When people perform experiments of any sort, the professional people only change one variable so that they know what is being changed. For instance if I compared the effect of the growth of flies, and one group had more food and cleaner water, and the other group had neither, it would be pointless. It would be pointless because even if the experimental results were a success, you wouldn't know for sure which variable caused the change, the water or the food variable. That's why there can only be one man-made variable at a time.
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Answer:
The Equinox (Vernal & Autumnal) There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a "nearly" equal amount of daylight and darkness at all latitudes. These events are referred to as Equinoxes.
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The northern hemisphere is located between the Equator and the North Pole.
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