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Tangent arcs alter shape as the sun's altitude changes. Mouse slowly over the slider to see them.
When the sun is on the horizon, both arcs are narrow 'Vs'. As the sun climbs the upper tangent arc opens and then droops like a gull's wings.
When the sun reaches 29º the upper arc links with the lower to become the "circumscribed halo".
The lower tangent arc narrows and then broadens again as the sun ascends. Of course, the lower arc is below the horizon when the sun is less than 22º high. Even so, it can still be seen from mountains or aircraft.