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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 49°N, 123°W, 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 25°40.7'
Index correction of -2.6' gives 26°38.1'
Divide by 2 for artificial horizon, gives Ha = 12°49.0'.

(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 12°44.8'.

Parallax correction for HP = 0.41' is +0.4', which gives Ho = 12°45.2'.

Sight was taken at 0520 on May 10 (GMT). 20m is 1/3 of the hour.
At 0500, GHA was 220°47.6' and Dec was 27°41.9'; 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 5°0.6'. This gives a GHA of 225°48.2', and LHA = GHA - APLong = 103° gives an AP longitude of 122°48.2' 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 = 12°11'
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 = 12°46.0'.

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