Nature’s sight test in the night sky

How do we deal with patients who cannot come to see us and, for whatever reason, we cannot get to see them? Especially in these extraordinary times of the coronavirus pandemic, novel solutions for remote sight testing, such as downloadable Snellen charts, have come to the fore. But Bedouins and early Romans were using a free resource to test eyesight more than a thousand years ago – and it is still available, given a reasonably clear and dark night for those living in the northern hemisphere.

This test, also used in antiquity to gauge children’s vision, is often known as ‘the Arabic Test’ because it is known to be used by nomadic Arabs. It was also used in ancient Persia, where it was called simply ‘the Test’ or ‘the Riddle’, to measure warriors’ eyesight. In the Roman army it was necessary to pass this test to become an archer. ‘The Test’ makes use of a double star in the part of the constellation Ursa Major (the Great Bear) known as the Plough (or Big Dipper in North America). The Plough is one of the most familiar landmarks of the night sky and is visible all year round in the northern hemisphere at latitudes above around 40º. It has a roughly rectangular body of four stars, with a curved ‘handle’ of three stars.