Making Astronomical Observations

The simplest instruments used for making astronomical observations are the eyes! They have been used for millions of years and when humans finally started to observe the skies in a more systematic manner about 5000 years or so ago, they were the only instruments available. The Babylonians, Mayans, ancient Egyptians, ancient Indians, and Chinese measured all the basics of their time and calendar systems by naked eye. They did a remarkably accurate job of it too!

Early observers saw that the stars have different brightnesses and eventually categorised objects into brightness categories. The eyes of a keen observer with 20/20 vision can discern faint objects of magnitude 6 or so. For the average person, that is a shade under 6000 stars. Those numbers depend on the sky conditions. In a city, where there is dust and light pollution, you are unlikely to see objects much fainter than magnitude 3. Out in the country, with dark skies and clear air, you may be able to see fainter objects.

The resolving power of the average human eye with 20/20 vision is about one second of arc, objects closer together than that, to the naked eye, will appear as one. Some people have keener eyes than that and there have been reports that the Galilean Moons of Jupiter had been seen prior to their discovery by Galileo and his simple telescope. There is no proof of that however.

For an observer to view the maximum number of stars, planets, galaxies etc their eye must be night adapted. At the back of the eye is the retina. This is made up from light receptors called rods and cones. Vision is created when photons of light cause reversible chemical changes in the rods and cones, sending impulses to the brain which are then interpreted. The cones are the colour receptors whilst the rods are sensitive to low light conditions but give no sensation of colour.

The rods contain a chemical called rhodopsin which is sensitive enough to change in response to a single photon. During the day, the rhodopsin is largely unavailable. When you go insideĀ  from bright sunlight it is difficult to see. Gradually, the situation improves because the rhodopsin re-forms and so the rods can be used for vision as well as the cones.

The process of night adaption takes about 30 minutes to complete in most people’s eyes. At that point, the maximum amout of rhodopsin is available. There is an initial fast night adaption as the cones adjust but after that, it is down to the rods. That is why you cannot discern general colour as it gets darker – you need a relatively bright object to trigger the cones.

When your eyes are fully adapted to night vision, it is important to preserve that, if you need a torch, make sure it has a feeble red beam, it will be enough to read by but will not be sufficiently powerful to cause the biochemical changes that reduce the rhodopsin level in the rods like daylight does.