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Sightings of a double sun

At about 1950 GMT   on 3 August 2000, we

observed the setting sun through hazy cloud.There appeared to be two distinct suns, separated

by a thin band of dark lenticular cloud. The

upper sun appeared to be offset from the vertical

by perhaps 58

 to the right. There was no obviousred coloration. Obviously this was a refractioneffect ± perhaps by a lenticular mass of air of differ-

ent density (see Fig. 1). The phenomenon lasted

about 3 or 4 minutes after the first observation.

We would be interested in hearing any alternative

theory. We have never observed anything similar 

in 11 years here, or elsewhere.

Culbokie,   Pam Draper

Ross-shire   Laurence Draper

Jim Galvin replies:

Double suns are very rarely seen and little is

known about the theory of their formation. Theexplanation given by the Drapers is sensible, the

sun shining through a lens of low-density, moist

air, near saturation. Minnaert (1993) states thatseveral additional images of the sun may be seen

when the sun is just above the horizon. A typical

refraction angle is between about 1 and 28. They

occur when there is a variable amount of cloud inmore than one layer.

The 1200 GMT  radiosonde profile on 3 August

from Stornoway, on the Isle of Lewis, shows thatthere was a very stable layer of air, near saturation,

between about 600 and 1500 m above sea-level.This corresponds to the cloud layers seen on and

above Ben Wyvis. Above this moist layer, the air was dry to an altitude of about 6000 m, above

which layers of cirrus and cirrostratus cloud

would be expected ± the hazy cloud observed bythe Drapers. The ice crystals of these clouds may

also have had a role to play in the refraction of 

light to form a double sun. The clockwise rotation

of the secondary image by about 58   is difficult to

explain and may be an optical illusion, due to thecurvature of the hill top and its cap cloud.

Most often, the secondary image is visible above

the sun, as partial refraction through the lower-

density air will deflect the sunlight upwards. Thiscan be seen in Fig. 2(b) (p. 292), received from

Prof. Paolo Candy of Viterbo, Italy. However,

occasionally the secondary image appears belowthe sun, as can be seen in Fig. 2(a).

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Acknowledgement

Ian Macgregor of the National MeteorologicalArchive provided radiosonde profiles for Storno-

way.

Reference

Minnaert, M. G. J. (1993)  Light and color in the out-

doors. Springer-Verlag, New York 

Fig. 1 Sketch of the double sun seen over Ben Wyvis, looking north-west from Culbokie, Black Isle, Ross-shire

(57830 ’ N, 48W)

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