Aberration or better astronomical aberration is a phenomenon that makes a star, observed through a telescope, appear in a place slightly different from the expected. Aberration had been first observed in 1727 by the English astronomer James Bradley. In the course of many surveys, he noticed that stars seemed to be subject to a slight movement within a period of one sidereal year. He thought that this movement depended on the position of the star inside the celestial sphere.
Aberration and the speed of the starlight
There is a diurnal aberration that is usually considered negligible, caused, as they say, by the rotation of the Earth around its axis. There is an annual aberration as well, that is considered to be the consequence of the motion of the Earth around the sun.
We can explain this phenomenon considering the way the light of a star enters a telescope. Since the light speed, though really fast, is limited and not infinite, it takes a short time to reach the eye of the observer. In addition, during this infinitesimal lapse of time the Earth is moving around the sun with an average speed of about 30 km/s, that is 1/1000 of the speed of light.
The speed of the starlight, thus, will show to be under the influence of the speed of the Earth. This would be the cause generating the aberration, an apparent change in the position of the star.
Aberration: its annual constant
A star perpendicular to the orbital plane of the Earth has an aberrant circular movement inside the periodicity of one year. A star that is seen exactly on the plane of the ecliptic has an apparent rectilinear movement. On the other hand, in the intermediate positions, this movement appears to be elliptical. The maximum aberration value measured during the year is 20”,49. This is called annual aberration constant.
Aberration: a classical example
I want here to present a classical example generally used to describe the aberration phenomenon. Consider a man with an umbrella under the rain. When the man stands still in a place, he can see the rain falling vertically. But, when he starts running, he will see the rain falling diagonally. This simply will be an apparent phenomenon due to the composition of two velocities: the one of the rain falling and the speed of the man running.
Aberration: not possible on a motionless earth
This phenomenon was considered to be one of the first experimental proofs that the Earth is moving around the sun and not the contrary. In fact, if the Earth were motionless, we couldn’t observe the aberration. The basic problem with this situation is the periodicity. Actually, when considering the phenomenon, as we have already noticed, there is a periodical movement presenting a cycle of one year.
Aberration and the wind of ether
Some people, while sustaining the belief that the Earth is flat, have tried to give a further explanation. They say the aberration is not caused by the motion of the Earth around the sun, but by a wind of ether. They posit this wind is blowing over the Earth with a speed of 30km/s. So they can imagine it can drag the light with it, thus producing the phenomenon we are just examining. You could easily find a number of videos on the web giving this sort of explanation. But is this the correct reasoning?
I dare say that, definitely, this kind of hypothesis cannot be acceptable. And this was proved experimentally. The Michelson Morley’s procedure couldn’t detect any ether wind in respect of the Earth surface. If you are interested in this inquiry, please check the article I’ve written on this subject. The link will be the following: An explanation of Michelson Morley experiment edited on 2016/12/07 by earthmeasured.com However, Airy’s investigation has proved that it is not in the motion of the Earth the cause of the aberration of light.
Let’s briefly explain in what Airy’s experiment consisted. Airy knew that he had to tilt slightly his telescope to see a star. He was convinced this tilt was necessary due to the motion of the Earth. Then he filled his telescope with water. Being now the medium denser, he thought the light could have a lower speed. This could be the basic reason why the star beam should not be able to reach the eye of the observer. He really thought he could expect such a consequence as a result.
He posited that the beam of light would take some infinitesimal fraction of time to reach the eye. In the same moment, he reasoned, the Earth was moving at the speed of 30km/s. Thus the beam of light would fail to reach the eye of whatsoever observer. As a matter of facts, it should be noted that a light beam goes through some bending in the water with an angle of somewhat entity. Airy repeated many times this experiment, but he could never detect a difference, whether the telescope was filled or not filled with water. He was always in a position to see the star, being the telescope filled with water or not. This would be the evidence, someone could deduce, that there is a wind of ether dragging the light beam in the air and in the water.
A short Abstract with pictures
Aberration: at a deadlock
So, we are at a deadlock: Airy’s experiment is a full evidence of the fact that the Earth is not moving around the sun while the Michelson Morley experiment proves that there isn’t any ether wind. We shall explore however some other hypothesis to avoid the idea of an ether moving in respect with the Earth.
Referring to the diurnal aberration, it is clear that the phenomenon is caused by the motion of the stars in the movable wall of the dome. Considering the diameter of that part of the dome we could get an idea of the speed of the stars. We know that the walls of the rotor of the dome start setting up at a radius of 26640km. The firmament takes 23 hours 54 minutes to make a turn. We discover thus that stars move at a speed of about 1940 m/s = 6990 Km/h. Due to this speed, we get a very little aberration: the diurnal aberration.
Annual aberration: a non-existing phenomenon
Referring to the annual aberration, the only thing I can say is that it is a non-existing phenomenon. The fact that, in the course of six months, you can detect a periodical difference in the position of a star can only be due to refraction. It could be due, in addition, to the Van Allen belt which is at one with the Moebius strip. Such is the machinery sustaining the movement of the sun and the moon. The plasma constituting this belt could in fact enormously increase many refraction phenomena. Let’s try an in-depth analysis of the refraction problem associated with aberration.
In six months, the aberration rate passes from a minimum to a maximum. Moreover, this cycle is repeated every year. Scientists have always kept on saying that the Earth is moving around the sun. This could explain the aberration, but astronomers believe as well that the sun keeps on moving inside the galaxy in the direction of Vega.
The aberration movement, in this case, shouldn’t be an ellipse but a spiral.
However, aberration has really been measured. So, how can this periodical, mysterious, apparent movement be explained?
|Aberration: experimentally measured or simply theoretically calculated?
Aberration angles are very small and it is quite difficult to think that they have been measured avoiding errors due to refraction. Thus the incredible match between measured values and theoretical ones appears astonishing. Let’s see the theory. Consider a telescope 1 meter long. The time light takes to make that distance is :
During this time the Earth covers the following distance:
So we have this situation (see also the figure):
A refraction phenomenon
It is noteworthy the fact that Bradley himself recognized that this phenomenon was the same for all the stars. At first, Bradley thought that it was caused by the parallax, i.e. an optical error due to the different positions of the Earth during the year. But if the modification of the position is the same for all the stars, this could be caused by the parallax only if stars were all at the same distance from the Earth, thing considered absurd by Bradley himself. He reached, thus, the conclusion that the phenomenon was caused by the aberration of light.
We really know, considering our flat model of the Earth, that this apparent change in the position of the stars can’t be caused by parallax because the earth is motionless, and the stars, month after month, are at the same distance from the Earth, so no parallax is possible.
It is important to consider that this phenomenon is cyclic and manifests the maximum gradient in six months. Could this be explained simply as a refractive optical phenomenon? Let’s see.
A ray of light that, from a star, reaches the Earth, passes through the atmosphere that owns a little, but anyway sensitive, refractive power. Thus, if the ray of light is not perfectly perpendicular to the Earth, it is bent with a small angle called astronomic refraction that can be thus calculated:
where r0 is the refraction angle expressed in minutes of degree and Hc is the height angle of the star.This formula is valid for an atmospheric pressure of 1010mbar and a temperature of 10°C. If temperature and pressure are different this formula should be used:
and refraction will be R=R0*f.
The true height of the star is H=Hc+R.
The maximum value of aberration measured by Bradley is 20”,49 that is called annual constant of aberration and corresponds to the major semi-axis of the aberration ellipse.
The refraction angle can assume a maximum value of 35’,4 on the horizon but it is 3′, only 3’ already at 17,5°. Notice that this value changes with the temperature (as with temperature the air density also changes) and the temperature changes with the seasons, and …mumble mumble…the maximum climatic difference with seasons is cyclic and recurring every six months.
The value of refraction angle changes of about 1% for each 3°C of temperature variation. If we consider a temperature variation from summer to winter of 30°C we have a 10% of variation on the refraction angle.
We arrive thus at 0,3’ i.e. 20” of apparent deviation due to refraction if we consider a star at 17° high, that corresponds quite well to the value of aberration.
We believe that the Earth is motionless and have already proved it in this blog. Thus the aberration cannot exist and this is confirmed by the fact that it has a precise periodicity that doesn’t follow the movement of the sun in the galaxy but the movement of the seasons. Aberration can be, however, easily included among the astronomical refraction phenomena. And, according to the above-given calculations, we have proved that the angles calculated (aberration and refraction) are very similar, so refraction is a possible reason for the astronomical aberration.