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post #21 of Old 05-10-2012
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Re: practicing with sextant on land

Whew, just finished reducing my sight of Venus, shot 40 minutes ago :P Actually it's my first sight reduction done on one of my own sights, and my first star sight! I used an artificial horizon but unfortunately there was quite a breeze running... oh well.

Anyway, since you were mentioning sight reduction the other day I thought I'd share the process I used, transcribed from the front and back of an envelope

I'm thinking I will keep a sort of running fix. I'm starting with a DR of 49N, 123W, and will update it whenever I can take a new sight, always shooting from my patio.

Index error was 2.6' on the arc.
Hs at 10:20 pm PDT was 2540.7'
Index correction of -2.6' gives 2638.1'
Divide by 2 for artificial horizon, gives Ha = 1249.0'.

(NB: not sure if that's the right order, but I *think* index error "only applies once" in an artificial horizon measurement)

Correction for refraction (not corrected for temp or pressure since it's probably close enough) is -4.2', which gives 1244.8'.

Parallax correction for HP = 0.41' is +0.4', which gives Ho = 1245.2'.

Sight was taken at 0520 on May 10 (GMT). 20m is 1/3 of the hour.
At 0500, GHA was 22047.6' and Dec was 2741.9'; d=0.1' so no interpolation is necessary for Dec.
v is 1.9', so GHA correction is 1/3 of 15 + 1/3 of 1.9', or 50.6'. This gives a GHA of 22548.2', and LHA = GHA - APLong = 103 gives an AP longitude of 12248.2' W.

Page 69 of Pub 249 (87 in the PDF) is for Lat 49 Dec (15-29) SAME name as latitude. I look in the 27 column for row 103 and find
Hc = 1211'
d = 46'
Z = 63 (Zn = 360 - Z = 297)

The d correction of 46' per degree of declination, combined with 42' of declination, makes for a correction to Hc of +35', so Hc = 1246.0'.

Hc > Ho gives an intercept of 0.8' away from 297, which is not great considering where my apartment actually is :P Partially it's the wavy surface of the water I used, and partially I'm not just particularly good at this yet, but I also think there are some uncorrected errors in my sextant. The intercept should have been something like 12 or 13 nm toward.

Maybe tomorrow I'll photocopy a chart from the library and start plotting LOPs on it.

s/v Laelia - 1978 Pearson 365 ketch
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