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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Pub 249 extract in CelNav Ed 2

Author Topic: Pub 249 extract in CelNav Ed 2

 - posted April 28, 2021 03:46 PM      Profile for Lance           Edit/Delete Post 
Short Question: Does anybody know the epoch of the extract from Pub 249 given on page 185 of CelNav 2nd Edition?

Longer Discussion:
The reason I ask is that last night I was revising the use of the Pub. 249 Vol 1 tables on page 185 of CelNav 2nd Ed. Hc is only given to the nearest minute, so I thought the entries would have to be assumed to be up to 0.5 min out.

To check this, I took the entry for Mirfak highlighted in figure 11.27-1, i.e. Hc=20° 28' (and Zn = 039) and recalculated for LHA of Aries 296° and latitude 45°N. Using table T-1 (1978 July 24-26) I find Mirfak had SHA 309° 18.3' and Dec. N49° 46.9' in 1978, so

LHA Mirfak = LHA Aries + SHA Mirfak
= 296° + 309° 18.3' - 360°
= 245° 18.3'

For the combination of LHA 245° 18.3', Lat 45°N and Dec. N49° 46.9' I calculated Hc=20° 26.25' using a calculator.

If my calculation is correct, why the 1.75' difference, well over my expected 0.5' or less.
Is is possible that the extract from Pub 249 is not from the same epoch as the 1978 almanac extract in Table T-1?

I compared the position of Mirfak in 1978 from the Almanac to that in 2021 and found that today it's near SHA 308° 33.1', Dec. N49° 56.1', a variation of roughly 1' of SHA per year, so if the epoch for the Pub 249 extract was not near 1978, that would probably explain the difference.

From: Hobart, Australia
David Burch

 - posted May 08, 2021 01:47 PM      Profile for David Burch           Edit/Delete Post 
That page of the book shows this Vol 1 table selection working out a sight from October 25, 1978—when we first started teaching cel nav.

Based on that, my guess is the Epoch is the one that includes 1978. These come out every 5 years good, for ± 4 years. So this could be from Epoch 1980 or Epoch 1975. My guess is it would be the 1975 edition. You could use a program like StarPilot or others to go back and spot check a few of the values in the table.

From: Starpath, Seattle, WA

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