What is the farthest distance visible with the unaided eye?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

03 December 2006

I was recently asked what the farthest distance the human eye can see is. My quick answer to that was: infinity, of course! But before I explain why that is so, let me first clarify that the question can have several meanings: It may have been asking for the distance to the horizon, or the distance to the distance to the farthest object visible from earth, or the distance to the edge of the universe etc.

As for the distance to the horizon; that depends on how high your eyes are from the ground and how “flat” the ground is. The word “flat” is in quotes because, strictly, what we call flat ground is actually curved. Remember: the Earth is round.

Nevertheless, if you are at a very flat place (for example, at the sea shore) the horizon appears about five kilometres away. This is the point where the Earth seems to end. But if you look carefully, you will notice that there is a second horizon – where the clouds seem to touch the ground.

Now the clouds horizon depends on how high they are from the ground.  This height varies greatly from less than two kilometres to more than six. Consequently, the second horizon can be between 100 and 250 kilometres away. However, they all appear to touch the ground at the same place.

The reason for is that our eyes are not to gauge very large distances. That is, after a certain limit, everything seems to be the same distance away. For example, the sun and the moon appear to be the same “height” from Earth even though in reality the former is 300 times farther than the latter.

The answer to the second meaning of the question (how far is the most distant object visible by the human eye?) is 2.5 million light years. This is the distance to the Andromeda galaxy – 24 billion-billion kilometres! That is so far away that even though the galaxy has over 2 billion stars, it looks like one star when viewed from Earth with the unaided eye.

Now the original question is a phrased the wrong way. It implies that the visibility of an object is determined by it distance from the observer. This is not so; visibility depends on brightness. No matter how far it is, an object will be visible if it is bright enough. For example, we can see many stars that are billions of kilometres away but not a pin placed 50 metres from our eyes!

So the question really is: What is the dimmest light visible by the unaided eye? The answer to that is also not straightforward. It depends on the wavelength (or the colour) of the light…and that is a story for another day.

Going back to the original question (what is the farthest distance visible by the human eye?), the answer is anything from a few millimetres to many billions of light-years. It really depends on whether there is something to be seen. Thus my quick answer was infinity.