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On 10 Dec 2004 at 15:45, Fred Hebard wrote:

> On Dec 10, 2004, at 3:36 PM, Frank Reed wrote:
>
> > Fred H wrote:
> > "At this point, it does not appear that they account for much of the
> > variation."
> >
> > But how did you reach this conclusion, Fred? Have you looked at the
> > almanac inaccuracies for 1800?
>
> Fair point Frank.  Most of your samples were in error in the range of
> 0.1' of arc.  I was extrapolating from that.  But it's speculation.

Below are the errors in the almanac lunar distances for October 1800 at
noon (astronomical time). The table gives the date, the error for stars
East of the moon (the value from the nautical almanac is subtracted from
the value in Frank's online almanac, given in seconds), and the error for
stars West of the moon. I haven't done this for the other 7 values given
for each date.

1   37
2   38  -55
3   40  -58
4   39  -45
5   41  -39
6   12  -53
7   15  -48
8   34  -49
9   34  -49
10  33  -45
11  40  -45
12  35  -41
13  32  -37
14  24    7
15  15   14
16       -6
17
18
19
20  44
21  48  -33
22  45  -30
23  40  -23
24  40  -19
25  28  -21
26  34  -27
27  39  -33
28  42  -39
29  42  -56
30  40  543
31  39  -43

The large error on the 30th is real. The almanac gives 71?20'16" while the
modern value is 71?29'19". The "0" was probably meant to be an "8".

Presumably the error bounces around a bit more over the year (or else they
would have noticed pretty quickly), but the magnitude is enough to explain
longitudes with +/- 1/2 a degree.

Ken Muldrew.

   

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