The measurements of light from Mars can show a pedagogically clear and convincing way of presenting the speed of light to the students. The expected results are supposed to show that the wavelength of light varies with the speed of Mars relative to the Earth according to Doppler’s law.
The speed of Mars does not affect the wavelength of light from Mars
In the literature we find measurements of a wavelength of light from moving sources. However, we do not find a measurement to measure the influence of the speed of the planets on the wavelength of light from them. On the planets, we know well the conditions of the light source and the speed of the light source.
Based on the postulate of the speed of light, we expect that the speed of the light source affects the wavelength of light according to Doppler’s law. On the other hand, several measurements cast doubt on whether the speed of the source affects the wavelength of light. I cite two measurements; it is a measurement of the wavelength of light from a comet and a measurement of the wavelength of light from the Sun.
The wavelength of light from a comet does not depend on the speed of the comet.
In 1997, the wavelength of the spectral line from comet Hale-BOPP was measured. The authors of the measurement measured that the wavelength does not change with the speed of the comet to a significant part of the light of the spectral line.
They felt that the wavelength of light changes the atmosphere. Light from a comet in the atmosphere hits air molecules. These absorb it and then immediately emit it with such a wavelength that the wavelength on the instrument does not depend on the speed of the comet.
The light absorbed by the atmosphere and then immediately emitted is scattered on all sides. The image of a comet is shown to us only by light coming directly from the comet, which is not absorbed and emitted by the atmosphere. This, in turn, means that the photons showing us the image of the comet did not react with the air.
Measurements of the wavelength of light from the Sun.
To observe the Sun, ESA and NASA launched the SOHO satellite into Earth orbit. This contains a Fabry-Pérot interferometer (LASCO C1) for measuring the wavelength of light.
There are turbulences on the surface of the Sun, which cause the particles of the solar corona to move at a speed of a few tens of km/s. During solar flares, the plasma of the solar corona reaches a speed of more than a thousand km/s.
Despite the high plasma velocities, the LASCO C1 meter on the SOHO satellite detects only a slight shift (scatter) of the wavelength of the spectral lines. It does not detect a shift of the spectral line in the range as one would expect according to Doppler’s law.
In the case of the postulate of the speed of light, at turbulent speeds of molecules (light sources) on the Sun, both the frequency and the wavelength of light would change greatly. The speed of the light source would extend the spectral line beyond recognition.
The measured brightness of the Sun, that is, the dimming of individual wavelengths of light, is shown in the diagram in the figure.
A set of spectral lines (dimming) of light from the Sun.
Clearly recognizable forms dimming indicate that the speed of the light source insignificant effect on the wavelength of the light. In many measurements, the authors recognize dimming of the spectral lines, which almost does not change depending on the speed of the sun’s corona. They concluded that either the solar corona moves very slowly, that it is very calm, or even that it is dormant. Some even concluded that their instrument was not working.
Measurement of incoming light from the planets.
The planets move at a speed of up to a few km / s relative to the Earth. The speed between Earth and Mars reaches up to 6 km/s, and between Earth and Jupiter up to 15 km/s. The speeds of the planets relative to the Earth vary according to the position of the planets. The velocities of the planets are sufficient to measure how the speed between the planet and the Earth affects the wavelength of light.
In the literature,, we find several articles on wavelength measurements of light from Mars and Jupiter. I focus on measuring the spectral lines of light from Mars, which is described in the article Spectra of reflected sunlight from planets. The measurement result is given in the form of a diagram in the figure.
Light spectrum of Mars. White frames show the light created by Mars, and shaded frames show sunlight reflected from Mars.
The measurement results do not show any shifts in the wavelength of light that would result from the motion of Mars. Even otherwise, we do not find records in the literature that would indicate that the speed of Mars would in any way affect the wavelength of the light arriving from it.
We measure light from Mars
How to understand the results of these measurements could be discussed indefinitely. Such discussions can be avoided by measuring the wavelength of light from Mars, where we know in detail both the light source and the motion of the light source. The measurement offers an unambiguous answer to the dilemmas described above.
The spectral lines from Mars are technologically measurable. Measurement of light from Mars is expected to show that the speed of Mars does not affect the wavelength of light coming from it. The wavelength of light from Mars is the same regardless of the speed of Mars. However, such a measurement has not yet been performed according to the standards and requirements of physics, nor have the results of the measurements been expertly assessed.
Light wavelength measuring instruments can be sensitive to both frequency and wavelength of light. For the purposes of this measurement, it is essential to use an instrument that is sensitive only to the wavelength of light and not to the frequency of light. One such instrument for measuring wavelength is the Fabry – Pérot interferometer.
We really know as much about nature as the measurements tell us about it. We need to measure everything that is measured.
The results of the measurements must be checked in several places. If a group of experts makes measurements of the wavelength of light from Mars in just one laboratory, physicists in general may not pay attention to them, at least initially. The results of this measurement are a serious objection that the speed of light is always the same.
Physics can make progress in its knowledge only when a significant part of physicists want to learn more about the topic. In this case, it is a superfluous question to measure or not to measure. Measurements emerge in laboratories of many institutes in several places and the measurement results are disseminated to the public.
The purpose of this article is to encourage astrophysicists to measure the influence of the speed of Mars on the wavelength of light from Mars, publish the results, and thus consolidate one of the key issues of modern physics.
These comprehensible measurements to a wide range of readers can replace less comprehensible measurements to prove the postulate of the speed of light, such as measurements of the decay of a basic meson particle, GPS system explanation, and similar proofs of the speed of light.
 6300 Large Aperture Photometry of Comet Hale-BOPP.
 Michael Richmond: The shape of spectral lines.
- Measurement of the solar corona from the SOHO satellite.
- Robbrecht: New techniques for the characterization of Dynamical Phenomena in Solar Coronal Images, Katholishe Universiteit Leoven, februar 2007.
- B. Delone, E. A. Makarova, G. V. Yakunina: ”Evidence for Moving Features in the Corona from Emission Line Profiles Observed During Eclipses”, v: Journal of Astrophysics and Astronomyvolume 9, str. 41–47, Moskva, 1988.
- P. Raju, J. Singh, S. Muralishanker: ”Fabry-Parot Interfereometric Observation of the Solar Corona in the Green line”,
v: Kodaikanal Obs. Bull., Vol. 13, str. 41–46, Indijski inštitut za astrofiziko, Indija, 1997.
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 C. C. Kiess: High-dispersion Spectra of Jupitor, Washington, Astrophysical Journal, vol. 132, p.221, 1960.
 Lewis D. Kaplan: An Analisys of the Spectrum of Mars, California Institute of Technology, 1964
 Publications of The Korean Astronomical Society, september 2015.