NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A Lunars Game
From: Frank Reed CT
Date: 2003 Dec 18, 03:37 EST
From: Frank Reed CT
Date: 2003 Dec 18, 03:37 EST
Trevor, you wrote:
"I have a nasty feeling that you and the other lunarians are going to
tell me that I am way off"
Not even close! Not even close about my reaction, that is. You took all the steps I was hoping you would, and I'm really pleased to read your analysis.
And:
"His zenith is roughly 45% of the difference in declination from Canopus
to Sirius. I take it that means 45%, not 40% or 50%"
Exactly. Do you think that's reasonable by "eyeball" estimation (from land, of course)?
And you wrote:
"With Venus and the Moon low in the sky, the parallels of declination
passing through them will make an angle with the observer's vertical
that is equal to his latitude."
This is critical, too. How many navigators do you think know this?
And:
"HP for the Moon at about the time of the observation is 58', to the nearest minute, and to that degree of precision is steady throughout the hours that the observation could have been made. The parallax in altitude is, therefore one degree, to the sort of precision we are dealing with."
And notice that you don't need the Almanac HP to do this. It's gonna be nearly a degree no matter what. The altitude is rather low. So the change in height due to HP is a bit less than a degree. No more details necessary...
You concluded:
"The only islands in the block of ocean between 35°30'S 10°W and 37°30'S
40°W are the Tristan da Cunha group at about 37°30'S 13°W."
Bravo!
" Since you described a helicopter ride following your contestant's flight but did not mention any urban development on the island, I'll guess that you did not mean Tristan itself but one of its small sister islands (Inaccessible and Nightingale?)."
The contestant can work that out at sunrise since all the islands in the group would be visible anywhere in the group.
You concluded:
"Maybe I am way off in my estimations. Even if I am not, I dare say that
the lunarians on this list can get a neater and more precise longitude.
However, given your choice of latitude and the placement of your
contestant on an island, even these very crude eyeball estimates seem
sufficient to answer your puzzle."
Indeed. Admittedly, this was a contrived example, but these neat cases where "eyeball" lunars would work are not really all that rare --maybe once a week from any location.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"I have a nasty feeling that you and the other lunarians are going to
tell me that I am way off"
Not even close! Not even close about my reaction, that is. You took all the steps I was hoping you would, and I'm really pleased to read your analysis.
And:
"His zenith is roughly 45% of the difference in declination from Canopus
to Sirius. I take it that means 45%, not 40% or 50%"
Exactly. Do you think that's reasonable by "eyeball" estimation (from land, of course)?
And you wrote:
"With Venus and the Moon low in the sky, the parallels of declination
passing through them will make an angle with the observer's vertical
that is equal to his latitude."
This is critical, too. How many navigators do you think know this?
And:
"HP for the Moon at about the time of the observation is 58', to the nearest minute, and to that degree of precision is steady throughout the hours that the observation could have been made. The parallax in altitude is, therefore one degree, to the sort of precision we are dealing with."
And notice that you don't need the Almanac HP to do this. It's gonna be nearly a degree no matter what. The altitude is rather low. So the change in height due to HP is a bit less than a degree. No more details necessary...
You concluded:
"The only islands in the block of ocean between 35°30'S 10°W and 37°30'S
40°W are the Tristan da Cunha group at about 37°30'S 13°W."
Bravo!
" Since you described a helicopter ride following your contestant's flight but did not mention any urban development on the island, I'll guess that you did not mean Tristan itself but one of its small sister islands (Inaccessible and Nightingale?)."
The contestant can work that out at sunrise since all the islands in the group would be visible anywhere in the group.
You concluded:
"Maybe I am way off in my estimations. Even if I am not, I dare say that
the lunarians on this list can get a neater and more precise longitude.
However, given your choice of latitude and the placement of your
contestant on an island, even these very crude eyeball estimates seem
sufficient to answer your puzzle."
Indeed. Admittedly, this was a contrived example, but these neat cases where "eyeball" lunars would work are not really all that rare --maybe once a week from any location.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
