Re: [asa] Rudwick does it again (back to Adam)

I haven't tried to check the math, but I have stood on the sea shore on a
clear day and seen just the tops of the masts of a vessel, or seen a
vessel sail over the horizon. To what extent does the refraction of the
atmosphere affect this?
Dave (ASA)

On Wed, 20 Aug 2008 22:01:55 -0800 "Don Winterstein"
<dfwinterstein@msn.com> writes:
"The distance to the horizon of an observer at sea depends on the
observer's height above the water. That is why sailing ships had a crow's
nest for a lookout.... ...as a ship approaches land, the top of a tower
on the shore becomes visible before the bottom. "

I suspect the invention of the crow's nest had to do with overcoming line
of sight blockage by waves, etc. People can always see farther from
higher up because there's less line of sight blockage.

This has little--or nothing, in most cases--to do with Earth's surface
curvature. Trigonometry and a knowledge of Earth's radius show that the
distance an object 30 miles away from an observer would "sink" below a
line tangent to the surface at the observer's location is less than 1.5
inches when both the observer and the object are at sea level. Unaided
human eyes can't resolve towers or much else besides mountains at a
distance of 30 miles. In short, there's no way unaided human eyes can
detect Earth's curvature by observing how objects "sink" as they move
away at sea. The rate of increase of "sinking" with distance is far too
slow.

Don

 
----- Original Message -----
From: gordon brown
To: asa@calvin.edu
Sent: Tuesday, August 19, 2008 7:28 AM
Subject: Re: [asa] Rudwick does it again (back to Adam)

On Mon, 18 Aug 2008, Jim Armstrong wrote:

> I'm not sure about what "common sense" might look like in the time,
> particularly among those familiar with the seas. With the horizon on a
> body of water a mere 13 miles (as I recall) away, and the existence of
> fishing boats and extended boat travel in the era, it absolutely could
> not be unknown that there was something at least curve-ish or even
> dome-ish to the shape of the surface of large bodies of water. Think
> about it. Surely there were many folks who mulled over the reasons for
> observing the "sinking" of ships as they move far from the shoreline,
> while the crew and passengers experienced no sinking at all. While I
> would not go so far as to assert that there was anything like a
> widespread popular understanding of a spherical earth, there must have
> been some thought about how to explain that non-flat behavior or the
very
> familiar seas extending from their shorelines. The most common of
sailors
> on the more significant seas would surely be hard to disabuse of the
idea
> of a perfectly flat ocean. That said, it is admittedly still a leap to
> extend such speculations to a spherical earth.
>
> Or so it seemeth to me. JimA
>

First, a correction to the above: The distance to the horizon of an
observer at sea depends on the observer's height above the water. That is

why sailing ships had a crow's nest for a lookout.

Next, some observations about evidence for the earth's sphericity: This
was accepted by Greek science as far back as the fourth or fifth century
before Christ. The earth's circumference was calculated by Eratosthenes
in
the third century B.C. The first century (B.C. and A.D.) geographer
Strabo's works contain a lengthy discussion of evidences for the
sphericity of the earth. One that he mentions is the fact that as a ship
approaches land, the top of a tower on the shore becomes visible before
the bottom. One that I don't recall seeing in Strabo is that in a lunar
eclipse the earth's shadow is always circular. This was used by the
Greeks
to determine the size and distance of the moon.

There is another observation that I have not seen mentioned anywhere, but

I wonder what the ancients may have thought about it. That is that no
matter where you are, the highest point in the dome of the sky is exactly

overhead.

Gordon Brown (ASA member)

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Received on Thu Aug 21 13:36:48 2008