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Frank suggested I try a Lunar distance with the Sun, and today is the first in these last couple of weeks I have had chance to try it as the Moon has been unobservable.

This afternoon in Bosham the sky is clear; there is a fingernail and very faint Moon visible in the S. with the Sun now in the WSW. The Moon was difficult to find even knowing where it is supposed to be, it appeared so faintly being a very thin crescent.

Results:
Lunar - Solar Distances
Observations.

Observer's position:
(WSG84)
Lat: North 50deg-49.910
Long: West 000deg-51.300

Monday 11th October 2010.
All times GMT. (UTC)

14Hr-45'-00" 51Deg-00'.8
48'-10" 02'.3
52-10 03'.9
55-10 05'.3
59-30 06'.1
----------------------------

Arithmetic mean:
At time 14Hr-52'-15"
Observed Limb to Limb: 51Deg-03'.7
------
Apparent Altitudes (from ICE)
Moon: 14Deg-59'.4 HP: 57.9 SD: 15.8
Sun: 20Deg-05'.6 SD: 16.0
------
Observed (Apparent) Luni-Solar Distance
(Semi Diameters added for Sun and Moon)
= 51Deg-35'.5
------

Cleared Luni-Solar Distance = 51Deg-23'.01
equiv = 14Hr-53'-37".5 in time.
(Interpolated from Oliv Soft Lunar Distance Table)

Hence error is +1 minute - 22".5 in time.
==========================================

Interestingly this error is very close to that found with previous Lunar/Star/Jupiter measurements I made a few weeks ago and in the same sense.
(i.e.Close to +1min-20" in time)
It suggests I am reading by 0.7' too great. I shall have to try to determine if this is an 'observer's personal error' or residual index error.

I shall also have to eat my words to some extent about what I expected to be poor accuracy of Lunars, but which are apparently more accurate than I gave credence for.
It must not be forgotten though, this experiment of mine is nevertheless an attempt to get as good as possible accuracy on land under the 'best' conditions, with a steadied 'standard' sextant with telescope, and calculated altitudes used, so it should be the 'best' possible or close to it. I am interested mainly in the original time accuracy problem from using Lunar Distances to obtain longitude as per nineteenth century.

I still expect it is a different matter altogether at sea however, and having to measure the altitudes too will reduce accuracy, unless they are calculated - but that presupposes a known time is available which in the 19th C. was not the case without chronometers. Some experiments are needed at sea. I shall have to try to arrange that in due course.

Douglas Denny.
Chichester. England.
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