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Re: Sun's instrumental altitude with artificial horizon
From: Bill Noyce
Date: 2003 Feb 21, 10:59 -0500
From: Bill Noyce
Date: 2003 Feb 21, 10:59 -0500
> Now, about Sun and Moon... what is a preferable way to set images (oil/sky > image)through view and just what corrections should I use in order to > finally calculate the departure? I think these are your choices: #1. Bring the sun's image down so it is superimposed over its reflection in the artificial horizon. In other words, the two disks merge into one. This is likely to be the most accurate way to make the observation. To convert Hs to Ho in this case, first correct for index error in the usual way, then divide by two, then correct for refraction (using the Almanac table for stars). There is no correction for semidiameter in this case. For extra refinement, you could add 0.1' for the sun's parallax if the altitude is less than about 60 degrees. #2. Bring the sun's image down so that its lower limb just meets the near side of the reflection. (This is the reflection of the lower limb, but it appears on the upper side of the image.) In other words, the two disks look like a snowman or a figure 8. To convert Hs to Ho in this case, first correct for index error, then divide by two, then apply corrections for refraction, semidiameter (lower limb), and parallax using the normal "Sun LL" table. #3. Bring the sun's image down so that its upper limb just meets its own reflection -- another snowman, but upside down from the case above. To convert Hs to Ho in this case, first correct for index error, then divide by two, then apply corrections for refraction, semidiameter (upper limb), and parallax using the normal "Sun UL" table. When observing the moon, my guess is that procedure #2 or #3 (based on whichever limb is visible) will give a more precise observation than #1, but I can't claim any experience. Also, in case #2 and #3, the NA tables correct for parallax at the same time as for semidiameter, whereas in case #1 you have to do it separately, and I don't think the NA makes that easy. -- Bill