[devilincarnate]

Originally Posted by Russ

The first picture, on the left - is that the natural curvature of the Earth or just a 'fish-eye' lens? If it's the Earth then he was brown-trouserly high

This could give a clue?

If you didn't know that the Earth is a sphere, there are three common observations you could use to convince yourself that it is.
The first common observation is the shape of the moon. First, the face of the full moon is circular, and that would lead you to believe that it is a sphere rather than a disc. When the moon eclipses the sun, the shape of the shadow is always circular, which clinches a spherical shape for the moon. By extrapolation, you could assume that the Earth is a sphere also.
Also notice that when the moon is being eclipsed by the Earth (a lunar eclipse), the part of the moon that is eclipsed is actually the shadow of the Earth. This shadow tells you that the Earth is a sphere just like the moon.
A third way to see that the Earth is a sphere is to look at how objects in the distance "disappear" as you get farther away. For example, a 100-foot-tall ship that is 15 miles away is not visible. That's because it is blocked by the curvature of the Earth. As it approaches, it "rises." First the tip of the mast is visible, then more and more of the ship comes into view as the ship gets closer.

http://science.howstuffworks.com/question65.htm

There are 2 six foot men. What would the distance be between them before one could not be seen because of the curvature of the earth?

My grandson, who is stationed in Baghdad, Iraq, asked the question. This is one of the fun problems his unit was discussing. Thanks for your time.

Submitted by Shirley



Hi Shirley,
In an earlier question on the curvature of the Earth Harley to showed that the earth curves approximately 8 inches per mile. Since 6 feet is 72 inches and 98 = 72, the Earth curves approximately 6 feet in 9 miles. Thus the two men would have to be approximately 18 miles apart.

Cheers,
Penny

In April 2004 we received an message from Jerry, a retired Engineer, who pointed out an error in my solution. What follows is a slightly edited version of what Jerry sent.

It is true that Harley showed quite correctly that the earth curves approximately 8 inches in one mile. The solution presented then goes on to find out over how many miles does the earth curve 72 inches or 6 feet. Then this distance is doubled which is required (and would be easy to forget), because each man looks this distance to his horizon, where the line of sight is tangent to the earth's surface midway between them. All of this reasoning is correct so far.

However, it turns out that while the earth does curve 8 inches in one mile, it does not take 9 miles to curve 72 inches. To show this, let us return to the Pythagorean Theorem method used by Harley, but using 6 feet for the curvature. Here is a copy of Harley's diagram with the 1 in the diagram replaced by x, since in this case the distance is unknown.



Again, using the theorem of Pythagoras

a2 = 39632 + x2 = 15705369 + x2

Solving for x,

x2 = a2 - 15705369

a must be 3963 miles + 6 feet (Let's say the men are actually 6'3", so their eyes are six feet above ground.). Thus
a = 3963.001136 miles

x2 = 15705378 - 15705369 = 9
x = 3 miles

Now, remember that each man looks 3 miles to the horizon, giving their distance from each other as 6 miles.

This shows that at eye level of 6 ft. the horizon is 3 miles (at sea or on a level plain).

A rule-of-thumb for line of sight problems such as this, where the distance is small in comparison to the size of the earth is

c = (2/3) times x2, where x is distance in miles and c is curvature in feet.

For the problem at hand, we then have x2 = (3/2)c

x2 = (3/2) 6 = 9
x = 3

This is the same result that the more lengthy solution yielded.

--Jerry

Thanks Jerry,

Penny

http://mathcentral.uregina.ca/qq/dat.../shirley3.html

Hope this helps?