Sun, observer and anti-solar point

The exact position at which an atmospheric rainbow will appear in the sky can be anticipated by imagining a straight line that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.

  • The imaginary line that joins the Sun, observer and the centre of the rainbow is called the rainbow axis.
  • The point on the rainbow axis around which a rainbow appears is called the anti-solar point. The centre of a rainbow coincides with the anti-solar point.
  • Stand with the Sun on your back and look at the ground on a sunny day, the shadow of your head marks the point called the antisolar point, it is 180° away from the Sun.
  • The red arc of a primary bow forms at an angle of 42.40 from the rainbow axis.
  • Seen from an observer’s point of view, the angle outwards from the rainbow axis to the coloured arcs is called the viewing angle.
  • In diagrams, the same angle between the axis and a line extended from an observer’s eyes to the arcs of a rainbow is called the angular distance.
  • With the Sun behind you, spread out your arms to either side or up and down to get a sense of where a rainbow should appear if the conditions are right.
  • Unless seen from the air, the centre of a rainbow and the anti-solar point will always be below the horizon.
  • The centre of a secondary rainbow is always on the same axis as the primary bow and shares the same anti-solar point.
  • To see a secondary rainbow look for the primary bow first – it has red on the outside. The secondary bow will be a bit larger with violet on the outside at an angle of 53.40 and red on the inside.