We could say that the annual parallax of a star is the angle defined by the sun, the star and the Earth. (considering the star perpendicular to the line that unites the Sun and the Earth).
π is the angle SÂE in the picture below. (annual parallax angle).
The star considered to be nearer to the Earth is Proxima Centauri that has an official parallax angle of 0.75 seconds of degree. The parallax effect due to the movement of the observer on the Earth orbit around the sun means a periodical movement of the star on the celestial sphere. The ellipse thus projected by this movement on the celestial sphere by is called parallax ellipse and has a periodicity of one year.
Annual parallax is a hoax
Warning: once again the so said “aligned astronomers” don’t consider the movement of the sun in the galaxy, but only the rotation of the Earth around the sun. The parallax thus shouldn’t originate an ellipse but a spiral during the year. |
How do astronomers determinate the parallax angle? This determination is one of the most difficult but most important key point of sidereal astronomy.
This issue is so important because when knowing the parallax of one star, its distance from the Earth can be determined . We can thus understand the efforts of astronomers in their attempts to be absolutely accurate in determining the parallax angles of stars.
The first parallax to be determined was 61 Cygni. It was calculated by Bessel in Konigsberg in 1837-38.
There is more than one method to determine the parallax. Here you will read about the trigonometric method.
To determine the parallax of a star S the astronomers chose two stars A and B with parallax almost equal to zero because they are very far. A and B must be aligned on a parallel to the ecliptic one on a side and one on the other side of the star S. During the year A and B will remain fixed in the celestial sphere while S, nearer to the Earth, will move toward A for six months and toward B for the rest of the year. By measuring for one year the amount of these movements it is possible to determine the parallax. These very little angles where measured by using an eliometer. Today the preferred method is photography that “consents much precision”. The idea is simple: when the star S is at one extreme of the ellipse, one picture is made, another when the star is on the other side of the ellipse, after six month, and another picture of control is made after one year. Pictures are checked and from the movements of S in respect of all the other stars the parallax is determined. Parallax is considered to be a strong evidence of the rotation of the Earth around the sun. If Earth were motionless this phenomenon wouldn’t exist. |
A consideration I have to do is that the parallax angle is really small, always smaller than one second of degree.
Consider a circle, divide it in 360°. Then take one single segment and divide it 3600 times. Well, the parallax angle of the nearest star is even smaller. |
This angle is even smaller than the aberration angle (you certainly remember it was calculated as 20”,45). But both these angles are smaller of the refraction angle. So we have three ellipses (the parallax, the aberration and the refraction ellipses) that superpose one over the other. The refraction ellipse, the greater one, is very changeable during the year depending on temperature and pressure of the air. Also the aberration depends on the air temperature, since light speed depends on the dielectric constant of the mean and consequently on temperature.
So how is it possible to evaluate with a photograph the contribution of the aberration ellipse, and even more difficult, the contribution of refraction when it could be sufficient a slight hot current of air in the moment the picture is made to change all the results?
The conclusion is that the annual parallax doesn’t exist, cannot be measured and absolutely cannot be used to determine distances of the stars or of planets.
Real accurate maethods to measure the height of planets and stars
This doesn’t mean that it is not possible to measure the distance of a star from the Earth and in this blog we have already shown some example when we have calculated the height of the sun or the height of Polaris. We have not used the annual parallax obviously but a parallax calculation that we call triangulation. We can remember these two cases:
Height of the sun.
To make a triangulation we need two observers that from two distant places (parallax) on the Earth can measure the angle of the sun. With these two angles we can track two lines that define the height of the sun. If you are alone you can consider that on solstice the sun will be vertical over the Capricorn and make thus the triangulation with that point. |
Height of Polaris.
Also with Polaris triangulation is easy. Polaris is in fact exactly on the North Pole. When we have measured the angle from our latitude, since we know the distance from the pole, we can easily calculate the height of Polaris. |
Next post will be about planets: we can use tha magical square to know something more about their height and trajectory .