The reason the equator experiences the least amount of seasonal variation is because

Distance of the sun from equator and parallel nature of sun rays are the two factors due to which equator experiences the least amount of seasonal variation

Explanation:

The equator of earth is quite far away from sun and also most of the sun rays are parallel to each other and thus they either affect the pole or the equator since poles are more close to sun therefore most of the time solar energy impacts the poles and not the equator. Thus , during most of the year, the climate at the equator remains hot with least seasonal variation.  

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