Re: [Paddlewise] Longitude and astronomy

On Sun, Aug 8, 2010 at 5:48 PM, Robert Livingston <bearboat2_at_comcast.net>wrote:

>
>
> What does local time have to do with figuring out longitude?  You need a
> clock accurate enough to detect that if you sail 100 miles west the day is
> longer than if you travel 100 miles east. That is not a sand clock.
>

Your line of longitude is exactly under the sun at its zenith measured from
Greenwich.  Your line of latitude is calculated from the elevation of the
sun at zenith. You can get an approximation of your longitude if you take a
noon sight. The trouble being that the sun "hangs" at zenith for a period of
time that makes this calculation only an approximation. And you'd need the
time.

So measuring your longitude from Greenwich does need accurate time. But you
can get a bit more sloppy if you are only measuring a couple hundred miles
from one location to another. You wouldn't need any real concept of
"longitude" either... just a concept of the earth being round and some
reference points.

Local noon plus or minus the elapsed time would give you a rough
approximation of how far east or west you traveled. It wouldn't be very
accurate... and it would have to be referenced to a departure point - but it
could give you some idea of when you might expect to see your destination
over a relatively short passage.

For instance.... if they had a way of measuring 24 hours then if the sun
were at zenith exactly 24 hours after they started their "clock" then they
would not have moved much in longitude. They almost certainly didn't
understand degrees... but they could understand concepts such as "the sun
begins to descend 1 hour after the "clock" reaches zero" would be, perhaps,
the next group of atolls west.

And they did not have sand clocks anyway. They had no glass. You try and
> make a even vaguely accurate sand clock out of the materials that they had
> and sell it to your tribe as something that was useful.


Ok... how about a sand coconut?

I'm merely offering up suggestions that seem to me to be more plausible than
reading the pattern of the waves or somehow "just knowing" where they were.
They could clearly move around in primitive craft over an area of thousands
of square miles. Running down the latitude really only works well for going
downwind. Upwind, where you need to tack, you'd also need to keep track of
time somehow. Otherwise you risk missing the island because you extended one
leg of the tack too far. Although it probably works ok for continents. :P


Craig Jungers
Moses Lake, WA
www.nwkayaking.net
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