The first OPOD goes to Gerhard Hüdepohl for this shot of the Green Ray on the lunar disk
The green ray is an atmospheric optical phenomenon belonging to the mirage family.
The physical causes indicated are:
Atmospheric refraction: light is bent while crossing air layers with different properties.
Dispersion: different colors are refracted at slightly different angles.
Selective absorption: the atmosphere attenuates the various components of light in a non-identical way.
Rayleigh scattering, which contributes above all to the selection of observable colors.
The particularly interesting part is the thermal stratification of the atmosphere. If near the horizon there are air layers at different temperatures, their refractive index changes. This can produce a mirage that distorts, flattens and can even multiply the image of the solar limb. Under certain conditions the green rim appears, which can turn into the green flash.
The San Diego State University astronomer Andrew T. Young states: “Green flashes all derive from the effects of variations in astronomical refraction near the horizon. Although there are several types of green flash, each of these is a by-product of a corresponding mirage and each type is always a magnified part of the green edge produced by atmospheric dispersion. This means that a separate explanation is needed for each type of green flash, so understanding mirages is essential to understand green flashes“.
📷 Photo: Gerhard Hüdepohl (https://atacamaphoto.com/)
📍 Location: Atacama Desert
🌙 Moon: 0° above the horizon
🔬 Phenomenon: green flash
⭐ Observation difficulty: rare
Column curated by Marco Meniero and Marcella Botti.
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