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>Frank Reed wrote:
>
>> All you need is a careful observation of the sidereal time (which is
>> the right ascension of the zenith). With a fixed installation, you
>> could mount a sight tube. For a temporary setup, you could hang a
>> couple of plumb lines and stand between them extending the lines. This
>> should give local sidereal time within two minutes or better. Assuming
>> roughly two minutes error in eclipse event timings and two minutes
>> error in local time, the average expected error would be about three
>> minutes --less than a degree error in longitude.

Herbert Prinz has examined the question of error in local time. Here, I
wish to question how realistic is Frank's assumption of "roughly two
minutes error in eclipse event timings." I think this question has come up
previously on Nav-l. How good an assumption is it that an observer can time
some moment in an eclipse, and agree with another observer who is timing
the same eclipse, to within two minutes? Also, how precisely will they
agree with the mid-time as predicted in the almanac?

The trouble is that none of the shadow-edges is really sharp, but somewhat
fuzzy, due to the fact that the Sun isn't a point-source, but subtends half
a degree. Assume that each observer has a timepiece, so that they can
estimate the mid-point between the last flash of light at the edge of the
Moon, and its reappearance on the other side.

Has such a test between observers ever been reported? They can be at the
same position, or at differing (known) longitudes: it doesn't matter much,
as long as they are out of immediate contact with each other.

The recent eclipse would have provided a good opportunity for Nav-l members
to indulge in such an exercise: at least,those enjoying clear sky, which
excluded me. Perhaps, it would be an interesting try-out at the next lunar
eclipse.

Of course, timing such an event was easier on land than it ever was at sea,
before the days of deck-watches. Star time-sights didn't work when the
horizon was dark. You couldn't set up a North-South transit line, as could
be done on a beach with two sticks in the sand. And presumably water-clocks
didn't work at sea. So timing an eclipse, though useful for longitude on
land, wasn't very helpful out at sea.

George.

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