The history of the investigations of Zodiacal light could be referred to three big periods. The beginning of the first period is connected with the name of Cassini who gave the first scientific explanation of this event. After the Cassini's work the Zodiacal light was observed only visually, its brightness and color were measured and its cone was drawn. Its boundaries were determined.
The second period starts with the work of V. G. Fesenkov, who at the beginning of the 20th century was the first who used systematical instrumental measurements of the brightness of the Zodiacal light with the help of a photometer which was constructed specially for this aim. Later photographic and photoelectric photometry was used.
The third period is related with the use of orbital instruments excluding the influence of the Earth's atmosphere.
What kind of scientific investigations are used?
- visual observation
- photometry - optical and CCD
- spectroscopy in visual, IR and UV wavelength
- measurement of the Doppler Shift
- measurement of the polarization of the Zodiacal light
- observations of the meteors and meteorites
- observations of comets and asteroids
- space investigations of the dust
- analysis of the dust particles from the Space.
The results from these investigations are so many, but we can summarize them in some different directions.
The brightness of the Zodiacal light is a very important parameter. It depends on the intensity of the light reflected by the dust particles namely on the angle of the reflection. From the observations of the brightness the dependence between concentration of interplanetary dust and distance to the Sun is determined. A relation between brightness of the Zodiacal light and activity of the Sun particularly of the solar flares. It means that the distribution of interplanetary dust depends on the solar corpuscular emission and electromagnetic field of the Sun. Seasonal and annual variation of the brightness of the Zodiacal light is noticed.
The position of the symmetry axis of the Zodiacal light is very substantial, because this is the plane of the Dust clouds in our Solar system. But the results are disputable yet.
Maxwell Hall shows the smallness of the deviation of the central line from the ecliptic. When smoothed out the maximum latitude is less than 3o, which seems to preclude the coincidence of the central plane of the light with that of the Sun's equator. But the observations do not extend continuously throughout the year, and do not include a sufficient length of the central line on each evening to enable us to distinguish certainly the heliocentric latitude of the central line, as distinct from its apparent geocentric position. Hall also reaches the interesting conclusion that the plane in question seems to lie near the invariable plane of the solar system, a result which might be expected if the light proceeded from a swarm of independent meteoric particles moving around the Sun.
Chaplain Jones concludes that the central line of the arch made an angle of 3o 20' with the ecliptic, the ascending node being in Taurus, near longitude 62o. This is about 40o from the ascending node of the invariable plane so that there is a well-marked deviation of his results from those of Hail.
Yet more divergent are the conclusions of Francis J. Bayldon who places the ascending node at the vernal equinox. He found that as the observer moved to the north or south, the axis of the light appeared to be displaced in the direction of the motion, which is the opposite of the effect due to parallax, but in the same sense as the effect of the greater atmospheric absorption of the light on the side nearest the horizon. He also describes the moon as adding to the Zodiacal light during her first and last quarters, a result so difficult to explain that it needs confirmation.
The results from the two Helios spacecraft, COBE-Dirbe and Clementine suggest that there is at least a 2o difference between the ecliptic and symmetry planes and that the plane of symmetry may not be uniform.
Indeed the real question is, which plane coincides with this axis - the ecliptic plane, the solar equator, the orbit of Venus or the orbit of Jupiter?
The color of the Zodiacal light is difficult to be determined visually because it is observed in night time when it is difficult for the human eye to distinguish colors. The color is determined by the distribution of intensity of the light in the different wavelengths. By photometric researches, it is established that the color of the Zodiacal light is near to the color of the Sun. Implicit, the dust particles scatter the sunlight without changing its spectrum. It means that the dimensions of the particles are bigger than the dimensions of the molecules.
The spectrum of the Zodiacal light is the same as the solar spectrum, but some changed from the Doppler shift due to the motion of the interplanetary dust particles.
The Infrared spectrum of the Zodiacal light cannot be observed from the ground, but it has been found that the intensity in this part of the spectrum is bigger from the expectations. In the 3-70 micron wavelength regime, the Infrared sky is dominated by thermal emission from interplanetary dust particles. Consequently the dust particles do not reflect the light only, but they are heated from it.
The measurements place a strong constrain on models of interplanetary dust concerning constituents and size distributions. The observed slight dependence of the temperature of the Zodiacal light on ecliptic position will be modeled in the context of the three-dimensional density and temperature distribution of the interplanetary dust cloud.
Infrared Space Observatory (ISO) performed an extensive observing program on the Infrared Zodiacal light, including multi-filter photometry of the global brightness distribution, observations of the asteroidal bands and commentary dust trails, and investigation of the small scale brightness fluctuation. The detailed determination of the spectral energy distribution of the Zodiacal light, including mid-Infrared spectrophotometry, opens the possibility for the separation of the main components of the Infrared sky. The mid-Infrared spectrum may provide information on the nature of the constituents, and on the size distribution of the interplanetary grains.
The Ultraviolet emission of Zodiacal light is also vastly and identical with the solar emission.
Polarization of the Zodiacal light is very difficult to measure because the other components of the light of the night sky are polarized too. The Zodiacal light appears to be quite strongly polarized (about 30%), but it is not the same everywhere on the ecliptic. It helps the scientists to determine the particles' distribution according to their size. To a great extend the polarization of the light is due to the electrons from the solar wind. But it is also possible for the specific shape or the chemical nature of the particles to determine this polarization.
Obviously extra investigations are needed to answer to the numerous questions about the Zodiacal light.

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Designed by:
Georgi Kokotanekov - gdk_pmg@yahoo.com
Dimitar Velevski - d_velevski@abv.bg
Dimitar Indzhov - d_indzhov@abv.bg
Joanna Kokotanekova - joanna@astro.bas.bg
Astronomical Observatory - Haskovo, BULGARIA - ac_helios@mail.bg