<P> Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and are centred on the sun itself . They are about 127 ° (violet) to 130 ° (red) wide . Since this is more than 90 °, they are seen on the same side of the sky as the primary rainbow, about 10 ° above it at apparent angles of 50--53 ° . As a result of the "inside" of the secondary bow being "up" to the observer, the colours appear reversed compared to the primary bow . The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky . Each rainbow reflects white light inside its coloured bands, but that is "down" for the primary and "up" for the secondary . The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it . </P> <P> Unlike a double rainbow that consists of two separate and concentric rainbow arcs, the very rare twinned rainbow appears as two rainbow arcs that split from a single base . The colours in the second bow, rather than reversing as in a secondary rainbow, appear in the same order as the primary rainbow . A "normal" secondary rainbow may be present as well . Twinned rainbows can look similar to, but should not be confused with supernumerary bands . The two phenomena may be told apart by their difference in colour profile: supernumerary bands consist of subdued pastel hues (mainly pink, purple and green), while the twinned rainbow shows the same spectrum as a regular rainbow . The cause of a twinned rainbow is the combination of different sizes of water drops falling from the sky . Due to air resistance, raindrops flatten as they fall, and flattening is more prominent in larger water drops . When two rain showers with different - sized raindrops combine, they each produce slightly different rainbows which may combine and form a twinned rainbow . A numerical ray tracing study showed that a twinned rainbow on a photo could be explained by a mixture of 0.40 and 0.45 mm droplets . That small difference in droplet size resulted in a small difference in flattening of the droplet shape, and a large difference in flattening of the rainbow top . </P> <P> Meanwhile, the even rarer case of a rainbow split into three branches was observed and photographed in nature . </P> <P> In theory, every rainbow is a circle, but from the ground, only its upper half can be seen . Since the rainbow's centre is diametrically opposed to the sun's position in the sky, more of the circle comes into view as the sun approaches the horizon, meaning that the largest section of the circle normally seen is about 50% during sunset or sunrise . Viewing the rainbow's lower half requires the presence of water droplets below the observer's horizon, as well as sunlight that is able to reach them . These requirements are not usually met when the viewer is at ground level, either because droplets are absent in the required position, or because the sunlight is obstructed by the landscape behind the observer . From a high viewpoint such as a high building or an aircraft, however, the requirements can be met and the full - circle rainbow can be seen . Like a partial rainbow, the circular rainbow can have a secondary bow or supernumerary bows as well . It is possible to produce the full circle when standing on the ground, for example by spraying a water mist from a garden hose while facing away from the sun . </P>

When did the rainbow appear in the sky