What Is the Depletion of Ozone Hole?
In 1996 studies conducted by US National Aeronautics and Space Administration revealed a big hole greater than the total surface area of North America over the...
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When Was the Sextant Invented?
The principle of the instrument was first implemented around 1731 by John Hadley (1682–1744) and Thomas Godfrey (1704–1749), but it was also found later in the...
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What Is Twilight?
Twilight is the faint light which appears a little before sunrise and again after sunset before it gets really dark. On the Moon there is no twilight. Darkness comes...
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What Makes People from Different Parts of the World Have Different Skin Colors?
What Makes People from Different Parts of the World Have Different Skin Colors? People noticed long ago that people from different parts of the world often had...
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Where Did Calendars Begin?
Where Did Calendars Begin? Early in history man began counting time by days, months and seasons and so had the beginnings of a calendar. When he studied the supposed...
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