Eratosthenes calculated the distance to the sun in 200BC, having already deduced the circumference of the Earth.
The distance to the moon was also known and a few centuries later, the sun was known to be at the center of our solar system with the planets in orbit around it.
Incredibly, these discoveries and others like them were made long before the invention of the telescope in 1608! Without the use of space-piercing telescopes, early astronomers had to rely on natures own optical instrument: the human eye.
Of all the eyes in the animal kingdom, the human eye isn't the most efficient but it is still amazingly adaptable. With a few tips and tricks you can actually learn to see more in the night sky using your naked eye.
The Naked Eye
Before launching into the all important night-gazing tips it's appropriate to look at the eye and see how it works. For the purposes of our discussion, the key parts of the eye are the lens, the iris and the retina (where we find the rods and cones).
The protective bit at the front of the eye is the cornea and once light passes through this membrane, it passes through the iris. The iris works just like the iris on a camera; if it's a bright day, and lots of light tries to enter the eye, then the iris will close down limiting the amount of light that can enter. If it's dark, it opens fully, letting in as much light as possible.
The light then passes through the lens where it is bought to a focus on the retina at the back of the eyeball and the image formed is transmitted as electrical impulses down the optical nerve to the brain.
Now we have basically seen how the eye works we can look at its use in studying the night sky.
Look, No Telescope!
Newcomers to astronomy can still see an amazing amount of detail in the sky even though the view is not magnified by a telescope. One thing you might notice the first time you look up is that, over a period of time, you can slowly see more and more stars. Look around you and you will notice you can even see more objects around you in the dark. This is called "dark adaptation" and it takes your eye up to an hour to become fully adapted to seeing in low light conditions.
There are two things that happen to allow dark adaptation, the first is that the iris will open to its fullest and this takes just a few seconds. After this, a chemical change inside the eyeball slowly makes the eye even more capable of seeing faint detail, but it's a process that can take up to an hour.
The tip here is that if you have just come out of the bright environment in your house, then you need to give your eyes plenty of time to adjust to seeing in the dark to fully appreciate the night sky. You'll be amazed how much more you can see. A further hint is to use red torches to give yourself a little extra light if needed. Red light, unlike white light, isn’t as harmful to dark adaptation. Exposure to bright lights, even for a second, can instantly destroy your dark adaptation, leaving you with another hour to adjust again.
Once you are dark adapted, there is another little trick that can help you pick up even fainter objects and it relies on the structure of the retina. The light sensitive receivers are found on the retina and can be likened to the pixels of a digital camera chip and are separated out into two sorts: rods and cones.
The Sensitive Periphery
The cones are concentrated around the center of the retina and the rods around them; there are a total of around 130 million light sensitive receivers. During normal light, the cones (particularly good at detecting color) are the sensors in use while the rods (much more sensitive but not good at detecting color) provide periphery vision.
At night, it's the extra-sensitive rods that take over -- this is why it's difficult to detect color in low levels of light.
Now, when you normally look at something, you look directly at it and so the image forms on the cones in the center, not the rods. This works fine during everyday life, but for an astronomer looking at a faint object, this is a problem.
It would be great if you could somehow use the rods instead of the cones to make use of their extra sensitivity. There actually is a way you can do this and its called "averted vision." All you have to do is to focus your gaze to one side of the object you want to look at and the image will be formed on the rods rather than the cones giving you a brighter image than you would get looking directly at it. It's a tricky thing to do but with practice you'll get used to it.
Unfortunately, because the rods are the ones that do all the hard-work at showing us faint objects and structures in the sky, and since they are rubbish at showing us color, we rarely see color in the night sky, except perhaps for planets and some bright stars.
Revealing the glorious color of the sky, I'm afraid, will remain in the domain of cameras. Nonetheless, following these tips will ensure you get the most out of your eyes and will help you to see the amazing treasures that await you in the night sky.