particles concentrated near the orbit of the stream's parent object, which is presumably related to Comet 2P/Encke. As the orbits of the material constituting this narrow, dense core have been subject to perturbations over thousands of years, it may be inferred that intense bombardment episodes have resulted at epochs when the material reaches Earth intersection. Dynamical calculations show that, as a Taurid-like orbit precesses, the northern daytime intersection occurs just a little (a few centuries) before the southern nighttime one, and the southern daytime one just before the northern nighttime one. That is, the four intersections occur in two pairs, and the influx of material to Earth is enhanced during epochs lasting a few centuries and spaced by a few millennia. The term "coherent catastrophism" has been used by astronomers at Armagh and elsewhere to describe the idea that there are strong patterns in the influx of extraterrestrial material to Earth.
>Let me first summarize from a preprint of a paper that Duncan Steel
>gave me. The paper is titled "The Tapanui region of New Zealand: a
>Tunguska' of 800 years ago?" by Duncan Steel and Peter Snow. Some of
>this will be practically verbatim: First they provide some evidence
>supporting the fact that the Earth is struck quite frequently by
>astronomical objects. The frequency of encounters with an object capable
>of producing a Tunguska-like explosion is of the order of every few
>centuries or so (Morrison and Chapman, 1989), although the data on which
>these rates are based are fuzzy.
>They suggest that the southern part of the South Island of New Zealand
>may have been the target of such a projectile about 800 years ago or so
>and that scientific study of the region is warranted.
For much of the past 70,000 years, the Sahara has closely resembled the desert it is today. Some 12,000 years ago, however, a wobble in the Earth's axis and other factors caused Africa's seasonal monsoons to shift slightly north