by Joe Murray

Created on: August 14, 2008   Last Updated: October 12, 2011

The theory of relative time arose through the six classic pillars of science: a field observation, supporting verifiable measurement, explained by a relevant theory (paradigm), a follow-up experiment, which, failing to support the relevant theory, led to its overthrow and replacement by another theory. See, measure, explain, test, refute, re-interpret.

And it all started with a failed field observation.

One wintry night in 1725 James Bradley and Samuel Molyneux aimed a 20 foot telescope at gamma Draconis in an attempt to discover stellar parallax. To their astonishment, the "image" of the star Eltanin was shifting in the OPPOSITE direction.

Story has it that Bradley was sitting on the banks of the Thames watching sailboats tacking across the river when the solution hit him.

The speed of light was known to be high, but finite, having been first measured in 1675 from the occultations of the Jupiter's moons across a known distance of space. Olaus Roemer's first calculation was off by a whopping 30 percent, but it had the advantage of having a correct and repeatable methodology.

Bradley realized that because of the Earth's 30 km/sec (or 30 kps) speed around the Sun the telescope tube had to be leaned into the starlight, rather like motoring upstream at an angle against the current in order to travel dirtectly across a river. However small the angle (let a = 20.5 seconds of arc, or about .0057 degrees), it just happened to be the trigonometric ratio of the Earth's speed (v) divided by the speed of light (c), tan(a) = v/c, so that by solving for c, Bradley calculated the speed of light to well within 1 percent.

The irony was that parallactic displacement was still outside their instrument error - Alpha Centauri, our second closest star 4.3 light years distant, requires the scope to be aimed at an accuracy well under .8 seconds of arc - so they never found what they were really looking for.

The effect of stellar aberration is not constant for every star in the sky - remember, we're whizzing around the galaxy at over 270 km/sec yielding "stellar aberration" - but for the stars in our lane of the galaxy, the effect is constant enough to postulate one particularly interesting morsel of information: the speed of light is INDEPENDENT of the speed of the emitter.

That is, a speeding star does not "push" its light ahead of it; it will shorten the wavelength and increase the frequency of the light, but the star's speed is not added to the light. We know this because all telescope

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