Heiligenschein
It seems that it is a prerogative of Nature to light up the antisolar point, as we have already seen in the Brocken Spectre or in the Gegenschein. There is, however, an optical phenomenon that creates a bright area around the shadow of the observer’s head which appears white but, unlike the Gloria, occurs in the absence of clouds. We are talking about the Heiligenschein. It depends on spherical dew drops, hanging from blades of grass, which act as microlenses. These tiny water spheres channel sunlight through a geometric-optics mechanism into an approximate focus about 20% of their diameter beyond their rear surface. Some drops rest on the tip of tiny hairs and do not touch the leaf surface. When the drops and hairs have the right size, sunlight is focused into a bright spot on the leaf. Light from the spot spreads in all directions, but some rays pass again through the drop tracing the path in reverse. The effect produced in each drop is called backscatter and sends light back toward the observer more brightly than in other directions. Thus backscatter makes the dew shine more intensely at the antisolar point.
Exceptionally, the droplets can also form a bright arc with a forty-degree radius, visible if the meadow is large enough. The Heiligenschein should look green because the droplets sit on leaves, but it appears white because the individual points of light saturate the eyes’ color receptors.
The two photos were taken from the balcony of a tower; in the shot on the left you can see the glow around the left shadow, in fact I was standing on the left side of the tower. To take the right shot I stood on the right balcony and in this case I captured the glow on the right shadow.
Sylvanshine
A very similar, but much rarer phenomenon is Sylvanshine, an optical phenomenon in which dew lies on the wax coating some types of leaves and reflects, via the backscatter mechanism, the beams of light it receives. This effect makes trees look as if covered with snow during summer nights. The phenomenon was explained in 1994 by Professor Alistair Fraser of Pennsylvania State University , an expert in meteorological optics. According to his analysis of the phenomenon, the wax on leaves causes water to form beads, which in effect become microlenses. These lenses focus light onto a point on the leaf surface and the image of this point is then directed in the opposite direction.
Il Sylvanshine e l’Heiligenschein sono due fenomeni ottici osservabili in prossimità del punto antisolare, ma differiscono per il meccanismo fisico che li produce e per le condizioni in cui si manifestano.
1. Sylvanshine
È un fenomeno luminoso osservabile in un bosco o in una vegetazione ricoperta di goccioline d’acqua, quando il Sole si trova alle spalle dell’osservatore. Le gocce presenti sulle foglie riflettono la luce solare in direzione dell’osservatore, producendo una brillantezza diffusa della vegetazione. Il nome deriva dall’inglese sylvan, relativo al bosco.
2. Heiligenschein
È un alone luminoso che circonda l’ombra della testa dell’osservatore, proiettata su erba o vegetazione ricoperta di rugiada. È dovuto soprattutto alla riflessione della luce all’interno delle goccioline, che agiscono come piccole lenti, e alla sua riflessione da parte delle superfici vegetali sottostanti. La luce viene rinviata preferenzialmente verso la sorgente, rendendo particolarmente luminosa la zona attorno al punto antisolare.
La differenza fondamentale
Sylvanshine: la luminosità è distribuita sulla vegetazione bagnata, spesso su un’intera porzione di bosco.
Heiligenschein: la luminosità è concentrata intorno al punto antisolare, in corrispondenza dell’ombra della testa dell’osservatore.
Entrambi richiedono condizioni di illuminazione e superfici favorevoli, ma l’Heiligenschein presenta una geometria più localizzata e caratteristica, mentre il Sylvanshine è un fenomeno più esteso.
Opposition Effect
In nature, besides water drops, there are also minerals and crystals that retroreflect light. This is why the bright formation we talked about can also appear without the presence of water, so we should not be surprised that it has been observed even on the Moon, on Mars and on some terrains. However, when it forms thanks to crystals it is called the “opposition effect”, or “Seeliger effect”. Famous is a photo of it that appears around the head of the shadow of Buzz Aldrin due to the retroreflective properties of the lunar regolith. The photo was taken by Neil Armostrong, who appears himself on Aldrin’s visor.
Opposition Effect on MARS
Mars Exploration Rover 'Opportunity' descending onto Meridiani Planum
24 January 2004
Summary
- The opposition effect is an illumination of the antisolar point due to the backscattering of light within crystals or water drops.
- The Heiligenschein is an "opposition effect" in water drops.
- The Sylvanshine is a Heiligenschein that occurs on plants.
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