Light is naturally available in our surrounding, either in day time or at night. Such light may derive from the natural or artificial resources. When we see an object located far from our standing place, we think that what we see is its actual appearance. We are frequently unaware that what we see is actually not the real appearance of such object.
For example, once upon a time when we are at a beach and admiring the beauty of nature before the sun set. The sun looks moving down slowly, and in a certain time the lower part of the sun touches the edge of the sky or horizon. The panorama is so beautiful. However, when we see that beautiful panorama we are not aware that the actual sun has already fallen under the horizon. So, what we see is not the actual sun, but the apparent sun, or the sun on its ‘apparent position’. Even, the horizon or the sky edge that we see is not the actual sky edge, but the illusive sky edge.
Such phenomena is caused by the occurrence of light refraction reaching to our eyes. The light refraction causing the presence of apparent sun is called ‘astronomical light curve’ or ‘astronomical refraction’, whereas the thing causing the presence of the illusive sky edge is called ‘earthy light curve’ or “Terrestrial Refraction”. The said terrestrial refraction causes the phenomena of ‘Mirage’. And mirage is not an optical illusion, but an actual physical phenomena.
And so is at night time when the sky is clear and we can see and admire the stars sprinkle and spread at the sky. All those space objects are not in their true conditions, but on their apparent position and all of them are caused by the astronomical refraction.
From the above explanation a question arises: Cannot we ever see in our bold eyes a star in the sky on its true position condition? Such a chance is available, though limited, and will be found at the following discussion.
The ray curve occurs because the lights of an object reaching to our eyes / observers are not transmitted in the form of straight lines, but deviated by a medium all along its track, including the deviation by the earth atmosphere. The ray curve is an angle occurring between the apparent position direction and the true position direction of the said object.
The light of the stars in the sky reaches the earth passing through a very long distance so far away, and has already passed through various kinds of medium respectively having different densities. The classical scientists such as Aristotle, Rene Descartes, Sir Isaac Newton and others believed that the light of the stars reaching us on earth crept spreading through a medium the so-called luminiferous aether. However various kinds of experiments had been made, among other was an experiment conducted by the American Scientists Michelson and Morley in the 19th century, and all of those experiments failed to detect the presence of luminiferous ether, so that the ether is deemed non-existent. There is a possibility that luminiferous ether truly exists, but it cannot be proven.
It is quite clear that the lights of objects in the sky reaching the earth have passed through layers of the terrestrial atmosphere, known as having different air density. Closer to the earth surface, the air is denser compared to the density of the air layer above it. The density is getting looser or weaker when it is getting higher.
The Snell’s law on light refraction declares that if a ray of lights passes through from one medium to the others with different densities, such ray of lights will be refracted. The magnitude of refraction angle depends of density of its medium. For example, a ray of lights is passed through water, the said ray of lights will be reflected closer to the normal.
The angle between the direction of the light and normal is called the ‘ angle of incidence ‘ and the angle between the direction of refraction and normal is called the ‘ angle of refraction”. Snell’s law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities the two media, or equivalent to the reciprocal of the ratio of the indices of refraction
A ray of lights is not refracted if its track is at the same direction with the normal. This statement answers the above question, that the change and the only chance to see a star on its true position is at the moment when the said star is precisely located straight above our head as an observer or exactly at the Zenith point.
At the above picture, the difference between air density and water density is sufficiently big or in a sudden, therefore the light track in the air and in water looks like a broken line. It is completely different from the light track at the earth atmosphere. The air density at the layers of earth atmosphere changes gradually and regularly. This causes the light refraction in the form of a curve. And the effect of such curve, the apparent position of a star will always look higher than its true position.
A curve of ray or light deflection is also known at the theory of Einstein, namely a deflection of light when passing through a gravity field of a massive object. According to this theory, when the light of a star passes through a gravity field of the sun, the said light will be deflected inwards, so that there will also be the ‘apparent position’ and the ‘true position’ of a star.
If Einstein’s theory of relativity was correct, then the light from stars that passed closest to the sun would show the greatest degree of “bending”. And the stars whose light tracks are very far from the sun have their lights not being bent or deflected. The stars whose lights are not deflected means that there is no difference between the apparent position and the true position of the stars. If being consistent with this theory, it means that all stars visible at night time are at the appearance of the stars on their true positions, because the said stars do not pass through the field of gravity. This is certainly incorrect if it is seen from the astronomical scientific point of view in astronomy.
The presence of ‘apparent position’ and ‘true position’ of a star is caused by the presence of ray curve, either the astronomical refraction or the terrestrial refraction, and not caused by the influence of gravity field. And due to the presence of ray curve, the ‘apparent position’ of a star always looks higher than its ‘true position’. It is different from illustration on the deflection of light caused by gravity field of the sun as stated at the reference above.