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Sunday, 10 April 2016

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Under the Sun in April

Our earth is a minute solid particle in the universe which spreads in space with dimensions of billions of billions of kilometers. In the primitive stages, the man was unable to understand the reality of the universe even though they used to observe the twinkling dots in the sky. As they could not make any sense about what they observed, they developed many myths and concepts about the stars, the Sun, moon and the planets. They did not fail to name some powerful body or a character in relation with many concepts.

Facts about the universe were not known to man even in the periods of great philosophers such as the Buddha and Jesus Christ and as a result, most of the concepts and beliefs about the universe have intruded into many religious teaching too.

Universe

But the great philosophers, and scientists, mathematicians, especially those who lived after the 15th century, could change the history of man in understanding the reality of the universe through their excavations of universe with mind and technology. The stories of what has been happening during the past 15 billion years and what is happening now in the universe are available for reading through pressing of the keyboard of a laptop.

As the whole world is aware now, our planet, the earth, makes one vibration around its axis in 23 hours, 56 minutes and 4.1 seconds (not in a 24 hour period as many people believe) while it moves along an elliptical orbit around the Sun with least distance of 147 million of kilometer and largest distance of 152 million of kilometers from the pole of the ellipse where the Sun is located. The period of one journey of the Earth around the Sun is 365.242 days (not in a 365-day period as many people believe). Since the axis of the earth is inclined to the normal direction of the plate of motion through an angle of 23.5 degrees, the point, or place, on the earth surface above which the sun rises overhead is not fixed but is varying throughout the year. This point is named 'Sub Solar Point' (SSP) in Astronomy and Meteorology. The SSP lies on different latitudes in one hemisphere through a period of 6 months and it lies on different latitudes through next 6 months in the other hemisphere. The location of the SSP changes from one hemisphere to the other in every six months and it is the nature.

As it could be obtained either from Websites or from the Meteorological institutes, the Sun rose overhead over the equator, the line dividing the globe into two hemispheres, on 20th March 2016 at 10.00 hours Sri Lanka time on its northward movement and now the SSP is gradually moving northward and the sun will rise overhead over Sri Lanka landmass between 4th April (Southern tip) and 14th April (Northern tip) this year.

Preparation

Global astronomical institutes are responsible for preparation of the tables with dates and times of solar overhead positions inter alia other astronomical data. Once the computed data are available, it is the responsibility of regional institutes to compute the dates/times of positions of the Sun relevant to local positions. Sri Lanka Meteorological Department has served the purpose and the Table 1 gives the data for some locations in our country.

We have to understand some important facts in order to answer the questions arising in our minds.As the SSP changes all the moments in North-South direction over the globe, and its displacement in 24hour time is several kilometers, Sun never rise overhead at all the points and places on the land. Only the cities with Sun risen overhead in April 2016 are given in the table.

Behaviour

Also please do not feel embarrassing to query about the universal behaviour. There are no special good or bad effectsby the Sun by rising overhead to your village or area except for the uncomfortable feeling due to direct solar radiation. This will cause increase of convection leading to development of convective clouds which will produce thunderstorms. Thunderstorms usually are accompanied by strong gusty winds, lightning, tornado and water spouts.

With these solar equinoxes, the thunderstorms may be very intense producing hazardous Tornado and also ice rain with minute ice particles. The tornado or water spouts may result in rain with colourful water in some areas, rain with small fish and frog in some areas.

You must not feel wonder to observe the above mentioned physical events because these things are very common in April in every year. Please do not regard them as unusual phenomena caused by any unseen power. Let us understand the reality.

(The writer is a Visiting lecturer, Post Graduate Institute of Science (PGIS), University of Peradeniya (Retired Director of Meteorology)

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