Author: Arvind Gupta
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As we saw in the earlier video cause of season is the tilt of earth of 23.5 degrees. This leads to sun's ray fall more directly over northern hemisphere than southern hemisphere. So northern hemisphere has summer and at the same time southern hemisphere has winters. As we enjoy the lovely mangoes here in India during our summers we hardly realize that summers are actually longer for us in Northern hemisphere by 3 almost days. Correspondingly the people living in the Southern Hemispjere (Australians) shiver in winters by 3 extra days. Let’s look at reasons behind this inequality. The root cause of this inequality lies in the theory discovered by Johannes Kepler in 1609. In his first law he proved that orbit of earth is slightly elliptical and the sun is at a slight offset from the middle, at the focus of ellipse. What this means is that sun will closer to sun at some point (which is called the perihelion). The farthest point is called aphelion when the northern hemisphere is enjoying summers. In his second law Kepler also said the earth sweeps equal areas in orbit in equal time. It means that to cover equal areas in same interval of time, when earth is far from sun it has to move less distance than when it is closer. This might sound complicated, but what it essentially boils down to is that speed of earth is faster when it is closer to sun and slower when far away. Currently the perihelion or the closes point to sun happens when the northern hemisphere is enjoying winters. So the speed of earth around the sun is faster when we in Northern Hemisphere have winters. So the winters in Northern hemisphere are shorter and summers longer. Correspondingly it is slower when Northern Hemisphere has summers as earth is close to farthest point called the aphelion. Since the earth orbit is not slightly elliptical this difference is not large and we get to enjoy longer summers by 3 days. If our orbit would like Mars which is around 10% elliptical orbit the difference between summers and winters would be much larger 25 days. But there's a flip side to this because that means that the Northern Hemisphere summer is four days longer than the southern hemisphere summer. Which helps explain why July is our planet's warmest month, At that point the Northern continents that are pointing towards the sun get a few days more to bake in the sun and raise the average temperature of the entire globe? I hope you enjoyed the reasoning behind slightly longer summers or winters as the case may be depending which part of the hemisphere you live on. This work was supported by IUCAA and Tata Trust. TATA Trust: Education is one of the key focus areas for Tata Trusts, aiming towards enabling access of quality education to the underprivileged population in India. To facilitate quality in teaching and learning of Science education through workshops, capacity building and resource creation, Tata Trusts have been supporting Muktangan Vigyan Shodhika (MVS), IUCAA's Children’s Science Centre, since inception. To know more about other initiatives of Tata Trusts, please visit www.tatatrusts.org