85. The Cordouan Light at a height of 207 feet should not be visible from a ship 31 miles away on a spherical Earth

“The Cordonan Light on the west coast of France is 207 feet high and visible from 31 miles away, where it should be 280 feet below the line of sight.”

Probably a bogus observation, but in any case Dubay and Samuel Rowbotham can't do trigonometry.

For evidence that the quoted observation is bogus, click here:

Why Dubay's lighthouse quotes are bogus


However, if you want to take the observation as genuine read on to see why this still does not add up to a proof.

Using the correct calculation, the observer on the ship would have to be at a height of 119 feet.
From the deck of a ship that would mean that the observer would be about 103 feet too low to see the lighthouse.

However, as  with so many of Dubays "proofs" this is taken from Samuel Rowbotham's "Zetetic Astronomy: Earth Not a Globe" published in 1881.

The relevant paragraphs from the book are:

"Many instances could be given of lights being visible at sea for distances which would be utterly impossible upon a globular surface of 25,000 miles in circumference.  The following are examples:-
.... 

 Allowing 16 feet for the altitude of the observer .....

The Cordonan Light on the River Gironde, west coast of France, is visible 31 statute miles, and its altitude is 207 feet, which would give its depression below the horizon  as nearly 280 feet."

See: https://books.google.co.uk/books?id=ETxNCgAAQBAJ&pg=PT24&lpg=PT24#v=onepage&f=false

Now the observation can make sense on spherical Earth.  In 1881 the ships that would be observing the lighthouse would be sailing ships or steam ships that still had sails as a back-up.  Look-outs on those ships would be placed in a crows nest near the top of the main mast.

"... masts were still to be found on many merchant and passenger ships well into the 1900s"

Source:  https://www.uh.edu/engines/epi1338.htm

The 119 feet of elevation required to see the Cordouan (not Cordonan) Light on a spherical Earth is absolutely consistent with the elevation we could expect for look outs stationed on the main mast.

Here is a typical ship of the time with a mast height of 145 feet.
https://en.wikipedia.org/wiki/Balclutha_(1886)

So in summary, Samuel Rowbotham may have quoted an accurate account of how far out to sea the lighthouse could be seen, but he was still as ignorant of trigonometry as Dubay.  He performed the incorrect calculation and used this to incorrectly conclude that the Earth could not be a sphere.  Although it would have made no difference to his conclusion, he also makes the mistake of assuming that the ships lookout would be standing on the deck when the best vantage point on a sailing ship is from the top of the main mast / crows nest.


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