Celestial correction for Great Lakes?

[Bigjim]

Originally Posted by El Pinguino

Consider this..

'2) For every ten degree increase in the surface air temperature
above the value of the surface water temperature, the value of the dip is to
be decreased by 1 minute of arc.'

So if Lake Michigan sea temp is 45*F and the air blowing off the land is 75* then the correction to dip is -3'.

So let us say... sextant altitude is 40* 30' .... dip correction from your tables is - 4'.... from above ^^ reduce dip correction by -3' dip to use is - 1'.
Now instead of having a corrected altitude of 40* 26' we have a corrected altitude of 40* 29' .... the altitude is greater... so we are closer to the body's G.P.... 3' closer.....

( I have ignored all the other corrections in the interests of clarity.. and I hope I have this the right way round...)



So.... taking the two sights that had an intercept of maybe 5' away.... we now have intercepts of only 2' away........ looking better all the time....

Temperature induced errors could possibly be the cause.

One of the sightings was from April. The water temp in Lake Michigan was pretty cold. Definitely in the 40s. We took the readings outside with jackets or coats on so the air temperature was not warm by any stretch.

Probably 50s. So, maybe 10 degrees warmer.

I think your theory would also have to take into account which way the wind was blowing. Coming off the warmer land would provide a greater differential.

If the wind was coming from the east, the air temperature was probably closer to the water temperature.

In Nov, the water was still in the 50s or 60s. Air temperature on those days was probably 60-70. We could look in the local papers or NOAA records for actual weather reports.

Very interesting.

[SeanPatrick]

Originally Posted by Bigjim

Our exact position was N42:21.4 W87:49.5

Another couple of questions: was everyone in your party familiar with the technique of swinging the sextant about the longitudinal axis of the telescope to ensure that the sextant is vertical when the measurement is made?


I noticed that, from your position, the Moon and Mars sights you posted were at an azimuth that had you looking almost right down the south-western shore of Lake Michigan. Do you think it is at all possible that this may have affected the way those sights were taken?





Your Sun sight, which was at a more easterly azimuth, had a much smaller intercept from your position. The azimuth of the Mars sight was closest to the shore and it also had the largest intercept.

[Bigjim]

Originally Posted by SeanPatrick

Another couple of questions: was everyone in your party familiar with the technique of swinging the sextant about the longitudinal axis of the telescope to ensure that the sextant is vertical when the measurement is made?


I noticed that, from your position, the Moon and Mars sights you posted were at an azimuth that had you looking almost right down the south-western shore of Lake Michigan. Do you think it is at all possible that this may have affected the way those sights were taken?

I took all three sightings. Each student worked out their own individually, with my help. I have been shooting celestial fixes for 40 years. So, yes, I'm familiar with all aspects of using the sextant.

[SeanPatrick]

Hmmm ... well, I'll keep thinking about this and see if I can come up with any other possibilities.

[rbrake]

I would also ask what type of sextant you are using. I taught celestial for 30 years and I could easily find 5-7 miles consistent error in the plastic filters of even expensive plastic sextants on a regular basis.

Work the problem backwards from a known LOP on a rock stable shore and see what you should be reading.

[Paul Howard]

I used to teach a celestial navigation course during the winter and, for the last class we would go to the north shore of Lake Ontario for a day to do some practical sights with a variety of plastic and metal sextants.
The range of visible horizon for Lake Ontario, at 242ft. above sea level, was a couple hours either side of noon. Using the HO 249 tables and nautical almanac the sights were within the acceptable range of a circle. It was during April, when the declination of the sun is far to the south and the temperature still quite cold. We constantly checked the sextant error, as it changes quite fast, especially on the plastic models, but also with the metal sextants.
Those studying celestial navigation were planning to make ocean passages and were not interested in using celestial navigation on the Great Lakes as a navigational tool, thus to teach a separate correction for the Great Lakes would be counter productive when teaching for ocean sailing.

[SeaBreeze-1]

Just want to verify something with you... Are you using DIP corrections for height of eye above the "Horizon" or above "Sea Level"? I have taken all sorts of sextant shots on the Great Lakes, DIP corrected for height of eye above the "Horizon" ONLY and have never encountered the curious problem you are experiencing.

As an accurate timepiece, I use time based on a clock that receives its signal from the Cesium Fountain Atomic Clock in Boulder, Colorado and any other typical corrections are applied to the celestial body and conditions observed at the time, based on the yearly Nautical Almanac compiled by the UK Hydrographic and U.S. Naval Observations. If the errors you are encountering occur regardless of which sextant you are using, there is definitely a man-made error in the calculations somewhere or the distance to one of your horizons may not be what you need it to be.

It would be interesting if, once you've tracked down the "gremlin", you can share it with us.

[barryglewis]

Where are you deriving the actual position from?
With a professional navigator overseeing, multiple sextants, multiple observers, multiple people calculating multiple sights all showing consistent discrepancy then perhaps the postion you are comparing the sights to has an issue.
Possibilities that come to mind;
The actual position derives from GPS, so a chart datum issue (although wouldnt expect it to be such a big difference),
or a local gravitational anomaly. Such things were known in the central Pacific, and were causing discrepancies of several miles in the central Gilbert isles.

[Bigjim]

Originally Posted by SeaBreeze-1

Just want to verify something with you... Are you using DIP corrections for height of eye above the "Horizon" or above "Sea Level"? I have taken all sorts of sextant shots on the Great Lakes, DIP corrected for height of eye above the "Horizon" ONLY and have never encountered the curious problem you are experiencing.

As an accurate timepiece, I use time based on a clock that receives its signal from the Cesium Fountain Atomic Clock in Boulder, Colorado and any other typical corrections are applied to the celestial body and conditions observed at the time, based on the yearly Nautical Almanac compiled by the UK Hydrographic and U.S. Naval Observations. If the errors you are encountering occur regardless of which sextant you are using, there is definitely a man-made error in the calculations somewhere or the distance to one of your horizons may not be what you need it to be.

It would be interesting if, once you've tracked down the "gremlin", you can share it with us.

DIP corrections are always figured from height above water or sea level. If we tried to use the correct for our altitude, the correction would be huge. The error or offset in this case is only 3-5 miles and ALWAYS away or north. I cannot see how any human error could produce the SAME error every single time. Statistically, such an error would produce random errors.

For example, taking a warm plastic sextant outside when it's cold can produce a dramatic index error unless you allow it to acclimate to the temperature. On one day, the IC was 4 degrees.

In every siting, we always check for IC. Unfortunately, we don't have time to let the sextants acclimate before shooting. Metal sextants should not change that dramatically.

If I track down the cause, I will definitely post it.

[Bigjim]

Originally Posted by barryglewis

Where are you deriving the actual position from?
With a professional navigator overseeing, multiple sextants, multiple observers, multiple people calculating multiple sights all showing consistent discrepancy then perhaps the postion you are comparing the sights to has an issue.
Possibilities that come to mind;
The actual position derives from GPS, so a chart datum issue (although wouldnt expect it to be such a big difference),
or a local gravitational anomaly. Such things were known in the central Pacific, and were causing discrepancies of several miles in the central Gilbert isles.

All observations were taken from the deck outside our yacht club. It is a fixed location that has not moved or shifted for 50 years. Unless the chart we're using is out of whack, we know exactly where we are located.

[Gruz]

Hello there,

10 ft. or 600 ft., terrestrial altitude doesn't matter as long as you have a good horizon. Thus a high altitude lake at least three miles across gives as good a horizon as the ocean, and requires only standard dip. If your body of water is less than three miles across use the table for "Dip Short of the Horizon" to adjust dip in minutes of arc on the sextant reading . But the altitude of the lake does not matter.

[barryglewis]

Has this happened again, or was it just this one lesson?

If just the once then probably abnormal refraction, or maybe ie incorrect.
I initially dismissed your thought re ie changing with temperature going out on the deck from the class room, but on reflection realise yachts I've taken sights from in near 0 degrees outside were similar temp below! Can you do a check on ie in similar weather to when you had your problem, and see how much it changes over time after taking sextant from hot classroom to cold yc deck?

If it happens consistently over weeks or months, what chart do you use to obtain the yacht club position? What is its chart datum?


Barry
Risky Business

[Smftdr2002]

Hi,

I ran the sights in Starpath StarPilot and using your location as the DR and your Hs for the three bodies, got a fix by computation right on top of your DR. The computer makes no corrections for altitude above sea level and I made no changes to atmospheric conditions from the default settings (ie sea level pressure, temp 50 degrees F) in the computer. Your sextant sights must have been very accurate, small intercepts and the fix was dead on. Of course fix by plotting leaves room for interpretation as to where to place the fix in the cocked hat. But the plot looked just like yours.

Thanks for the fun problem,

Ken

[SeanPatrick]

Originally Posted by Smftdr2002

...got a fix by computation right on top of your DR.

42°41.0' N, 87°25.4' W is about twenty-six nautical miles (almost half a degree) from his stated DR of 42°21.4' N, 87°49.5' W. Hardly "right on top".



However, it is very close to my plotted fix - which means I didn't screw anything up, either.

[SeanPatrick]

Originally Posted by SeanPatrick

Your Sun sight, which was at a more easterly azimuth, had a much smaller intercept from your position. The azimuth of the Mars sight was closest to the shore and it also had the largest intercept.

It has also occurred to me that the Sun sight was taken in April, while the two others were taken in October. On April 8, 2018 at about 10 am, the temperature in Waukegan was 31°F and the pressure was 29.30 inHg. This results in a refraction correction of 102%.



On October 18, 2018 at about 6 pm, the temperature was 52°F and the pressure was 29.50 inHg. This results in a refraction correction of 98%.


I have no way of knowing what the temperature of the lake was at these times.