Learning to See the Sky at Night: User's Manual For Your Eyes

Your eyes are the most powerful astronomical instruments you will ever own. Learn how to use them!


If I’ve tempted you into trying to find objects in the night sky, you may have already discovered that there are challenges in store for the casual astronomical observer. On top of the difficulty of just finding your way around the night sky as the visible constellations change their locations change every hour as the Earth rotates, and every month as the Earth makes it way around its orbit, there is the additional challenge of learning how to use the instruments of astronomy. And the most fundamental instrument you will use is the most sophisticated of all, and does not come with a user’s manual — your eyes.

Using your eyes to observe astronomical objects is an acquired skill, requiring both knowledge of how the eye works, and training. In my classes, I spend a good deal of time training the students how to see what I am showing them. By the end of the course they can see galaxies that were completely invisible to them in our first observations. Whether you are just staring directly at the night sky, or looking through the largest of telescopes, your success in personally experiencing the wonders of the night sky comes down to how well you make use of your vision. 

Let’s start with a quick overview of how your eye works. (I’ll warn you now that I am not an ophthalmologist, so this description may not be medically accurate — I’m going for functional accuracy here.) Light enters your eye through the pupil – a physical hole at the outermost edge of each eye. The light then passes through a lens of gelatinous material, and is focused down onto your retina — an array of light-sensitive cells, curved around the back interior of your eyeball. You can think of the retina as the sensor of a digital camera. 

The human eye is in fact an astoundingly versatile instrument. The ratio of the brightest scene that can be viewed, to the faintest objects that can be detected, is about a billion to one. (No camera can come close to accomplishing this). In a famous set of experiments performed in the 1940s, it was determined that a visual experience occurs when only 9 photons reach the retina at the same instant! To allow for such a wide range of sensitivity, the retina must be protected against overexposure to bright scenes. This is accomplished by a combination of chemical sensitivities in the retina itself (think ISO setting on the camera), and with the iris, which surrounds the pupil and controls the amount of light that enters the eye (think aperture setting). 

In astronomy, we are usually interested in viewing dim objects. To get the best views, we need to ensure that our irises are opened as far as they will go, and that our retinas are maximally adapted to darkness. Since eyes are damaged by bright light, but not by dim light, our eyes adapt very rapidly to bright lights — shutting down the retina sensitivity and slamming the iris down to 1-2 millimeters in diameter in a small fraction of a second — but much more slowly to dim light.  The iris will open to its maximum size in about 10-15 minutes, and the chemical changes in the retina will take up to 30 minutes to complete. Both of these times grow longer with age.

To observe dim objects in the night sky, we need to eliminate as much of the other light in our surroundings as possible. Turn off all of the lights in your control, and try hard not to look at other lights around you. If a source of light is needed, deep red light has the least effect on dark adaptation — a flashlight covered with red crepe paper, or with almost dead batteries is ideal. Remember that you will lose your night vision in a second, and it will take at least 10 minutes to get it back again! I know astronomy enthusiasts who observe surrounded by partitions, or even sport a pirate patch over the eye they put to the telescope. 

So let’s say you’ve managed to stay away from lights for a good 10 minutes, you’ve found the constellation you need to find, and you are searching for that dim object Aaron said you could find between this and that star.  The next trick is to not look directly at the object! 

In your retina, there are two kinds of light-receptive cells. Cones convert light into the sensation of color, while rods operate strictly in shades of gray. The cones are less sensitive to light than the rods — which is why you see little or no color at night. However, near the center of the retina, where your center of vision is focused, cones far outnumber rods, and therefore at the center of your vision you are less able to see the dimmest of objects.  Astronomers become trained in using “averted vision” to find the hardest objects — looking and studying the objects out of the corner of their eyes.

Before I wrap up this lesson in seeing, I want to discuss the purpose of binoculars and telescopes in astronomy. Many think of these instruments as being great magnifiers, bringing objects “closer” to use to give us clearer views of the heavenly bodies. In fact, the goal of any astronomical instrument is to stuff as many photons from the object being observed through the pupil of your eye as possible. This is done by creating a bigger pupil, and focusing the light coming through that larger pupil into a cone that passes through the pupil of your eye. 

So it is the diameter of the telescope or binocular that matters — not its magnifying power. In fact, in many cases the magnification is more a hassle than a desire, because stars will forever appear as points of light no matter how magnified the image becomes, but the higher the magnification the less area of the sky you can see in one image.

A typical pair of binoculars has a 50mm diameter lens. The human eye, when the iris is maximally open, has a diameter of about 5mm (a little bigger for children).  Since the area of the hole through which the light passes is what matters, the 50mm diameter binoculars will bring about 100 times more light to your retina than your naked eye. You will be able to see objects about 100 times dimmer through a $20 pair of binoculars. You may find it is well worth the investment!

Thanks for reading, and I hope to see some of you this fall in our upcoming classes — I still have several spots available if you wish to join us! Check it out at www.turnerclasses.com.

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