In the 1880s, when electromagnetism was better understood, interest shifted to the problem of aberration. At the time, Fresnel`s and Stokes` theories were known to be erroneous. Fresnel`s theory required that the relative velocity of ether and matter for light of different colors must be different, and it was shown that the boundary conditions that Stokes had assumed in his theory were not consistent with his hypothesis of the irrotation flow. [1] [26] [27] At the same time, modern theories of electromagnetic ether could not explain the aberration at all. Many scientists, such as Maxwell, Heaviside, and Hertz, tried unsuccessfully to solve these problems by incorporating Fresnel or Stokes theories into Maxwell`s new electromagnetic laws. In contrast, stellar aberration is independent of the distance of a celestial object from the observer and depends only on the instantaneous transverse velocity of the observer relative to the beam of light incident at the time of observation. The light beam of a distant object cannot itself have a transverse velocity component, or it could not (by definition) be seen by the observer because it would miss the observer. Therefore, any transverse velocity of the emitting source plays no role in the aberration. Another way of saying this is that the transmitting object may have a transverse velocity relative to the observer, but any beam of light emitted by it that reaches the observer cannot, as it must have been previously emitted in a direction for which its transverse component has been “corrected”. Such a beam must meet the “right” observer along a line that connects the observer to the position of the object when it emits light. [1] The position shift of a nearby star caused by parallax turned out to be much smaller than the change in position due to stellar aberration, which, unlike parallax, does not vary with the distance of a star. Even for nearby stars, parallax is so small that it was only successfully measured in 1838, when German astronomer Friedrich Wilhelm Bessel discovered it for the star 61 Cygni.
The parallax he measured was 0.314 arcseconds, which is about 65 times smaller than the displacement due to stellar aberration. The position shift of 61 Cygni due to parallax corresponds to a width of 2 cm at a distance of 12 km. Because parallax was so difficult to detect, in 1900 only 60 nearby stars had their parallax measured. It was only with the development of machines to accurately measure the position of stars on photographic plates in the twentieth century that a large number of stellar parallaxes were calculated Planetary aberration is the combination of light aberration (due to the speed of the Earth) and light time correction (due to the movement and distance of the object) calculated in the resting frame of the solar system. Both are determined at the moment when the light from the moving object reaches the moving observer on Earth. It is so called because it is generally applied to planets and other objects in the solar system whose motion and distance are well known. In astronomy, aberration (also known as astronomical aberration, stellar aberration, or velocity aberration) is a phenomenon that produces an apparent motion of celestial objects around their actual positions, depending on the speed of the observer. It gives the impression that the objects are shifted in the direction of the observer`s movement from the moment when the observer is motionless. The change of angle is of the order of v/c, where c is the speed of light and v is the speed of the observer.
In the case of a “stellar” or “annual” aberration, the apparent position of a star to an observer on Earth varies periodically over the course of a year, as the speed of the Earth changes as it rotates around the Sun, from a maximum angle of about 20 arc seconds in right ascension or declination. Fresnel and Stokes theories were popular. However, the question of aberration was dismissed for much of the second half of the 19th century, when the study focused on the electromagnetic properties of ether. The relationship between these phenomena is valid only if the frames of the observer and the source are inertial frames. Since the Earth is not an inertial rest frame, but experiences centripetal acceleration towards the Sun, many aberrational effects such as annual aberration on Earth cannot be considered corrections of light time. However, if the time between the emission and detection of light is short compared to the Earth`s orbital period, the Earth can be approached as an inertial framework and the aberrational effects are equivalent to light time corrections. A special case of annual aberration is the almost constant deviation of the sun from its actual position around κ to the west (view of the Earth), unlike the apparent movement of the sun along the ecliptic. This constant deviation is often wrongly explained as due to the movement of the Earth during the 8.3 minutes it takes for light to travel from the Sun to the Earth: this is a valid explanation, provided that it is given in the Earth`s reference system, while in the Sun`s reference system, the same phenomenon must be described as the aberration of light. Therefore, it is no coincidence that the annual aberration angle is equal to the path traveled by the Sun along the ecliptic in the time it takes for light to travel from it to Earth (8.316746 minutes divided by a sidereal year (365.25636 days) is 20.49265″, very close to κ). Similarly, the apparent motion of the sun against the background of fixed stars could be explained as a (very large) parallax effect.
Hendrik Lorentz has made considerable efforts in this regard. After working on this problem for a decade, problems with Stokes` theory led him to abandon them and follow Fresnel`s proposal of a (mostly) stationary ether (1892, 1895). In Lorentz`s model, however, the ether was completely immobile, like the electromagnetic ethers of Cauchy, Green and Maxwell, and unlike the Fresnel ether. He obtained the Fresnel drag coefficient from changes in Maxwell`s electromagnetic theory, including a change in temporal coordinates in moving images (“Local Time”). To explain the Michelson-Morley experiment (1887), which apparently contradicted Fresnel and Lorentz`s theories of immobile ether and apparently confirmed Stokes` complete resistance to ether, Lorentz (1892) theorized that objects undergo a “contraction of length” by a factor of 1 − v 2 / c 2 {displaystyle {sqrt {1-v^{2}/c^{2}}}} in the direction of motion through the ether. In this way, aberration (and all associated optical phenomena) can be explained in the context of immobile ether. Lorentz`s theory became the basis for much research over the next decade and beyond. His predictions for aberration are identical to those of relativistic theory. [26] [28] The apparent position of a star or other very distant object is the direction in which it is seen by an observer on the moving Earth.
The actual position (or geometric position) is the direction of the straight line between the observer and the star at the time of observation. The difference between these two positions is mainly caused by the aberration. The sun and solar system revolve around the center of the galaxy, as do other nearby stars. It is therefore possible to conceive of an aberrational effect on the apparent positions of other stars and on extragalactic objects. However, the change in the speed of the solar system relative to the center of the galaxy varies over a very long time scale, and the resulting change in aberration would be extremely difficult to observe. Therefore, this so-called secular aberration is usually ignored when it comes to the positions of stars. In a final twist, Bradley later discovered the existence of the Nutation of the Earth`s axis – the effect he had initially considered the cause of the aberration. Looking at the phenomena of stellar aberration, I am inclined to believe that the luminous ether permeates the substance of all material bodies with little or no resistance, as freely as the wind blows through a grove of trees. Bradley, like the majority of eighteenth-century physicists, believed in a theory of particles of light. In the nineteenth century, a wave theory of light was generally accepted. Wave theory was better able to explain phenomena such as diffraction and interference patterns.
However, when this theory was used to try to explain stellar aberration, it encountered some difficulties. The age-old component of the aberration caused by the movement of the solar system in space has been divided into several components: aberration, which results from the movement of the barycenter of the solar system around the center of our galaxy, aberration, which results from the movement of the galaxy relative to the local group, and aberration, which results from the movement of the local group relative to the cosmic microwave background. [11]: 6 Secular aberration affects the apparent positions of stars and extragalactic objects.