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On 5/7/2013 2:15 PM, Jeremy C wrote:
> FER - "The (presumably) nearest to correct rule would be dip + 34'(for
> mean refraction) - 16'(for SD) or in other words dip + 18' (and to be
> yet more accurate, +/- a few minutes of arc for non-standard temperature
> and pressure). Considering you're shooting from a greater than average
> height of eye, do you have a rule of thumb that incorporates a higher
> dip value?"

Regarding lower-limb altitude for amplitude, thanks for the HC=0 answer.
That nugget cleared it up for me.

Referencing the NA 0-10 altitude-correction tables seeking the ideal LL
altitude for amplitude at STP with no dip correction using inspection
and mental interpolation I arrived at approx. 14’ for HC=0. (LL altitude
– refraction). I used an average of the averaged seasonal refraction
values to arrive at the 14' target. As Frank suggested, a baseline plus
dip and pressure/temp correction should be optimal.

Figures I ran for a 10 minute period prior to sunset at 45N 86W on 6 May
2013 averaged 0d 9!7 HC change per minute, and 0d 10!5 per minute for
azimuth. Clearly LL altitude being off by 5’is not a huge deal in mid
latitudes, keeping in mind refraction will amplify the change in HC. As
mentioned, the further you are from the equator the more critical timing
becomes. In the bigger picture refraction is the fuzzy variable.

By my calculations, the average height of the squished sun with a 14’ LL
altitude is 27’, so an RCH over 1 SD should work just fine. More on
squish later.

Bill B

   

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