"Our shadow will follow us everywhere", says an old Tamil proverb. On two days in a year, however, this does not hold true. On 'Zero Shadow Days' no shadow of vertical objects is cast as the sun is at the zenith at noon on two particular days. Stick a vertical pole on the ground or stand erect, no shadow will be seen for a fleeting moment on zero shadow days.
We all believe that the sun rises in the east; the sun is on top of our head at noon; and we have twelve hours each of day and night. All the three statements appear simple truths but are just fallacies in scientific terms.
Shadows have helped humans to understand the passage of time since time immemorial. It is by studying the behaviour of shadows that humans began to understand bigger cycles of time. A day can be easily recognised and measured as the time interval between two successive sunrises and a month one cycle of lunation of the moon - from one full moon or poornima to another.
A year has been defined as the time taken for the earth to make one round around the sun.But how do we mark the beginning and end and actually compute its period? For ancient civilisations, shadow cast by the sun came in handy.
"On 'Zero Shadow Days' no shadow of vertical objects is cast as the sun is at the zenith at noon on two particular days. Stick a vertical pole on the ground or stand erect, no shadow will be seen for a fleeting moment on zero shadow days,"
Consider an imaginary point at the equator. If you stick a vertical pole and observe its shadow, it will be as long as 200 times the height of the pole at the time of sun rises. Now if you watch the direction of the shadow at sunrise, first it would be away from the sun, and it would not be pointing towards due west on most days of the year. By measuring the angle of the shadow at the sunrise time, one can compute the sunrise point. It would be evident that the position of the sunrise point is north-east or south-east. This means the sun does not always rise from the east.
Any keen observer could notice that the sunrise point at the horizon achieves maximum north on June 21. After this day, the sunrise point would slowly and steadily shift towards the south - in a phenomenon known as dakshinayan. While the sunrise point is in dakshinayan, the sunrise point would be true east on September 21. The next day, the sunrise point would be towards southeast, and the sunrise point would reach the southernmost point on December 21. After December 22, the sunrise point would appear to move north - marking the uttarayan. During the uttarayan period, again on March 21, the sun will rise at true east. Thus one cycle of north-south movement of sunrise point gives the year.
Now consider the case of the shadow cast by the vertical pole when the sun crosses the meridian. As the sunrise point is usually not east, it is clear that the sun will not come overhead every day. Precisely on March 21 and September 21, the day of equinoxes, that sun will rise exactly in the east and set exactly in the west and hence will be on top of the vertical pole at the local noon. However, between March 22 and September 20, the sunrise point is northeast, and hence the sun would appear to be slightly north of the pole even at the local noon. This implies the shadow at noon will be towards the south. Similarly, it is easy to make out that the shadow of the pole at midday would be pointing to the north between September 21 and March 20. Thus at the equator, only on two days in a year - March 21 and September 21 - the day of equinoxes a vertical pole will not cast any shadow, giving us zero shadow days.
While the sun is on top of the equator on March 21, it will be north of equator from next day.This means it would be over some latitude north of the equator on that day and will cast now shadow at local midday in that latitude. Likewise, the position of the sun would be aligned with some latitude, and on June 6, it would be over 22.7 degree N latitude. Indore is situated in this latitude. At the local midday, the sun will cast no shadow in Indore.
The position of the sun will go further north, until it reaches the maximum on July 21, when it would go over the Tropic of Cancer. Subsequently, the position would move south and once again go over Indore on July 6, before again being on top of the equator on September 21,the day of the equinox. The Sun would be over the Tropic of Capricorn on December 22.Incidentally July 21 when the Sun's path in the sky is maximum north is called summer solstice and December 22, when it is maximum south is called winter solstice.
Why the sun's path does not go beyond 23.5 degrees? That is to do with the earth's tilt. This has implications for people in the northern hemisphere, living north of Tropic of Cancer. For then, there will be no day on which the sun will come right overhead. Sun will always be south of them.
One can mimic zero shadow day at home too. Stand right below an overhead lamp, you will feel intense light and perhaps warmth. Keep the same overhead light to your south, the intensity of the illumination will be reduced and it won't be that warm. When the sun is right on top of your latitude, or nearby, it is obvious it would be summer. When it is taking the southernmost path, and even during the midday it is south of you, it would be winter. That is why in the northern hemisphere we have summer during July and winter during December. It is not as most assume that in elliptical orbit the earth is near to the sun at one time and far away at another time. In fact, the earth's orbit is nearly circular that there is perhaps not more than 3% variation in the sun's rays. It is the tilt of the Earth and the apparent motion of the sun that causes seasons.
Zero Shadow Day is an opportunity for us all to understand the phenomenon and feel it. Just keep a vertical pole in an open ground, you will see no shadow on these two days. The Public Outreach and Education Committee of Astronomical Society of India and Vigyan Prasar are jointly organising many programmes on these two days. (India Science Wire)
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