This phenomenon is very weak to sunlight, so it was not until 1856 that Danish astronomer Bronson wrote the first record: "The brightest part of sunlight is just opposite to the position of the sun, so the estimation of the maximum brightness position is often within 1." The faintest light is enough to drown the sunshine, so only on a clear night without the moon, away from the city, and let your eyes fully adapt to the dark environment, can you hope to see the sunshine. The most favorable observation time is March and September; It is difficult to observe in winter and summer, because it just overlaps with the Milky Way.
The cause of sunshine is still inconclusive. There are four popular theories:
1, zodiacal light hypothesis, regards sunlight as a part of zodiacal light, and its brightness increases because the scattering function of particles in this direction has a maximum [backscattering]. At present, many people are inclined to this view.
2. According to the Gildan-Moreton hypothesis, there is a concentrated area of interplanetary dust around the anti-solar libration point 0.0 1 astronomical unit from the earth, and their scattering of solar light forms the opposite sunlight.
3. The dust tail hypothesis holds that under the action of solar wind and radiation pressure, the earth produces a dust Cloudtail, which points in a direction deviating from the anti-solar point, and the scattering intensity in this direction can be fully enhanced, thus producing sunlight.
4. The gas tail hypothesis holds that there is a "gas tail" on the earth, and the formation of sunlight is related to the emission of excited atoms and molecules in the gas tail.
Although there are different opinions, the solar spectrum has no emission line, which is similar to the solar spectrum, but slightly reddish. From this point of view, it seems certain that it is only an interplanetary substance that reflects sunlight.
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