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Mumbai, _www.dnaindia.comWednesday, January 21, 2009 epaper.dnaindia.com

ASTRONOMICAL UNIT MEASURES LARGE DISTANCES IN SPACE. HERE IS AN INDEPTH VIEW

Distance units in astro'1omy ow far is Delhi from Mumbai? A distance chart mayshow the answer to be something like 1407 kilometres. But suppose we answer the question by saying that Delhi is 1,407,000,000 millimetres from Mumbai! We would becorrect, of course, since our textbook tells us that one kilometre isequal to a million millimetres. But this answer is not verypractical. Thenumber is solarge as to makeit difficult to relate to our experience: And thenumber is toolarge becausethe unit used isvery small. Mil-limetre may beappropriate forr ~ ants navigatingacross- yourspecial kitchen floor,but for us humans intercity distances were best measured in milesor kilometres. ,Take for example, the Earth-Sundistance. Measurements show thatit is around 150 million kilometres.

I f we continue using the same unitof kilometre which serves us sowell on the Earth , then some othersolar system measures are evenlarger: Sun-Jupiter distance is 780million kilometres and NeptuneSun distance is 5,000 million kilo-

metres. And while we get snowed bythese large numbers we are re minded that we have still not leftthe solar system.To deal with the large distances ofthe so lar system, astronomers haveelected to have a new distance un it,the so-called Astronomical Unitwhich equals the mean Sun-Earthdistance. So we say that the Earth

is 1AU from the Sun, Jupite r 5.2 AUwhile Neptune is 30.1 AU.The AU will work well within thesolar system but not beyond. For ex-ample, the Earth is so far from theSun that light from the Sun takesabout" 500 seconds to come to us.How far is the nearest star from theSun? The stars Alpha and ProximaCentauri are so far that light from

the Sun will take 4.25 years getthere. Even in the AU unit, the answer is as large as 268056 .And as weprobe the universe farther and farther we would get even large'r numbers. So we need a bigger unit thaneven the AD. :One way to devise a new unit is touse the above example for guidance.Why not use the light speed as a con-

stant in describing how far away theobjects are? So a light-year is the distance travelled by light in one year.It equals approximately 9.46 millionmillion kilometres. By the same to-ken we have noted that one astronomical unit equals approximately500,light-seconds.While physicists would be happyto use a light year to measure astronomical distances, the astronomersuse another unit, called the parsec.This is approximately three lightyears and more exactly equals 30.8million million kilometres. To visualise the parsec we could draw anisosceles triangle with the smallthir d side equal to 2AU and the other two equal sides are of such alength that the small angle facingthe third side is 2 seconds. of arc.One second of arc is 3600th part ofthe degree that ts commonly usedfor measuring angles. This meansthat i f we saw a star located at thethird vertex today and siX' monthslater, we would find a change of itsdirection by 2 seconds of arc. Sucha star is then said to be at a parsec'sdistance from the Earth. The changeof direction of the sta r is called 'parallax'. Thus the word 'parsec' ismade up by joining part of 'parallax'with part of 'second' of arc.AstrophysicianJayant Narlikar and ThanuPadmanabhan, theoretical physi-Cist, will contribute to DNA's specialseries through'2009, designated theInternational Year of Astronomy.,This is the third article in the series

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