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A simple question, I hope
From: Stan K
Date: 2013 Apr 11, 22:17 -0400
From: Stan K
Date: 2013 Apr 11, 22:17 -0400
The SHA of stars changes very slowly, so the Nautical Almanac uses a single value for the three days of a daily page. I would have to believe that this is the value for 1200 UT of the middle day (rounded to 0.1'), but I have not been able to find this, or something contrary, documented anywhere.
This came up in a discussion of the difficulty in getting an "electronic" almanac to match the printed Nautical Almanac. It would appear that the rounded SHA value could transition to a 0.1' smaller value sometime on the third day, and could have transitioned from a 0.1' larger value some time on the first day.
This is still not as "bad" as the sun altitude correction in the Nautical Almanac being a six-month average (of some sort), which could cause a 0.2' (possibly 0.3'?) discrepancy with an electronic almanac that calculates based on the actual date and time.
Sure, there are ways around both these issues, but it is just not worth it. Most people using an electronic almanac are not trying to match the printed Almanac - they are trying to get an accurate result.
Nevertheless, if anyone could point me to where my suspicion is confirmed or contradicted, it would be appreciated.
Stan
This came up in a discussion of the difficulty in getting an "electronic" almanac to match the printed Nautical Almanac. It would appear that the rounded SHA value could transition to a 0.1' smaller value sometime on the third day, and could have transitioned from a 0.1' larger value some time on the first day.
This is still not as "bad" as the sun altitude correction in the Nautical Almanac being a six-month average (of some sort), which could cause a 0.2' (possibly 0.3'?) discrepancy with an electronic almanac that calculates based on the actual date and time.
Sure, there are ways around both these issues, but it is just not worth it. Most people using an electronic almanac are not trying to match the printed Almanac - they are trying to get an accurate result.
Nevertheless, if anyone could point me to where my suspicion is confirmed or contradicted, it would be appreciated.
Stan
