# How far can we see?

If the US Space Shuttle or Int’l Space Station are anywheres near you shortly before sunrise or shortly after sunset, the reflected sunlight is very easy to see. You can’t see any detail, just a swift point of light, but it’s several hundred miles away, and I think that posters to this thread might think the idea of seing it to be rather cool.

NASA has a site http://shuttle-mir.nasa.gov/realdata/sightings/index.html which will show you exactly when and where to look. For example, tomorrow, Tues Aug 13, it will become visible in Mexico City at 7:54 pm, in the NNE part of the sky, 35 degrees above the horizon. It will travel south, reach a maximum elevation of 39 degrees above the horizon, and then descend, disappearing at 7:57 pm, in the ESE part of the sky, at 12 degrees above the horizon.

If its not cloudy!

Can someone do the following calculations please?

Let’s say that the Great Wall, or a ship’s wake, or whatever, is 10 meters across. From 100 miles up, what angular size is that?

Now, a human eyeball has a diameter (from the lens to the back of the eye) of how much?

Given that angluar size, and that eyeball size, how big is the image of the object, as it appears on the back wall of the eye?

One last question: How big are the receptors in the eye which see these things?

Thanks.