# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Cook and equal altitude sights around noon
From: Frank Reed
Date: 2014 Feb 1, 12:13 -0800

Warren Blake, you wrote:
"I can add Historical justification for the use of Equal Altitudes of the Sun for determining Apparent Noon. Captain Cook in Endeavour 1768-71 used this method for Apparent Noon"

Certainly equal altitudes CAN be used to determine local time, though it's a rather time-consuming process best suited for sights on land. Ashore Cook and his astronomers no doubt used this method once in a while, but the standard method of getting local time, especially underway, was a simple altitude of the Sun when bearing more or less east or west. The altitude can be converted into local time using a simple, standard calculation known today as a "time sight". In effect, this process turns the sextant into a sundial. The key feature of it, compared to equal altitudes, is that we know the local time right away. So for example, if I have shot a "lunar" to determine Greenwich time by measuring the angle between the Sun and the Moon, I can use the Sun's altitude, taken (almost) at the same time to get local time. These two times, having been determined simultaneously, immediately yield the longitude from their difference.

And you wrote:
"This voyage was without Harrison's Chronometer. In the next two voyages (with this Clock) I am guessing he still used Equal Altitudes but did not need Lunar Distances."

Note that Cook did not have Harrison's chronometer (not the original) but Kendall's much simplified copy (though it could still be called "Harrison's" since he was the creator of the original). That chronometer was excellent, but it was still regularly checked by lunar observations. And by the way, in Cook's time, it was called a "time-keeper" or simply "watch" --the word 'chronometer' did not catch on for another thirty years.

You also wrote:
"As far as I can see at this early stage, he did not use Ex-meridian shots to provide Lines of Position, but am not sure."

That's right. There were no "line of position" sights until well into the 19th century. On the other hand, a "time sight" is functionally equivalent to a line of position sight. It differs only in the way that it is processed mathematically.

-FER

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