# How did the ancient Indian sages know speed of light?

There is a shloka in the Rig Veda, which mentions the value of speed of light.

तथा च स्मर्यते योजनानां सहस्त्रं द्वे द्वे शते द्वे च योजने एकेन निमिषार्धे- न क्रममाण नमोऽस्तुते- ॥ (Devnagari script)

English

tatha cha smaryate yojananam. sahasre dve dve shate dve cha yojane ekena nimishardhena kramaman namostute

Meaning

"[O Sun,] bow to you, you who traverse 2,202 yojanas in half a nimesha."

The yojan is a unit used to measure length and the nimesha is a unit used to measure time, in ancient times in India.

Definition of Yojan

In the Vedas, Yojana is a unit of distance.

1 Yojana = 8000 Dhanus (Bow string length, Average man’s height)

1 Yojana = 9 miles roughly

Definition of Nimish

The measures of time are thus defined in the Bhagavata Mahapurana:

15 nimesa = 1 kastha

30kastha= 1 kala

30 kala = 1 muhurta

30 muhurta = 1 diva-ratri (day-and-night)(24 hours).

Converting these units into modern SI units, we get $$c = 3.00 × 10^8 ms^{-1}$$ Which is quite close to the currently accepted value.

My question is how did the Indian sages predicted or found the value of speed of light to that high precision?

• They didn't. They speculated about the size of the universe (or rather the Solar system) and had the idea that light is supposed to traverse it in one day, which gives the vedic number. Although both ideas were wrong, the ratio happened to be in the right vicinity under one interpretation of the size of the units, see Kak, The Speed of Light and Puranic Cosmology. Still, they were right that the speed is finite, that came out of analogizing light to wind. – Conifold Jul 19 at 20:43
• Sorry, the claim "sonverting these units into modern SI units, we get 3x10^8 m/s" just looks like the history written backwards from the known value of speed of light. When and where were those units defined? Is there is any authentic book which shows the meaning of those units that lead to the current value of the speed of light? – M. Farooq Jul 19 at 21:39
• @M.Farooq I dont think the units were adjusted to fit c. – Deschele Schilder Jul 21 at 5:16
• @Conifold Although both ideas were wrong, this is wrong. In their view it is right. You could just as well argue that our view is wrong. – Deschele Schilder Jul 21 at 13:07
• Deschele Schilder, You have probably missed these BBC reports. bbc.com/news/world-asia-india-41344136 and bbc.com/news/world-asia-india-46778879 – M. Farooq Jul 21 at 15:58