I dont know whether it is authentic or fake i got this in one of the blog how could ancient people so accurately calculate the speed of light? see below info

A verse from RIG

VEDA(I,50-4).

In the verse Sun light speed is referred with the help of units called YOJAN.

"Yojananam Dwe Dwe Shate Dwe Cha Yojane Aken Nimishardhena Krammana Namostute"

In this verse sun is referred to with respect saying that sunlight moves 2202 Yojans in Half Nimish. Yojan is quite a common unit in India. It means 4 KOSE, each kose measuring 8000 BRITISH YARDS AND EACH YARD MEASURING 0.9144 M.

The Definition of time unit Nimish can be found in ShrimadhBhagwat(ii,11-3 TO 10) WHERE IT IS MENTIONED THAT 15 NIMISHAS MAKE 1 Kashta,15 kashtas make 1 laghu, 30 laghus make 1 muhurtas and 30 muhurtas make 1 Diva-ratri. A diva Ratri is day-night and 24 hours in modern language. When you convert 2202 YOJANS PER HALF NIMISH INTO SI UNITS YOU FIND IT TURNS OUT TO BE:

15 Nimish = 1 Kashth

30 Kashth = 1 Kaal ( 30 * 15 Nimish = 450 Nimish)

30 Kaal = 1 Muhurt ( 450 * 30 Nimish = 13500 Nimish)

30 Muhurt = 1 Ahoratr ( 13500 * 30 Nimish = 405000 Nimish)

1 Ahoratr = 1 Day (24 Hours)

1 Day = 24 * 60 * 60 Seconds = 86400 Seconds

86400 Seconds = 405000 Nimish

1 Second = 4.69 Nimish

1 Nimish = .214 Second

3X10^8 m/s upto two significant digits a value quite accurate as we know it today

A nimesa is therefore equal to 16/75 seconds. It does come very close to the correct figure of 186,000 miles per second.”

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source:

http://probedeep.blogspot.com/2008/03/speed-of-light-according-to-rigveda-how.html