|MadSci Network: Astronomy|
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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