Double Star over the Pyramids
The picture shows a Sun halo and a mock Sun gleaming over the Pyramid of King Khephren (Khafre). The Sun shines beyond the pyramid’s apex. The mock Sun shines as a fainter, smaller Sun to the left of the image, at the same altitude of the Sun. The Sun halo shines as a rose ring of light over the pyramid.
Photo by Aymen Ibrahem, BA Senior Astronomy Specialist

On 5 April 2008, magnificent patterns of high clouds drifted for hours, over Cairo, Egypt. BA Senior Astronomy Specialist, Aymen Ibrahem, recorded the fascinating optical atmospheric phenomena that are usually associated with high clouds, including Sun halos and mock Suns. Ibrahem took unique images showing Sun halos and mock Suns glowing over the mighty Pyramids of Giza.

Sun halos occur when high clouds veil, or pass near the Sun in the sky. A Sun halo appears as a ring or an arc of light around the Sun. Sun halos of various sizes arise due to the refraction of sunlight within the tiny hexagonal ice crystals contained in the clouds. The most common of halos is often 22 in radius, and hence is known as the 22 halo. The 22 halo is usually white in color, but under some conditions, it may be reddish or even resemble a rainbow.

The mock Sun, also known as the sundog, is usually associated with the 22 halo. It shines as a bright white patch, situated at least 22 from the Sun and at an altitude similar to that of the Sun. It may even resemble a smaller fainter Sun, giving the impression there are two Suns in the sky. The mock Sun may also be a colorful patch, shimmering like a segment of a rainbow.

Sometimes two mock Suns form simultaneously on either side of the Sun. In this case, the Sun may appear to have two smaller companion Suns! This phenomenon is relatively rare in Egypt. The mock Suns can often be observed in the morning or in the evening, when the Sun is near the horizon. The scientific term of the mock Sun is parhelion. The flat ice crystals forming the mock Sun are horizontally oriented, while those forming the Sun halo are randomly oriented.

The Circumzenithal Arc (CZA), also known as the Bravais’ arc, is a fascinating optical phenomenon associated with the 22 halo. It is a colorful arc that resembles an upside-down rainbow. The CZA occurs almost directly overhead in the form of a quarter of a circle centered on the zenith, the point directly head over, and facing the Sun.
Like mock Suns, the CZA arises due to the refraction of sunlight within plate-like, horizontally-oriented hexagonal ice crystals. Sunrays enter and leave the crystals forming the mock Sun through their vertical side faces. However, in the case of the CZA, Sunrays enter the crystal through the upper horizontal basal face, and leave through one of the vertical side faces.

The CZA forms only when the Sun’s altitude in the sky is low, between about 15 and 25. It can never be observed if the altitude of the Sun is greater than 32.2. On exceptionally rare occasions, the CZA is visible as a complete circle around the zenith. This phenomenon, known as the Kern arc, has been recorded in Finland, in November 2007.
“A few weeks earlier, I photographed Sun halos with each of the three Pyramids as a foreground,” said Ibrahem. “Interestingly, the pictures show graceful celestial and ancient Egyptian geometries. It is intriguing to speculate how the ancient Egyptian priests interpreted the halo phenomena.”

Ibrahem took over 400 images of the memorable halo display. Some of his photos were published by two international publishers. The picture gallery below features images of the three halo phenomena, taken on the Plateau of Giza. 

Picture Gallery

Fig (1)
Sun Halo, Mock Sun and CZA over the Pyramid of King Khephren
The Sun shines beyond the apex of the pyramid. The mock Sun shines as a fainter, smaller Sun to the left of the image, at the same altitude of the Sun. The Sun halo is an arc centered on the Sun. The CZA is visible above the halo, near the top of image, as a reddish arc. The presence of haze made the arcs relatively faint.
Photo by Aymen Ibrahem, BA Senior Astronomy Specialist

Fig (2)
This is an annotated, contrast-enhanced version of Figure (1).
Photo by Aymen Ibrahem, BA Senior Astronomy Specialist

Further Reading

Atmospheric Optics (a famous website by Les Cowley)
A Library in Space
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A Ringed Star Shines over the BA
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Bibliography
Color and Light in Nature, David Lynch and William Livingston, Cambridge University Press, 1995.

Aymen Mohamed Ibrahem
Senior Astronomy Specialist