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The exact position where a star appears in the sky does not only depends on the coordinates of the source observed, but also on the observer's relative velocity. The observer velocity is responsible for a phenomenon called "Bradley aberration" or "Stellar aberration". Stellar aberration is a well known phenomenon among astronomers. It was discovered by the astronomer James Bradley  in 1727. It is claimed to be caused by the relative transverse motion between the earth and the star emitting the photons.
Some authors [2-5] have shown that this prediction is not fully compatible with observations. There is no available explanation for the fact that, while the observational data on stellar aberration are compatible with a moving earth, the symmetric description, when the star (and not the observer) possesses the relative transverse motion, does not apparently lead to observations compatible with predictions.
Each component of relative motion between a source and a detector is discussed here separately. In the case of radial motion, it is well known that the relative motion between the source and the detector produces a change in wavelengths, explained by the "Doppler effect". There is then no change of direction of the photons. The radial velocity is compatible with the difference of radial velocities between the star and the detector. According to Einstein's relativity, the condition of invariance implies that there exists no absolute velocity of the source or of the detector. Observations have shown that in the case of radial motion, data are in full agreement with Einstein's predictions.
In the case of transverse motion between the two objects, another effect called "Aberration of Light" is predicted and observed. Following Einstein's relativity, a symmetrical situation is expected whether the source or the detector possesses the transverse velocity. Just as with the Doppler effect, the Einstein's principle of relativity also means that only the relative motion is relevant.
However, it has been demonstrated [2-5] that some observations are not compatible with those predictions. The seriousness of the problem has even been overlooked. Of course, the absence of a suitable explanation leaves the phenomenon of aberration of light without any rational solution. It has been claimed that the observed results, which depend on whether the earth or the star is moving, are experimental proofs of the failure of Einstein's principle. Let us examine that problem.
Description of the phenomenon.
One knows that the earth completes a full circumference around the Sun every year. Consequently, since the Earth-Sun radius (Re) is well known, it is easy to determine the earth tangential velocity (Vt) required to complete the circumference in twelve months (T seconds). We have:
We read in astronomy textbooks, that the relative velocity between photons (at velocity c) and the Earth (Vt), explains why a telescope "T" on Earth (see Fig. 1) must be pointing at the angle q , with respect to the Earth-star direction, to be able to point out at the star. Figure 1 shows that while the photons move in straight line toward the Earth, they will always remain in the axis of an inclined telescope, since it is moving sideways with the Earth. The angle q is equal to:
Apparent Lack of Symmetry.
A serious problem is revealed when one reads the description of that phenomenon. Scientific papers and textbooks on relativity  state or imply that stellar aberration is determined by the "relative velocity" between the light source and the detector. It has been clearly shown experimentally that it is not so. Many convincing arguments have been pointed out by H. Ives , Eisner , and Phipps , and Hayden , and several others to show that when the source (instead of the detector) is moving, the aberration no longer exists. Observations show clearly that, in contradiction with special relativity, stellar aberration does not depend on the relative motion between the source and the detector but exists only when the detector is moving. Textbooks explain the results solely when the observer is moving. There is no known explanation for the case when the source is moving.
That lack of symmetry, between the cases whether the source or the detector is moving is shown clearly  on the basis of the separation of binary stars. One can reliably determine the motion of individual stars of a binary system from their spectrum from the periodic Doppler shifting of the spectral lines of the star components. Therefore, stars of a binary system acquire sufficient relative transverse velocity to produce an important aberration. Such an aberration is not observed. Some papers report non realistic hypotheses to explain aberration when the star is moving. The explanation goes from the claim that we do not observe reality or the "Umbrella Analogy" mentioned by Eddington and some others.
Mechanism of Aberration.
Let us consider on figure 2, a stellar source S and a detector located on Earth. Source S emits photons in all directions at velocity c. In physics, photons are conceived as being particles and are represented as bullets ejected from the surface of the emitter S with a velocity c. After a short interval of time after emission, the photons emitted at one instant form a sphere around the star as shown (dotted circle) on figure 2. Let us consider now that the star S (instead of the Earth) has a transverse upward velocity as shown in figure 2.
We know that the resulting velocity U of the emitted bullets (photons) is the relativistic sum of the star velocity V and the speed of light c. From figure 2, we see that V is perpendicular to the direction of light going to earth. The general velocity composition formula giving the sum of V and c, as given by Møller  is:
Compatibility with Einstein's Theory?
This result must be examined in terms of relative motion. It is stated in relativity that there exists no absolute velocity. Only the relative velocity between two objects has a physical meaning. How can this be compatible with the above description of stellar aberration? This apparent paradox is solved when one considers more carefully what those two objects are. In the case described above, the aberration of light clearly involves the relative motion between the bullets (photons) and the detector. It does not involve the relative motion of the system that has fired the bullets (called source). In fact the star, from which the particles are emitted is no more than the support from which the particle (bullets) originated. Erroneously, the relative velocity between the star and the earth has been considered while one should consider the relative velocity between the incoming particles (bullets or photons) and the earth. Using that last consideration we find at last that the principle of relative motion described by Einstein can be applied in this case.
The error of interpretation discussed here is exactly similar to the situation that appears when a hunter is firing at his prey. Nobody ever claimed to be able to calculate the transverse velocity of a running hunter from the knowledge of the direction of the bullet moving toward the prey. The angle of penetration of the bullet into the prey depends on the relative velocity between the prey and the bullet and not the velocity relative to the hunter. If the hunter is running forward, he must point his gun with an angle having a backward component to reach his target.
One must conclude that it was an error to claim that the aberration of light is caused by the relative transverse velocity between the star and the Earth. One should say that it is the result of the relative transverse velocity between the Earth and the coming photons. Consequently, in this particular case, Einstein's principle of relativity can be applied.
Case for Other Models for Light.
It can be seen quite easily that the explanation given above about the aberration of light can be applied to the wave model of light just as well as to the model of photons. The consequences are quite identical. No aberration of light is expected when the emitter (instead of the wave) has a transverse velocity. One can also ask the question: "Can we find a similar interpretation for the aberration when light is described according to modern physics? One must recall that modern physics finds its interpretation with the use of the Copenhagen interpretation. The Copenhagen interpretation implies that photons cannot exist independently of the observer and are created at the moment of detection. Consequently, the explanations given here cannot be applied directly. In fact the question does not make sense since in modern physics, it is claimed that explanations do not necessarily have to be compatible with causality. This is stated clearly by Heisenberg himself when he writes: "The law of causality is no longer applied in quantum theory". When we do not accept the principle of causality it does not make sense to look for the cause of stellar aberration.
However, without changing the mathematics of modern physics, (but without using the Copenhagen Interpretation of Modern Physics), it is possible to show that physical phenomena can be described in a causal way. This has been shown in more details recently. It can be shown that, using a causal description of modern physics, the phenomenon of stellar aberration of light" is explained classically.
The author wishes to acknowledge the financial assistance of the National Science and Engineering Research Council of Canada, and exchange of correspondence with Dr. T. E. Phipps.
 J. Bradley "Account
New Discovered Motion of the Fixed Stars" Phil. Trans. 35
p. 637 (1728).
 H. E. Yves "Extrapolation from the Michelson-Morley Experiment" J. Opt. Soc. Am. 40, pp. 185-190 (1950)
Also - P. Marmet, “The Overlooked Phenomena in the Michelson-Morley Experiment”
 E. Eisner, "Aberration of Light from Binary Stars - a Paradox?" Am. J. Phys. 35, pp.817-819 (1967)
 T. E. Phipps "Relativity and Aberration," Am. J. Phys., 57, pp., 549-550 (1989) also Phipps T. E., Jr., "Stellar Aberration from the standpoint of the Radiation Convection Hypothesis." Phys. Essays 4, 368, (1991)
 H. C. Hayden, Stellar Aberration," Galilean Electrodynamics, 4. pp. 89-92 (1993)
 C. Moller, The Theory of Relativity, (Oxford 1972)
 W. Heisenberg, "Physics and Philosophy, the Revolution in Modern Science" New York, Harper and Row, (1966) p. 88
 P. Marmet, "Absurdities in Modern Physics: A Solution" Les Éditions du Nordir, Simard Hall, 165 Waller, Ottawa, K1N 6N5, Canada (1993).