The Sagnac Experiment was first conducted by French physicist Georges Sagnac in 1913. The experiment is essentially the same experiment as the Michelson-Morley Experiment, except that it is on a horizontal rotating turn-table. The results of the Sagnac Experiment show that when the table is rotated around on a horizontal axis during the experiment, the velocity of light indeed changes. This contradicts the Michelson-Morley Experiment that was designed to use the supposed movement of the earth to change position, and which showed no change in light velocity.
From a paper on the Sagnac Effect we read:
“ Since its discovery at the beginning of the XX century the Sagnac effect  has play an important role in the understanding and development of fundamental physics (for a review see ). The Sagnac effect is the dependence of the interference pattern of the rotating interferometer on the direction and speed of rotation. This phenomenon is universal and manifested for any kind of waves, including the matter waves and has found a variety of applications for the practical purposes and in the fundamental physics .
...In the context of the Sagnac effects the null result of the Michelson-Morley experiment is also not clear. Applying the same logic to Sun centered rotating frame in which Earth is fixed, one would expect different light speeds as seen from Earth. ”
A Critical Analysis
“ Einstein's theory of relativity is shown to be a physical theory of limited experimental validity. Twelve different experiments seem to disprove its two postulates. ”
On the subject of the Sagnac Effect, Dr. Monti states:
“ The effect was tested in 1913 by Sagnac(46), and Sagnac’s experimental results disproved the second postulate of special relativity. ”
The geocentrist Malcom Bowden describes the Sagnac Experiment in the following video:
“ An animated explanation of how Sagnac's experiment proved the existence of the aether, thus demolishing Einstein's Theory of Relativity. Complaints by two experienced physicists that when they were at university, they were never informed about this important experiment. When other experiments - Airy's Failure, Michelson-Gale's experiment (also not taught at universities!) and of course the famous Michelson-Morley experiment- a completely different picture of the cosmos emerges - with the Earth at the centre of the universe. ”
The video author's personal views on geocentricity aside, Mr. Bowden provides an accurate description of the Sagnac Experiment and its results.
General Relativity Framework
On the assertion that the Sagnac Effect is explained under the framework of General Relativity theory, Professor Ramzi Suleiman of the University of Haifa writes the following:
“ It is believed that the Sagnac effect does not contradict Special Relativity theory because it is manifest in non-inertial rotational motion; therefore, it should be treated in the framework of General Relativity theory. However, several well-designed studies have convincingly shown that a Sagnac Effect identical to the one manifest in rotational uniform motion is also manifest in transverse uniform motion. This result should have been sufficient to falsify Special Relativity theory. In the present article, we offer theoretical support to the experimental results by elucidating the notion that the dynamics of transverse and rotational types of motion are completely equivalent. Since the transverse Sagnac effect contradicts Special Relativity theory, it follows that the rotational Sagnac effect contradicts Special Relativity theory as well. ”
It has been claimed that Special Relativity does not apply to the Sagnac Effect because it involves rotational acceleration. In 2004, Dr. Ruyong Wang demonstrated that a non-accelerating observer can also measure a difference in lightspeed, falsifying Special Relativity's explanation of the Michelson-Morley Experiment.
Dr. Robert Bennett writes:
“ Belief that the Sagnac test of 1913 only applied to rotational motion was discounted when Ruyong Wang found the same results for linear motion in 2004. The Sagnac result has never been credibly explained, despite its wide application in modern technology. ”
From Challenging Modern Physics: Questioning Einstein's Relativity Theories by Al Kelly (bio) we find:
“ A recent ingenious test by Wang et al. (2003) shows that the Sagnac result is also achieved by sending out and back again light in a straight-line portion of the light path. This is what this author claimed above, but it is so much more convincing when an actual experiment has shown the same thing. I wonder what excuse will be trotted out now! Wang et al. achieved the seemingly impossible by reversing a light beam sent out on a straight line on a moving platform and measuring the difference in time for it to return. This author had the pleasure of meeting Wang in 1997 and corresponded with him during the rests he performed and since then.
In another paper (2005), Wang gives further details of the experiment; appended to this paper are comments on the experiment by Hatch and Van Flandern, confirming that Wang had succeeded in proving that the Sagnac effect applies to straight-line motion.
...Any claims that the Sagnac experiment upsets [Special Relativity] were heretofore brushed aside by a statement that Sagnac is a rotational experiment and that SR does not apply to rotational experiments. That defense is now shown to be groundless. ”
“ Since the realization of this experiment, which has been done with photons , electrons  and neutrons , many trials have been made to interpret the observed results seen, for instance, Selleri . Indeed, Sagnac utilized the habitual linear additive rule and with that he was able to correctly predict the observed results. Still, since his prediction lead to velocities greater than c and consequently are against relativity which claims that the maximal possible velocity is c this raised a large amount of arguing. In fact, many authors tried to explain the results of the experiment in the framework of relativity which assumed that the maximal possible velocity is c. As can be seen in the literature, there are almost as many explanations as the authors that have tried to explain the results in the framework of relativity. In some cases the same author  presents even more than one possible explanation. The complexity of the problem stems mainly from the fact that the experiment is done in a rotating platform. In such case, there may occur a possible accelerating effect leading the explanation of the experiment to fall in the framework of general relativity.
This controversy, whether Sagnac experiment is against or in accordance with relativity, was settled recently by R. Wang et al.  with a very interesting experimental setup they called linear Sagac interferometer. In this case the platform is still, what moves is a single mode optical fiber coil, Fig. 12.
They did the experiment with a 50 meter length linear interferometer with wheels of 30 cm. The observed relative phase shift difference for the two beams of light following in opposite directions along the optical fiber was indeed dependent only on the length of the interferometer and consequently independent of the angular velocity of the wheels. From the experimental results obtained with the linear Sagnac interferometer one is lead to conclude that in this particular case the linear additive rule applies. Consequently we may have velocities greater than c, which clearly shows that relativity is not adequate to describe this specific physical process. ”
In parting, Dr. Croca states:
“ As a final note, I would like to stress that these observed facts [do not] in any way deny the usefulness of relativity. Relativity is a good approach to describe reality at its proper scale of applicability. What is quite wrong is to claim that relativity is the last, the complete and final theory ever devised by mankind. ”
Defeated, and with the ever need to have an 'explanation' to keep the model alive, relativists seem to hint that relativity might operate differently at different scales. Dr. Croca suggests that Special Relativity is useful for explaining why light is consistent in experiments of the Earth going around the Sun (Michelson-Morley Experiment) and with the direction of Starlight (Airy's Failure), but that this theory is not applicable in other experiments such as Sagnac and Wang devices which have observed an inconsistency of light speed for different observing points, including light traveling faster than c—which is directly contradictory to Special Relativity's postulate of a speed consistency of light to all observers and the luminal speed limit of c.
One must remark that it is quite odd and remarkable that Special Relativity remains true despite a direct violation of its postulates with multiple experiments; apparently only applicable to certain situations involving the motion of the Earth as a heliocentric explanation for why the Earth has been tested to be horizontally motionless.