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In a recent posting, replying to Aubrey O'Callaghan about timekeeping in
Cook's second voyage, I wrote-

>However, the above creditable figures for the performance of the
>chronometer (Kendall's watch) should be taken with a large pinch of salt.
>By the time Resolution had reached the south seas, daily gains of the watch
>of 6 to 9 seconds of time a day were being measured.

Off-list, I've had a response from Aubrey which asks-

>How were they
>measuring 9 seconds a day error in the clock in the South Seas - what would
>have been their reference; I suppose averaged from total difference from
>Lunar measurements ?

I'm posting the answer on-list in case anyone else is puzzled. Here goes-

It's really quite simple. Not from on board, though. But having made a
landing, set up some sort of instrument to look along a South-North
direction. It doesn't need to be aligned at all accurately, though. An
astronomical quadrant or a pillar sextant would do the job. Set its
altitude to that of some bright star and wait for the moment when that star
passes the cross-wires, recording the time reading on the clock you want to
calibrate. Leave the instrument undisturbed and record the next crossing in
the same way, one sidereal day later, 23 hours 56 minutes 4.1 seconds. If
the time interval measured by the clock differs from that amount, that's
the daily error in the clock.

Note that this only measures the present daily going rate of the clock; it
gives little clue to how much error may have accumulated since it was last
checked. For that, you need to obtain the Greenwich time. From timings of
the disappearance of satellites of Jupiter as seen through a telescope,
perhaps, which can only be done from on land. Or from lunar distance
measurements, which can be done most accurately from on land.

George Huxtable


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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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