A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Winter Sextant Sight Accuracy?
From: George Huxtable
Date: 2002 Jan 11, 7:32 AM
From: George Huxtable
Date: 2002 Jan 11, 7:32 AM
On the effect of winter on the accuracy of sextant observations. I quote below a recent mailing from Brian Whatcott- >At 06:57 PM 1/10/02, George Huxtable wrote: >>Jared Sherman asks- >> >> >How tight a fix are other folks getting with a sextant in winter conditions? >>... >> >My last cluster of sights were within 1 mile (+-.5m) of each other, but >> >still off by nearly two miles in absolute position. > >/snip/ > >>Even in my latitude in the UK of 51 degrees-odd, we get a rather pale noon >>sun at about 16 degrees altitude in midwinter. Even at that altitude, the >>refraction correction is only 3 minute 30 sec and is rather well-known and >>predictable. At that altitude, it's inconceivable for temperature or >>pressure variation to be enough to account for the errors that Jared refers >>to. The refraction is rather insensitive to overall pressure or temperature >>variations. A change of 14 degrees Celsius in the temperature of the air >>changes the refraction by only 0.2 minutes of arc. see Norie's tables, for >>example. > >/snip/ > >>George Huxtable. > > >Probably teaching grannie how to suck eggs, but a UK base can be >a misleading basis for the effects of latitude on climatic temperature >extremes. >/war story follows/ > Playing with the kids many years ago, on a tobaggan down a snowy > Montreal slope (5 degrees more southerly) with a brisk 12 kt breeze >and an ambient of minus 40 is not the temperature George is likely > to see often. > > That gulf stream does a lot for the NE Atlantic coast. > > >Brian Whatcott > Altus OK Eureka! =========== Response to Brian Whatcott from George Huxtable- When I asked what Jared Sherman's latitude was, it was to establish the altitude of his winter Sun, not to estimate his temperature. Perhaps I didn't make myself clear. ============ Jared Sherman wrote in his second posting- >George- > Latitude about 43N and tht puts a late afternoon sun around 11-12d above >the horizon. > I am aware of the possible errors, causes, and effects and did in fact run >them all through a calculator to see just how far a change of 5F or 50mb or >eye height would affect things, I have notes on exactly how far each can >affect the readings. > And I am aware of sextant index error which I think was the root >problem--the index zero wheel may have loosened under my gloved hand, I will >be checking this again and have since re-zero'd on a star which gives me a >far tighter zero. > The effect of taking a warm sextant out of the car does not exist, after an >hour in my trunk near 35F the sextant is nicely prechilled thank you.
> > The question remains: Has anyone found that it is harder to get absolute >accuracy when shooting an afternoon sun from a lattitude 43N during the >winter, when refraction causes the three factors of eye height, air >temperature, and air pressue to be far greater in effect than they are for a >typical summer sight from these same lattitudes? ============ I have these comments to make. 1. I am puzzled by what Jared says about checking index error. Checking the index error is something that should be done AT THE TIME OF THE OBSERVATION. It is bad practice (in my view) to check the index error against a star at night, then go out next day and use that index error to correct a Sun altitude (if I have understood correctly what Jared is saying). 2. Jared writes- >during the >winter, when refraction causes the three factors of eye height, air >temperature, and air pressue to be far greater in effect than they are for a >typical summer sight from these same lattitudes? My response- These are NOT the effects of winter. Why should the effect of eye-height be "far greater" in winter? (the effect of eye-height doesn't vary with the altitude of the body being measured). As I have shown, the effects of air temperature are small. Air pressures are similar, in winter and summer. Refraction effects can be significantly greater in winter (for Sun altitude measurements only: not for other bodies) only because the Sun altitudes are in general less. Observers should choose their times, if possible, when the Sun is well up in the sky. At Jared's latitude of 43 degrees, at midwinter the midday Sun gets up to 24 degrees, and he has many hours available to him when the Sun is above 15 degrees. He is fortunate. The people that have real problems are winter astro-navigators around Scandinavia, but I guess they are now few and far between. At one time, not so many years ago, vessels sailed these Northern waters, without electronic assistance, throughout the year. Presumably star-navigation became more useful than the Sun in the depth of winter. I see no reason why a winter observation of the Sun at an altitude of 15 degrees altitude should be any more sensitive to refraction than is a summer observation of the Sun at 15 degrees. Perhaps Jared will explain further, if he thinks that it is. Jared added- >I also suspect that some mirage (from the ocean/air temperature difference) >and a rough horizon (either thermal mirage or waves at sea) were involved. I >wasn't getting a real crisp sight and I've gotten crisper before, I'm >wondering if that air/water temp difference causes enough distrotion in the >winter to make it something to routinely be mindful of. My reply- If you are looking at a rough horizon, from on shore, errors could be quite significant. You are seeing the tops of the waves only. On a small vessel in rough weather, the observer is recommended to measure altitude when his vessel is also at the top of a wave, to cancel out this error as far as possible. Standing on a beach, that can't be done. My guess is that it might be allowed for by subtracting from your height-of-eye on the beach an amount corresponding to half the peak-to-trough height of the waves at the horizon (if you knew what that height was, which is unlikely). If it's rough out at sea, it would be best to take a higher viewpoint. As for distortion caused by temperature gradients in the air close above the sea surface, that is always something to be "mindful of", but it is not a matter that's possible to make a rational allowance for. Instruments were developed in the early 20th century for measuring the actual dip of the observed horizon at sea, but they have never caught on. I have adapted a sextant to do this job, but it awaits proper testing next summer. In a further posting, Jared writes- changes the refraction by only 0.2 minutes of arc. [George Huxtable]> > >And those 14�C amount to 2/10 mile of difference in position from a sight >>reduction here, from the sample numbers I just ran at Ho 11�30' and 0�C >/vs/ >14�C but a difference of 4/10th mile when Ho is down to 5�30'. > >So the temperature difference alone can be .2-.4 mile if not factored >>correctly under winter conditions at 43N on a late afternoon. My reaction- Well, nobody should CHOOSE to measure Sun altitudes when they are as low as 5 deg 30 min, if he has any alternative. If it is essential, as it might be in midwinter toward the arctic circle, he has to be very aware of the uncertainties in the refractions. But I do not think these are problems that face Jared at 43 degrees latitude, even in midwinter. ======================== There's another factor that changes between summer and winter. The Sun's semidiameter varies between 15.8 min in Summer and 16.3 in winter (opposite for Southerners) so that correction alone could cause differences of half a minute. I hope that Jared (or his reduction program) takes this factor properly into account. To sum up, my opinion remains that winter itself does not add significantly to the errors in astro altitude measurements. For Sun altitudes, however, it limits the range of times over which the Sun can usefully be observed, depending on the observer's latitude. George Huxtable. ------------------------------ email@example.com George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. Tel. 01865 820222 or (int.) +44 1865 820222. ------------------------------