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> Fred wrote

> Here are the data with the mean date and time, UT1, and the altitudes.
> The location was 36*46.8N, 81*50.7W.  The observation for 18 April is
> listed as for 17 April in the original table, where the separate dates
> are zone time rather than UT1.  Most of the the time the sun and moon
> are higher than 20* altitude.

Fred

Your data does not seem to support my hunch that the error gets greater
later in the day.  Must be pilot error on my part.

Following are two Sun/moon sessions on 15 April, 2005 (zone date). They tend
to pretty close to both ends of the spectrum, other sessions usually falling
somewhere between. Both used the  Astra IIIB deluxe with half-silvered
horizon mirror, and in seated position.  Times listed are UT.  EST times
were approx. 3PM and  5PM.  IC was -0.5'.  All calculations were done with
Frank's online calculator.  All STDEV are n-1.

Location:  40d 27.7N, 86d 55.7W

SESSION 1

Temp 67 F, Reported pressure (which would have been adjusted to sea level)
30.31"

Approx. Sun Hc 48d       Approx. moon Hc: 45d

UT          Hs         Error'    Lon error'
19 44 36    81 32.6    0.7       21.2
19 51 02    81 33.2   -0.9      -27.9
19 54 49    81 35.9    0.4       13.3
20 04 40    81 37.6   -1.2      -36.3
20 09 14    81 41.0    0.6       18.9
20 11 17    81 42.1    1.0       31.2
20 12 54    81 42.6    1.0       30.6

Mean error: 0.23'  Error STDEV 0.90'
Mean longitude error 7.3'   STDEV 27.74'

Which pretty much puts the above in the "better lucky than good" category.
God bless averaging ;-)

SESSION 2

Temp 68 F, Reported pressure (which would have been adjusted to sea level)
30.29"

Approx. Sun Hc 25d       Approx. moon Hc: 68d

UT          Hs         Error'    Lon error'
22 06 35    82 15.8    0.0        1.3
22 06 25    82 16.4    0.5       15.2
22 07 52    82 16.8    0.4       12.3
22 11 25    82 17.6    0.2        7.3
22 12 54    82 18.4    0.6       19.0
22 14 44    82 18.7    0.4       13.2
22 15 49    82 19.2    0.6       19.3
22 18 04    82 20.0    0.8       25.0

Mean error: 0.44'  Error STDEV 0.24'
Mean longitude error 14.08'   Lon STDEV 7.41'

The frustrating part is that the above are representative of other series.
Late afternoon series with error STDEV as low as 0.18', but mean of long
27.78 off.  Early afternoon series with error STDEV of 0.36' but mean
longitude 3.75' off.

Bill

   

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