Mike Lorrey writes, regarding http://arxiv.org/abs/physics/9804020,
which describes ancient Indian knowledge of the speed of light:
> this is one more april fools joke. the speed of light was not first
> calculated by measuring the time differences that light took to travel
> from Io to Earth in the 1600's depending on whether it was moving toward
> or going away from earth (how would they measure these times? The could
> only have done so by redshift of spectral lines, which would have given
> us relativity long before Einstein).
First, it's clearly not an April Fool's joke. The abstract reads,
We survey early Indian ideas on the speed of light and the size
of the universe. A context is provided for Sayana's statement
(14th century)that the speed is 2,202 yojanas per half nimesha
(186,000 miles per second!). It is shown how this statement may
have emerged from early Puranic notions regarding the size of
the universe. Although this value can only be considered to be an
amazing coincidence, the Puranic cosmology at the basis of this
assertion illuminates many ancient ideas of space and time.
So the author himself is claiming only that this value is a coincidence.
Second, the story about measuring the speed of light in the 1600's is
real. You don't measure redshift due to Io's motion; you measure the
difference in travel times due to the Earth's motion around the sun.
Earth is 16 light-minutes closer to Jupiter at one point in its
orbit than six months later, and so the measured interval between
Io's orbits would slow for 6 months and then speed up for another 6.
An astronomer correctly deduced that this was because of light taking
more time to get to the Earth at some points in its orbit than at others.
See http://www.phys.virginia.edu/classes/109N/lectures/spedlite.html
for discussion on early measurements of the speed of light.
Hal
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