01-05-2016, 08:46 PM
(01-05-2016, 01:46 AM)stargazer Wrote: The sun has a large apparent disc covering a big area. The lower limb has more atmospheric refractive bending than the upper limb. By the way, the bent light makes it look high, just like shooting at a fish with an arrow. With the lower limb appearing a bigger percentage higher than the upper limb, it makes the sun look flattened as it approaches the horizon.
So how have others been adjusting for the centerline of the apparent disc? Somewhere between the upper limb and lower limb tabulated amounts, but not exactly half way which would not allow for the increased effect on the lower limb?
Heck, I'm not even sure how to figure the halfway point, which would at least be fairly close to correct. I added the two corrections, one is a negative number so I got a smaller negative number. Appears sensible but seems awfully easy, so I fear it may be wrong.
Anyone have any ideas or good/bad experiences?
The lowest I've shot the Sun in an AH is at about 5° off the horizon which would yield a 10° Hs. That's very low and I had to lie on the ground far away from the AH. People thought I was doing some weird prayer.
With an AH there are 3 different sights you can take of the Sun;
1- Upper Limb
2- Lower Limb
3- Center (superimposed)
From my experience here's what I've done in order or preference.
- Shoot the Lower Limb in the AH.
- Shoot the Upper Limb if clouds are in the way or something obscuring the
Lower Limb such as a tree branch.
- Shoot the sun superimposed in the AH.
I think what you're referring to as "half-way point" of the Sun is what is called the semi-diameter. It's not the diameter- it's only part of the diameter. Some genius, working on his doctoral thesis, probably came up with the term but it makes more sense than saying "radius".
If shooting the sun super-imposed in the AH no semi-diameter correction is necessary. All I do is correct for refraction and maybe parallax. Parallax at low altitudes is very small- about .14' from 0° to about 20° off the horizon.
Refraction is always subtracted and parallax is always added.
For refraction correction use TABLE 6. Refraction which is found in HO-249 Vol 2. page 15. Here- HO-249 Vol 2
(The same Table 6 is found in HO-249 Vol 3)
If you don't have Table 6 for refraction correction it can easily be calculated;
0.96 / Tan(Ha)
Meaning- 0.96 divided by the Tangent of Ha
TheNauticalAlmanac.com has a lot of formulas here- Celestial Navigation Formulas
When using Table 6 base the altitude of your location at sea level.
The total corrections necessary to any sextant sight can be summarized by D.R.I.P.S.
Dip
Refraction
Index error
Parallax
Semi-diameter
When using the AH, since there is no Dip correction, Altitude Corrections Table for Sun, Star & Planets makes the determination of semi-diameter, parallax and refraction very easy (but only when shooting the Upper or Lower Limbs).
I hope that answers your question.
Fred
P.S. too cold here to make any sights- the water in the AH would freeze before I could lift my sextant.