The human eye
5.2 The human eye
Our eyes are all that we need if we wish to observe the outlines of the constellations, the majestic sweep of the Milky Way or the fl eeting trail of a meteor arcing across the sky. However, to use them to view the heavens at their best, a little time is required for them to become dark-adapted . First, and most obviously, in dark conditions the pupils will dilate so allowing more light
to enter the eye (Figure 5.7). This happens over a period of 20 s or so and will thus almost immediately enable one to see fainter stars. The typical daylight size of the pupil is about 2.5–3 mm across but this extends to 5–7 mm under dark
conditions. Sadly, as one gets older, the maximum size is likely to be nearer 5 mm rather than 7 mm (a factor of 2 in area) so younger people will defi nitely be able to see better in the dark!
There is a second dark adaptation mechanism that takes about 20 min to come into effect. Without high light levels reaching the retina, vitamin
A is converted fi rst into retinene and then into rhodoposin (visual purple) that signifi cantly improves the sensitivity of the rods and cones which are the light sensitive receptors in the retina of the eye. Strong white light can quickly
Introduction to Astronomy and Cosmology
Figure 5.7 The human eye.
reverse this change, but red light is far less damaging which is why astronomers use red light to view their star charts. In daylight we tend to use a region of the retina called the fovea which is densely populated with cones that are colour sensitive but require high light levels to work well. This is why objects that can have beautiful colours when imaged photographically tend to appear as shades of grey when viewed through a telescope. In contrast, the rods are not colour sensitive but are more sensitive to light. Away from the central foveal region, the cones are fewer in number and the rods are more closely packed. We can thus see fainter objects by using ‘averted vision’ – directing our gaze to one side of the object of interest.
Even after dark adaptation, what we will actually be able to see on any given night depends critically on the amount of dust and water vapour in the atmosphere. These absorb and scatter the starlight so making it diffi cult to see fainter objects. The dust and vapour will also scatter back towards our eyes any light from the ground – called light pollution – making the sky appear brighter and the problem even worse. We use the term ‘transparency’ to defi ne how clear the sky is. At a dark site with a very transparent sky we would typically be able to see a 6–6.5 magnitude star looking towards the zenith. Light pollution com- bined with an atmosphere laden with dust and water vapour will reduce this value signifi cantly. To give the best chance of viewing the heavens one should try to be as far away from built up areas as possible – even a few miles will help signifi cantly.
Observing the Universe