360° Rainbow ~ Adam Kraft created the bow with a low sun and a garden hose. There is part of a secondary rainbow visible. Both bows are centred at the antisolar point, the location of his camera's shadow.  ©Adam Kraft, shown with permission.
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We almost never see a complete 360° natural rainbow because there are not enough raindrops between the eye and the ground to produce the lower part.

An intense hosepipe spray provides enough drops between the eye and ground. The drops making the bow are close - but the rainbow itself still appears like any other rainbow - an infinite distance away.

The "Rainbow Cone". Drops near its surface generate rainbow colours.

The coloured rim of the primary bow is produced by raindrops near the surface of a cone extending from the eye towards the antisolar point.  The cone half angle is ~42°.   

Drops near the surface refract light towards the eye. At right, a drop is refracting red light to contribute to the bow’s outer red rim.    It also refracts blue and other colours but these miss the eye.     Drops slightly deeper in the cone refract bright blue to the eye and fainter hues of all longer wavelength colours – the eye sees a blue inner edge.

Drops even deeper in the cone produce a white inner disk for rather complicated reasons.