MadSci Network: Astronomy

Re: Why does the Earth spin and what make it spin on its own?

Date: Tue Jun 23 14:10:43 1998
Posted By: Michael Martin-Smith, Other (pls. specify below), Family Physician, Fellow,BIS, amateur astronomer( BAA), British Interplanetary Society
Area of science: Astronomy
ID: 898580250.As

The Earth, Sun and planets condensed out of a nebula ( cloud) of gas and
dust, which was in turn rotating around the galactic centre. Its own
rotation, at first, was very slow, but, as the early nebula condensed,
the speed of rotation increased - This is just like the case of  a
ballet dancer whose rotation speed increases as she draws in her hands
from an outstretched to a "by your sides" position as she spins . This
is the principle of conservation of angular momentum- though I wouldn't
bring that up in talking to a 2-3 years old!
 It continues to this day because the Earth being very heavy, has
enormous momentum, and, Space being effectively a vacuum, offers no
resistance. However there is evidence that, over geological time, the
Earth's day has grown longer- it was less than 21 hours when the first
land animals appeared, and at formation was about  4 hours. One day it
will be 47 hours. One factor slowing down the Earth's rotation is the
presence of the Moon, whose gravitational pull  provides a small
resistance. The Moon , being much lighter, has slowed into lockstep with
Earth so that its day equals its orbital period ( a month)Why are the
clouds black or gray?
 The colour depends on the bulk of water which absorbs/reflects
different wavelengths of incoming light from the Sun -and also
particulte dust embedded within the cloud. The sky normally scatters
blue light and so appears blue, but the concentrated water vapiour and
dust in turn absorbs some of the blue wavelenths. If all is absorbed the
effect is black; with gray, it is a matter of degree.

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