Antitwilight Sky

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  • #611994
    Gianni
    Participant

    Cheers to you all

    For some years now I have been observing colours from the anti-solar sky at sunset and sunrise. The attached images are photographic observations I have made from the seashore (from Pescara city on the Adriatic Sea), from a hill (Roio hill) near my town (L’Aquila) and from my room window. All were taken at sunset time with a simple 6Mpixel camera in auto mode (f=5.8 mm). At least three areas are evident: Blue Belt (bottom); Belt of Venus (pinkish stripe) and Upper sky. However, the fine structure is more complex. The height above the horizon of the VB maximum brightness is indicative of the transparency of the atmosphere on a local-synoptic scale and correlates with the thickness of the stratospheric ozone layer.
    If this topic is of interest to anyone, I will gladly accept their contribution. For any further information, please contact me cross19@libero.it

    Greetings to all.

    Giovanni

    (Translated from Italian with DeepL Pro)

    Translated with http://www.DeepL.com/Translator (free version)

    #611998
    Grant Privett
    Participant

    Lovely images. The Belt of Venus is rather beautiful.

    I only see it occasionally in the UK. Does it require particularly transparent skies to appear?

    #612000
    Gianni
    Participant

    The Venus Belt, but also the other structures, appear if the sky is clear and clear around the horizon. Frigid and clear winter days, with a N wind, are the most favourable. All colours are most evident about 10 minutes after sunset (or sunrise). In summer, particularly the current very hot summer, the haze does not allow good observations of the phenomenon.

    Attachments:
    #612006
    Paul Leyland
    Participant

    The “blue belt” is also known as the “Earth shadow”. It can be very impressive at times.
    The “belt of Venus” corresponds to the red shades seen towards the opposite direction in the sky. It comes from back scattered red light.

    #612011
    Grant Privett
    Participant

    After seeing the pictures from Giovanni, I dug out an old picture….

    https://britastro.org/observations/observation.php?id=20220818_202738_4a8c0021466584a7

    #612018
    Gianni
    Participant

    Yes, the arc is called Earth’s Shadow, it persists until about 30 minutes after sunset and appears 30 minutes before the sun rises. However, this is not the true Earth’s Shadow, which may be visible, but with great difficulty, as a narrow ‘Dark Segment’ in contact with the horizon, and in any case only from high mountains. The antitwilight phenomenon is generated by the scattering (Rayleigh, and Mie) of sunlight that has passed through the atmosphere, grazing the planet. Therefore, position and intensity of colours contain information about the physical state of the atmosphere down to the synoptic scale (500-1000 km)
    I have been systematically observing this phenomenon for about three years. Observation is cheap, easy and interesting. Light pollution and air turbulence do not affect it.
    Perhaps, Leonardo da Vinci was the first to attempt a rational explanation of the colours in the sky. The French mathematician de Mairan in his Traité Physique et Historique de l’aurore boréale (page 400) expressed astonishment that such a striking effect, as old as the world, was only mentioned in Joh. Casp. Funccii and not in the books on Physics and Astronomy.

    #612019
    Nick James
    Participant

    Hi Gianni,

    Here’s an example from the top of a mountain showing its shadow at sunset. This was taken from the peak of La Palma.

    #612022
    Gianni
    Participant

    Well done and quite meaningful images, yours. Grant, the picture portrays the coloured areas very well, very evident the pinkish VB. The image taken by Nik portrays well the shadow that a mountain casts on the atmosphere and blends in with the Blue Belt. In both images the sky is cloudy below but very clear above. This is an interesting feature, but very rare in my observing locations. Many thanks to you.

    #612038
    Grant Privett
    Participant

    Cloud tends to be below you when you are at 2400m high. 🙂

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