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Re: Nautical astronomy was different
From: Frank Reed CT
Date: 2004 Oct 24, 02:06 EDT
From: Frank Reed CT
Date: 2004 Oct 24, 02:06 EDT
I wrote earlier:
"They also have had experience with the modern Nautical Almanac format which may give the erroneous impression that you must have exact Greenwich Time to pull any data out of the almanac. That change in format of the almanacs is only 60-70 years old --so convenient in practical terms yet perhaps confusing from an educational standpoint."
And Fred H replied:
"If you guys could expand on this point, I'd like that."
If you look at a modern "Nautical Almanac", the positions of the planets, Sun, and Moon are tabulated for every hour of GMT as GHA and Dec. This is advantageous from the point of view of modern short LOP sight reduction methods. But this layout can create the impression that the data in the almanac is nearly useless unless you know the GMT to the second. An experienced navigation or astronomy enthusiast will soon enough realize that this isn't true. The dizzying changes in the positions are mostly just the steady advance of GHA Aries. If the sight reduction methods depended on SHA and Dec, they would shift much more slowly and they could be tabulated at much larger intervals of time (once a day for Jupiter and Saturn, for example).
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"They also have had experience with the modern Nautical Almanac format which may give the erroneous impression that you must have exact Greenwich Time to pull any data out of the almanac. That change in format of the almanacs is only 60-70 years old --so convenient in practical terms yet perhaps confusing from an educational standpoint."
And Fred H replied:
"If you guys could expand on this point, I'd like that."
If you look at a modern "Nautical Almanac", the positions of the planets, Sun, and Moon are tabulated for every hour of GMT as GHA and Dec. This is advantageous from the point of view of modern short LOP sight reduction methods. But this layout can create the impression that the data in the almanac is nearly useless unless you know the GMT to the second. An experienced navigation or astronomy enthusiast will soon enough realize that this isn't true. The dizzying changes in the positions are mostly just the steady advance of GHA Aries. If the sight reduction methods depended on SHA and Dec, they would shift much more slowly and they could be tabulated at much larger intervals of time (once a day for Jupiter and Saturn, for example).
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
