The reason behind this phenomenon is that the amount of solar energy,i.e.,the rays of the sun, gets on decreasing as it reaches the poles. understand like this that in winters we get smaller days and bigger nights because the heat of the sun is far more less in winter than in summer season, similarly, the poles get very less sunlight than the equator, so variation in the period of day and night increases because of the lesser amount of sunlight. KEEP IN MIND THAT THE POLES ARE AWAY FROM THE SUN SO THEY ARE COOLER THAN THE EQUATORIAL REGION, AS THEY GET LESS SUNLIGHT.
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