NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Nautical astronomy was different
From: Bruce Stark
Date: 2004 Oct 24, 14:01 EDT
From: Bruce Stark
Date: 2004 Oct 24, 14:01 EDT
Fred,
Frank has now answered the question you asked four days ago. I figured he'd forgotten, so started this before dinner last night. May as well finish and send it along.
The question was:
"On Oct 20, 2004, at 6:26 PM, Frank Reed wrote:
> 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.
If you guys could expand on this point, I'd like that."
The change Frank is referring to is to the GHA system. To the best of my knowledge the Almanac was pretty much the same from 1767 until the 1830s, when changes began to be introduced now and then. I think it was mainly a matter of adding more information, like lunar distances of planets, accurate (rather than approximate) declination and RA coordinates of the moon and planets, giving declination and RA data at closer intervals, and so on. Other List members may be able to fill in or correct me here.
But the WAY data was presented remained the same until sometime in the first half of the twentieth century. Until then, Almanac data was independent of the earth's rotation. The Almanac gave the positions of the heavenly bodies in relation to the first point of Aries and the equinoctial. The only way Greenwich time came into it was that predictions were generally for the moment the sun would cross the transit wires at Greenwich.
Except for the rotation of the celestial sphere, nothing much is happening up there. The sun takes twelve hours to get out of its own tracks, and the stars move so slowly that the navigation manuals, rather than the Almanac, told were they were. To enter the Almanac, all you wanted was the approximate Greenwich time.
The moon, of course, is moving half a degree per hour. But normally you only used the moon for lunar distances. All you had to know was the name of the other body, whether it was east or west of the moon, and the month. The Almanac told YOU the Greenwich time.
Although you didn't need accurate time to enter the almanac, you needed accurate time to keep track of the celestial sphere's rotation. That's where local time came in. It's easily found by time sight, but not easy to keep track of. Since it shifts with every shift in longitude it depends not only on the watch, but on the dead reckoning. Fortunately it only had to be kept accurately during the elapsed time between the lunar and the time sight. A "well regulated" time keeper was one that had recently been compared with a time sight.
With steam ships, quick passages between places of known longitude, and an abundance of chronometers, the old way of thinking began to change. Perhaps it wasn't long before time sights came to be thought of as "timed sights."
The old style Almanac worked well when navigators could go by dead reckoning for weeks at a time, and were away from places of known longitude for months or years. But it didn't work well in air navigation. Someting different was needed. One of the innovations of air navigation was an almanac giving the angular distance of the celestial bodies from the meridian of Greenwich.
The GHA Almanac is easy to use and easy to teach, and soon became popular with surface navigators.
East-west data is extremely time dependent, but keeping accurate time is no problem now. And, thanks to time zones and excellent time keepers, GMT has, for more than a century, been THE time.
Things are different than they were in Bowdich's day.
Bruce
Frank has now answered the question you asked four days ago. I figured he'd forgotten, so started this before dinner last night. May as well finish and send it along.
The question was:
"On Oct 20, 2004, at 6:26 PM, Frank Reed wrote:
> 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.
If you guys could expand on this point, I'd like that."
The change Frank is referring to is to the GHA system. To the best of my knowledge the Almanac was pretty much the same from 1767 until the 1830s, when changes began to be introduced now and then. I think it was mainly a matter of adding more information, like lunar distances of planets, accurate (rather than approximate) declination and RA coordinates of the moon and planets, giving declination and RA data at closer intervals, and so on. Other List members may be able to fill in or correct me here.
But the WAY data was presented remained the same until sometime in the first half of the twentieth century. Until then, Almanac data was independent of the earth's rotation. The Almanac gave the positions of the heavenly bodies in relation to the first point of Aries and the equinoctial. The only way Greenwich time came into it was that predictions were generally for the moment the sun would cross the transit wires at Greenwich.
Except for the rotation of the celestial sphere, nothing much is happening up there. The sun takes twelve hours to get out of its own tracks, and the stars move so slowly that the navigation manuals, rather than the Almanac, told were they were. To enter the Almanac, all you wanted was the approximate Greenwich time.
The moon, of course, is moving half a degree per hour. But normally you only used the moon for lunar distances. All you had to know was the name of the other body, whether it was east or west of the moon, and the month. The Almanac told YOU the Greenwich time.
Although you didn't need accurate time to enter the almanac, you needed accurate time to keep track of the celestial sphere's rotation. That's where local time came in. It's easily found by time sight, but not easy to keep track of. Since it shifts with every shift in longitude it depends not only on the watch, but on the dead reckoning. Fortunately it only had to be kept accurately during the elapsed time between the lunar and the time sight. A "well regulated" time keeper was one that had recently been compared with a time sight.
With steam ships, quick passages between places of known longitude, and an abundance of chronometers, the old way of thinking began to change. Perhaps it wasn't long before time sights came to be thought of as "timed sights."
The old style Almanac worked well when navigators could go by dead reckoning for weeks at a time, and were away from places of known longitude for months or years. But it didn't work well in air navigation. Someting different was needed. One of the innovations of air navigation was an almanac giving the angular distance of the celestial bodies from the meridian of Greenwich.
The GHA Almanac is easy to use and easy to teach, and soon became popular with surface navigators.
East-west data is extremely time dependent, but keeping accurate time is no problem now. And, thanks to time zones and excellent time keepers, GMT has, for more than a century, been THE time.
Things are different than they were in Bowdich's day.
Bruce
