A primary rainbow is centered on the point opposite the Sun.

A rainbow is a viewing geometry, not a colored object fixed above the landscape. The droplets that send light to your eyes occupy particular directions relative to you and the Sun. Another observer sees light from a different set of droplets, even when both people describe the same apparent arc.

Sunlight bends as it enters a water droplet, reflects once inside, and bends again on leaving. Different wavelengths bend by different amounts. This dispersion separates the colors, while the geometry concentrates emerging light near the angles that make the bright primary bow.

The reference direction is the line from the Sun through your position and onward away from it. The rainbow forms a circular pattern around this antisolar direction. Red appears on the outside of the primary bow, while violet lies closer to its center.

The familiar arch is only the visible portion of a possible circle. On level ground, the horizon and nearby terrain normally prevent you from seeing the droplets that would complete the lower section. There is no physical requirement for a rainbow to end at the ground, and its apparent feet do not mark reachable locations.

From an aircraft, or from a sufficiently elevated viewpoint with illuminated droplets below, a full circular rainbow can sometimes be visible. Height alone does not guarantee this view: the Sun, observer, and droplets still need the correct arrangement. The circle remains centered opposite the Sun rather than on a particular mountain or cloud.

This geometry also explains why a higher Sun makes a ground observer's rainbow appear lower. As the Sun climbs, the antisolar point moves farther below the horizon. Under ordinary level-ground conditions, a primary bow is generally not visible once the Sun is higher than roughly its angular radius.

The quoted 42 degrees describes an angle at the observer, not the size of a droplet or the distance to the rain. A nearby spray and a distant rain shower can produce bows with similar angular radii. Their physical distances may differ greatly, but the optical paths select approximately the same viewing directions.

A secondary rainbow follows a different path involving two internal reflections. It appears outside the primary bow and reverses the color order. It should not be confused with a second copy of the primary circle; the extra reflection changes both the emerging angle and the amount of light available.

A rainbow is a direction-dependent pattern of light arriving at an observer, not a painted arch fixed to a location in the landscape. Different observers can see similar arcs while receiving light from different droplets. Moving toward an apparent end changes the geometry rather than reaching a physical object.