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.
‘The Test’ of antiquity was to discern Alcor next to Mizar in the constellation of the Plough
The two stars at the end of the body, Merak and Dubhe, are famously known as the ‘pointers’ since the imaginary line connecting them, if extended approximately five times the distance between them, locates the North Star (Polaris). But it is the middle star of the handle, Mizar, which concerns us, for it has a faint companion, Alcor, which, if able to be seen with the naked eye, means one has passed the Test. This method is referred to in 964CE by the Arab astronomer, al-Sufi, in his book Description of the Forty-Eight Constellations (also known as The Book of Fixed Stars): ‘Above Mizar, there is a very small star that is almost attached to it that the Arabs call Al-Suha [‘the forgotten one’]…. It is the one with which people test their vision.’
But how good is this test as a measure of vision? Can it be compared in any meaningful way to Snellen acuity, for instance? The angular separation of Mizar and Alcor is 12 minutes of arc, which is approximately equivalent to Snellen acuity of 6/60; on the face of it not a particularly difficult test to pass to be able to distinguish them. But remember that Alcor is very faint. To illustrate this, Alcor’s brightness is measured at around fourth magnitude. Third magnitude would be 2.5 times brighter, second magnitude 2.5 times brighter than third and so on. Mizar is a second magnitude star while, for comparison, Sirius, the brightest star in the sky, is of magnitude minus 1.44. Spotting a white object on a dark background is also considered to be more difficult than a dark object on a white background. There is also the problem of repeatability due to varying atmospheric conditions and ambient light pollution.
A small-scale attempt was made by Bohigian (An Ancient Eye Test – Using the Stars, Survey of Ophthalmology 2008, 53:5) to compare this star test with Snellen acuity. He took 10 subjects, all corrected to 6/6 or better, to a remote location on a moonless night. All were able to see Alcor with each eye separately. Lenses of increasing increments of +0.25DS were introduced to each eye until two stars could no longer be distinguished. On repeating the blur test the following day with a Snellen chart, it was found that the over-refraction required to blur the 6/6 line was approximately equal to that needed to extinguish resolution of Mizar and Alcor. No measurements of contrast sensitivity were taken but it was noted that one other subject, aged 65, was unable to resolve the stars even when corrected to 6/6, suggesting that physiological changes affecting that metric are a significant factor in passing the star test.
For those wishing to test their eyesight further, or who are located in the southern hemisphere, there is another test available at the Pleiades star cluster. Known as the Seven Sisters and located in the constellation of Taurus, it actually consists of more than seven stars. To see all seven main stars, it is said, would mark one out as a warrior suitable for battle; to be able to see five is considered good. The best way to locate the Pleiades is to start from the prominent winter constellation of Orion (the Hunter). His belt of three stars in a diagonal line is easy to find with a little practice. A line extended upwards along this diagonal passes just below the bright red (first, or sometimes brighter, magnitude) star, Aldebaran. Extending a parallel line from Aldebaran locates the Pleiades. Note that this exercise can be tried in the southern hemisphere too, but here Orion appears upside-down so the diagonal needs to be extended downwards and passes above Aldebaran.
And for the really adventurous, on a dark, clear summer’s night, go back to the Plough. Take the imaginary line between the two pointers but, instead of extending it upwards (north) in a straight line, extend it to the left at about 45 degrees; it will reach the bright star, Vega. Below that line, just before Vega, is the constellation, Hercules. Within his square body there are several fifth and sixth magnitude stars. These are right at the limit of what the human eye can see, so to be able to make out any stars there is about as good as it gets.
Stars are not the only celestial bodies by which eyesight can be measured. Some people can distinguish the phases of Venus, which typically appears as a bright star in the early morning or evening. Of more interest is a body much closer to home: the Moon. There are a couple of general things to look out for. One is when the newish Moon is a thin crescent; on a clear night someone with average eyesight can see the rest of the disc of the Moon faintly, dimly illuminated by reflected Earthlight. The other is irregularities in the Moon’s terminator (the line dividing the lunar day and night) when it falls across certain mountains or craters. One opportunity occurs when the Moon is around 10 days old; another, which occurs fortnightly when the terminator crosses two large craters, requires exceptional eyesight to observe it.
William Henry Pickering (1858-1938)
More useful, perhaps, is the graded lunar vision test devised by American astronomer, William Henry Pickering (1858-1938). Based mostly at the Harvard Observatory, and known mainly for his planetary observations, he mapped out 12 features of the lunar surface of increasingly difficult visibility that could in theory be observed with the naked eye. It might be mentioned that Pickering was noted to have exceptionally good eyesight; in his prime, in good conditions, he could locate 13 stars in the Pleiades (see above). The lunar features Pickering listed, in increasing difficulty of observation are as follows (see figure on this page from The Moon: A Guide, The Society for Popular Astronomy):
- Bright region around Copernicus
- Mare Nectaris
- Mare Humorum
- Bright region around Kepler
- Gassendi region
- Plinius region
- Mare Vaporum
- Lubiniezky region
- Sinus Medii
- Faintly shaded area near Sacrobosco
- Dark spot at foot of Mt Huyghens
- Riphean Mountains
This test is best attempted at dawn or dusk for the optimal viewing and contrast conditions. Good eyesight should enable visualisation of Mare Vaporum, while to see the Riphean Mountains, the last feature on the list, would require exceptional vision due to the light grey colouring of the mountain peaks and the fact that they subtend an angle of only two arcminutes. Some authorities argue that this feature is impossible to see with the naked eye.
There is so much to see in the unique beauty of the night sky, and nature helpfully has placed plenty of clues there to enable us to find out just how much we are able to see: a sort of stellar Snellen.
- Note: ‘The Test’ and some other man-made ‘free’ sight tests are mentioned in The Changing Cost of a Sight Test, Ophthalmic Antiques 154, p9, Jan 2021.