01-15-2017, 07:27 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?
Edit: I should have mentioned that I'm using a reflective artificial horizon.
The diameter of the Sun is approx half a degree (angular subtense at the eye).
Theoretically there is a difference in atmospheric refraction between the upper and lower limb due to the difference in altitude measured, but this is completely negligible in practical celestial navigation.
If you look at Table A2 in the Nautical Almanac: Altitude Corrections Tables 10 - 90,
you can see the effect is not significant and does not become highly significant even when altitudes are becoming much less than 10 degrees . . .even when one is measuring altitudes down to around only one or two degrees altitude.
It is not recommended sights should be taken below 10 degrees in general if best accuracy is wanted, because the absolute value of the refractive correction becomes uncertain due to local atmospheric conditions which can be very variable.
If lower than 10 degrees altitude sights are to be used, then additional corrections are obtained with Table A4 Altitude Correction Tables - Additional Corrections. I will be seen from this table that even at Apparent altitudes of only one degree to two degrees, the difference in correction is only 0.9' - where it follows for the Suns diameter of half a degree the difference between upper and lower limbs will be 0.45' . . . still only half a minute or error between upper and lower limb.
In Table A2 (used for most cases of sight correction for altitudes above 10 degrees), you will see as an example the difference in correction for refraction between altitude 9deg33' and 10deg33' (a whole degree), is only 0.4' (minute of arc).
It follows for the Sun's diameter of half a degree, the difference in refraction between upper and lower limb at altitude 10 degrees is therefore half of this i.e. 0.2 moa.
For greater altitudes measured the difference will be even less.
At 50deg43' to 54deg46' for example, the difference in correction is only 0.1 moa for an altitude difference of 4 degrees.
It follows the difference in upper and lower corrections at altitude 50 degrees for the Sun 's diameter of half a degree is one eighth of 0.1 moa i.e. 0.0125 moa (which is equal to 0.75 seconds of arc).
This kind of refractive problem and corrections are only a difficulty with scientific precision astronomy.
Douglas Denny. Bosham. England.