Author: Arvind Gupta
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Now let’s look at the real cause of seasons. Let’s take a source of light and shine it on a paper horizontally. We will see a circle of light. Now if we turn the paper we will see an ellipse. This is same as cutting the cylinder at an angle. The area of this ellipse is larger than that of a circle. With the same amount of light falling on a larger area, intensity of light on the surface decreases. Suppose we take the same situation at tropic of cancer when the sun rays are directly overhead. At this time Tropic of Capricorn will have light falling at 2 times 23.5 or 47 degrees. At 47 deg, area of an ellipse at Tropic of Capricorn is 1.5 times the area of the circle at tropic of cancer. Thus the energy received in the Southern hemisphere at Tropic of Capricorn is 1.5 times less than at energy received at Tropic of Cancer. This difference in received energy per unit area leads to higher and lower temperatures and hence causes seasons. Now, we have also seen that the length of days or nights change during the year. This is also because of the tilt of the earth. So when the earth’s axis is pointing towards the sun, at summer solstice, the northern hemisphere has larger days. This region gets heat for more time, thus raising the temperature, hence summer. The southern hemisphere has shorter days, longer nights, lower avgas temperatures hence winter. Now if we consider earth’s position at winter solstice, exactly reverse of the above happens. Rays are directly overhead the Tropic of Capricorn. So energy received is less in northern hemisphere. In addition, length of day is shorter in northern hemisphere causing an overall decrease in avgas temp and hence winter in northern hemisphere. Now let’s see equinox position from side view. The axis of rotation appears vertical from this view. At the equinox position, the sun rays are directly over head the equator. So heat received at same latitudes like 18 degree N and south will be same. And interestingly the length of days and nights for all latitudes is equal. In other words everyone on earth has 12 hr days and nights days/nights in this position of earth which happens twice a year at equinox. This position lies in the middle of the transitional phase between summer and winter every year. So we have seen that due to tilt of earth going around the sun we have seasons. These are combined effect of 1. Sun-light falling at various angles at different latitudes leading to different energy received at the place. 1. Duration of day or the amounts of time a place gets light. So I hope you enjoyed the reasoning behind the seasons. 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