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 Tue Aug 19 11:28:57 2008