"It is not an optical illusion, it just looks like one."
Secular Aberration and the MTLs
Secular aberration will now be revisited with respect to the MTLs. Aberration of starlight is a phenomenon that requires telescopes to be tilted slightly in order to track stars, but why do telescopes need to be tilted? There are actually two ways to describe why they need to be tilted.
Suppose there is a train that is traveling at 50 kph (kilometers per hour) under a bridge. On this train is a flatbed car that has a tall, thin bucket standing in the middle of this car. As the train goes underneath the bridge, someone who is standing on the bridge drops a drop of water such that it enters the exact center of the top of the bucket. Because of the motion of the train, this same drop of water will not hit the center of the bottom of the bucket. The bucket is attached to the train and the train is moving while the drop of water is moving down the bucket. In order for the drop of water to hit the center of the bottom of the bucket, the bucket must be tilted. If this train, on the next day, was traveling in the opposite direction, the bucket would need to be tilted in the opposite direction.
Note that the drop of water and the bucket/train entity move independently of each other. The drop of water is never "carried" or "dragged" with the train or the bucket.
The second way is to talk about rain and cars.
When we drive down the highway during a rain storm, even if the rain is coming straight down, it appears to the driver that the rain is coming down at an angle. This is an optical illusion. CCD chips that are used in telescopes are not subject to this kind of optical illusion because they only see starlight when it hits each pixel, and they see nothing until this light beam hits the CCD chip. The reason a person is confused is because he or she sees the light before it hits the windshield. Thus, the tilt of aberration of telescopes is not caused by the same kind of optical illusion as people encounter while driving in the rain.
To understand why aberration is an application of the MTLs, consider that the earth is orbiting the sun, and the target (i.e. the bottom of the telescope) is attached to the earth (the platform), just as the bucket above is attached to the train. Thus the target is moving around the sun with the earth at 30 kps. A tilt of the telescope is required so the light that hits the center of the top of the telescope also hits the center of the bottom of the telescope. Since the 1700s scientists have used aberration of starlight as evidence of the velocity of the earth in total space, because they felt the sun was at rest in the universe.
But now we know a lot more about the universe. We now know that our earth's total velocity in space is 370 kps. Suddenly, we know that the actual tilt of aberration of starlight must be based on our total velocity in space of 370 kps. To be more specific, it is based on our range of velocities from 340 kps to 400 kps.
Since the tilt for secular aberration is constant for a given star, and always causes telescopes to be tilted in exactly the same direction, we cannot isolate this tilt. We can only measure the tilt caused by our variable velocity of 340 kps to 400 kps.
The main point to this discussion is that because the bucket and drop of water are independent of each other, we can therefore conclude that photons (assuming they exist) move independently of the telescope (i.e. they are not dragged with the telescope), and thus independently of the earth. This observation, in fact, was one of the key arguments against the ether drag theory of light. To put it another way, the path of a photon, once in motion, moves relative to the 3D CMBR of the universe, totally independently of the earth in its motion in the 3D CMBR of the universe (i.e. the photon, unlike air, is not dragged with the earth). If it were not for this, there would be no aberration of starlight with the photon theory.
Before moving on another metaphor would be helpful, “The Glowing Suit Metaphor.”
The Glowing Suit Metaphor:
Let us consider a train that is traveling at 370 kph in a vacuum, meaning we can ignore all types of wind. The train tracks are straight. One of the cars on this train is a flatbed car that has a table on the middle of it. A person is running in circles around the table on this flatbed car at 30 kph, 3 meters from the table. The running person is carrying a tall, narrow bucket.
As this person runs in circles around the table, note that the table is always traveling at a perfectly constant velocity of 370 kph towards the train's destination. This means the person, if he were standing still, would also be traveling at a constant 370 kph down the train tracks. However, the person is not standing still. When he happens to be running in the same direction as the train, his total speed is 400 kph, relative to the ground, 370 kph from the motion of the train plus 30 kph from his motion running around the table. When he happens to be running in the opposite direction the train is headed, he is running at 340 kph, relative to the ground, 370 kph from the motion of the train minus 30 kph from his motion running around the table.
Image, for a moment that the table and the suit of the running person glow very brightly in the dark. Imagine that astronauts in space are looking at this train at night and they can only see the glowing table and the glowing suit of the running person. These astronauts would see the glowing table traveling in a straight line at a steady 370 kph. They would also see the glowing suit moving in an almost straight line, but not at a constant speed. They would probably think that the two objects were in a race down a highway. The glowing suit would travel in nearly a straight line, but it would slow down and speed up and at times would be in front of or behind or on different sides of the table. The astronauts would be very puzzled, particularly if they could not see the table (try to visualize that!).
Now consider two people that are standing far above our galaxy. The "ecliptic plane" is the 2D (2 dimensional) plane in space defined by the sun at its center, and by the orbit of the earth. In other words, the earth orbits the sun on the ecliptic plane by definition. The 12 zodiac constellations are all on the ecliptic plane, including Leo, the constellation we are headed for. Let us assume these two people are normal (i.e. perpendicular) to the infinitely wide ecliptic plane, but are totally stationary relative to Cosmic Microwave Background Radiation (CMBR). Suppose they stood in the same spot for a thousand years, and could only see the virtually linear motion of our sun and the motion of our earth in the cosmos (i.e. they could not see anything else in our galaxy). They would see almost exactly the same thing the astronauts just described would see from space.
If they could measure the velocities of the sun and earth they would note that the sun is moving at a constant 370 kps in a linear direction, but they would also note that the earth is not moving at a constant velocity. At times the earth is moving at 340 kps, at times it is moving at 400 kps (because it is going in circles around the sun), and at most times it is moving at some velocity between these two extremes. As with the astronauts, these observers would think that there was a race between our sun and our earth. At times the earth would be in front of the sun and at times it would be behind the sun in this race. At times it would be moving faster than the sun, and at times it would be moving slower.
Just because we don't "see" our 370 kps average linear speed in the cosmos on a daily basis (this is because of our "slow" speed relative to the vastness of the Universe) does not mean it is not happening. For many centuries before Kepler, no one believed our earth was rotating or that it was orbiting the sun. Their belief, no matter how popular or how sincere, did not stop our earth from rotating and orbiting the sun - and heading towards Leo. Between the time of Ptolemy and Kepler, the earth continued to rotate and orbit the sun and move with the sun towards Leo at 370 kps.
The important point to make with this metaphor is that the sun is traveling at a virtually constant velocity towards Leo (this is why secular aberration is generally ignored in celestial mechanics calculations), but our earth’s velocity towards Leo varies from 340 kps to 400 kps, depending on where we are in orbiting the sun, relative to our joint path towards Leo. Since Leo is on the ecliptic plane, the above example is very accurate.
Now lets talk about the bucket the running man is carrying. Suppose that high above the train and train tracks is a long pipe. On this pipe are occasional buckets that are full of water and each has a small hole in their bottom. The water is dripping slowly out of each of these buckets. These buckets are not moving, meaning each drop of water reaches the train perfectly vertical. Suppose each water drop hits the top of the bucket at the exact center of the top of the bucket, no matter where the running man is in his circular running around the table.
When a water drop hits the center of the top of the bucket, the train is moving at 370 kph and the running man is running at 30 kph. As just mentioned, the relative velocity of the running man to the ground varies between 340 kph and 400 kph. This means that the bucket is also moving at this range relative to the drops of water that are coming down. This is because the pipe is not attached to the train, it is attached to the ground. Thus, in the time that it takes the drop of water to travel from the top of this long, thin bucket (that is being carried by the man), to the bottom of the bucket, the bucket is moving with the train and running man. No drop will hit the center of the bottom of the bucket.
Because of the MTLs, in order for each drop to hit the center of the bottom of the bucket, the bucket will have to be tilted. However, because the man is running in circles around the table, his velocity is changing and the tilt of the bucket will need to be constantly changed, depending on where he is relative to the table at the time a drop of water hits the top of the bucket.
In a similar way, the tilt of aberration of starlight varies between 340 kps and 400 kps. It is this variance that is measurable and is caused by our earth's orbit velocity around the sun (remember this discussion is pertaining to the photon theory). We don't really care about the motion of each star or galaxy, what we do care about is the angle of this light relative to the telescope.
Note that with the running man, the tilt of the bucket is always based on two factors: first, the constant speed of the train, and second, the variable speed of the running man. If we were to measure only the change in the tilt of the bucket (and ignore the constant or absolute tilt caused by the train's motion), the change would only be caused by the variable speed of the running man. In other words, no part of the change is caused by the train's motion, because the train's motion is constant. The change in the tilt would be caused exclusively by the motion of the running man.
Likewise, with light, aberration of starlight is always based on two factors: first, the constant velocity of our solar system towards Leo at 370 kps, and second, our earth's orbit velocity around the sun at 30 kps. As with the running man, if we were to measure the change in the tilt of aberration (and ignore the absolute tilt caused by our solar system's motion towards Leo), the change would only be caused by the variable speed of the earth around the sun. No part of the change would be caused by secular aberration because secular aberration is constant. Thus, the USNO dictionary is quite right, secular aberration can justifiably be ignored.
The key point to all of this, and the point the reader needs to absorb, is that with the photon theory, actual aberration of starlight always includes our 370 kps motion towards Leo. There is absolutely nothing in the photon theory to challenge or contradict that secular aberration is actually observed, no matter what the source of light is.
If we think about the bucket mentioned above being horizontal, instead of vertical, and if we think about the water source as being in front of the train, little tilt would be needed for these drops. In other words, stars on or near the ecliptic plane have far less secular aberration or stellar aberration than stars normal to the ecliptic plane. But this does not negate that secular aberration exists for these stars, it simply means that because of their location much less tilt is necessary (i.e. the lack of tilt is not because of the lack of secular aberration, it is because of the angle at which the light arrives relative to our ecliptic plane or to be more accurate for secular aberration - the plane of our path towards Leo).
So why can’t we measure the absolute tilt of aberration? We could if we knew where a star really was. But we don’t know where any star really is, we only know where each star appears to be. We “see” the star in our telescope, and we think we know where it actually is, but in fact we don’t know where that star is really located.
The scientific community is willing to ignore knowing where stars are actually located because it is impossible to determine where they really are. In fact we will never know their exact location because there may be many other factors that affect the light between the star and our tiny planet.
Has the total 370 kps tilt of aberration ever been isolated and proven to exist? The answer is ‘no’ because we don’t know where the stars really are, thus we have no basis for calculating actual aberration. The term "secular aberration" was invented because our true 370 kps average velocity towards Leo must be accounted for in terms of aberration, no matter whether the photon theory or the ether theory is true. But the two theories account for secular aberration in vastly different ways, as will be seen as the book progresses.
In reality, it should be very easy to detect and isolate secular aberration, simply use terrestrial light, but things are rarely as simple as they seem.
With the ether drag theory, why is there any aberration of starlight? Late in this book there will be an entire chapter on aberration of starlight and ether drag. The conclusion of this chapter will be that aberration of starlight occurs at the boundary (i.e. the outside surface) of the ether drag. There are two different ways this can occur, but they will be mentioned later. This means that if the sun's ether drag extends beyond the orbit distance of the earth, that the 370 kps secular aberration actually occurs at the boundary of the sun's ether drag, millions of miles from earth. Only the 30 kps stellar or annual aberration occurs at the boundary of the earth's ether drag. In other words, with the ether drag theory, the total aberration of starlight is broken into two pieces, if the sun's ether drag extends beyond our orbit distance. A future chapter will detail this.