NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Question on Lunars
From: Frank Reed CT
Date: 2004 Oct 27, 20:22 EDT
From: Frank Reed CT
Date: 2004 Oct 27, 20:22 EDT
Alex E wrote:
"Just watch for the moment when the moon "collides with a star" on its normal way (there are so many stars around!)"
For land-based astronomers, this was an excellent method of getting longitude. Occultations can be timed with good accuracy. But the "so many stars" comment is a little tricky, of course. There are lots and lots of very faint stars, plenty of faint stars, and few bright stars. Even when we can see the occultations of faint stars, there is the problem of identifying them. Nineteenth century navigators had a hard enough time identifying the nine lunars stars. With the glare of the Moon interfering, it can be quite difficult to correctly identify stars near the Moon.
And wrote:
"Few days after this came to my mind, I found that
I was not the first:-) The method was proposed centuries ago and it is called
"occultations of the stars by the Moon".
It's practical on land but not so much at sea. Nonetheless, as early as the third edition of Bowditch's Navigator (I think it was the third), published in 1811, Bowditch included the details of the calculation of longitude from an occultation. It was a long calculation requiring considerable attention to detail and for that reason it was impopular.
And:
"Then why was not this practiced at least as much as the Lunars were?"
Mostly because favorable occultations are so rare. By contrast, lunar distance sights were available on any clear day except for four days or so around New Moon. Also because occultation calculations were tedious.
In the logbooks I've reviewed, I've seen one reference to an occultation --not a successful observation but at least the thought has occurred to a real navigator at sea. I posted in August a detailed account of navigation aboard the Weymouth. It's in the list archives here:
http://www.i-DEADLINK-com/lists/navigation/0408/0118.html
Here's the relevant section (my notes in parentheses, the rest is directly quoted):
"May 12, 1823
(some comments about depression and boredom and poor health followed by...)
The Sun and Moon are not in distance. No Occultations are visible where we are so that we have no means of obtaining the correct Longitude. The observations taken in the preceding days were obtained under the disadvantage of squally weather. (...) Roast Chicken and Eggs for Dinner.
Lat DR: 32 20N Lat Obs: 32 32N Lon by Lunar: 37 53W (again, by D Lon this is almost certainly a "lunar account" longitude)"
And Alex wrote:
"I sort of remember someone saying that Lunars give bad results when the distance is too small... In the case of occultation the distance is zero."
The calculation is much longer, but it can be done. In practice historically, it was rarely done.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"Just watch for the moment when the moon "collides with a star" on its normal way (there are so many stars around!)"
For land-based astronomers, this was an excellent method of getting longitude. Occultations can be timed with good accuracy. But the "so many stars" comment is a little tricky, of course. There are lots and lots of very faint stars, plenty of faint stars, and few bright stars. Even when we can see the occultations of faint stars, there is the problem of identifying them. Nineteenth century navigators had a hard enough time identifying the nine lunars stars. With the glare of the Moon interfering, it can be quite difficult to correctly identify stars near the Moon.
And wrote:
"Few days after this came to my mind, I found that
I was not the first:-) The method was proposed centuries ago and it is called
"occultations of the stars by the Moon".
It's practical on land but not so much at sea. Nonetheless, as early as the third edition of Bowditch's Navigator (I think it was the third), published in 1811, Bowditch included the details of the calculation of longitude from an occultation. It was a long calculation requiring considerable attention to detail and for that reason it was impopular.
And:
"Then why was not this practiced at least as much as the Lunars were?"
Mostly because favorable occultations are so rare. By contrast, lunar distance sights were available on any clear day except for four days or so around New Moon. Also because occultation calculations were tedious.
In the logbooks I've reviewed, I've seen one reference to an occultation --not a successful observation but at least the thought has occurred to a real navigator at sea. I posted in August a detailed account of navigation aboard the Weymouth. It's in the list archives here:
http://www.i-DEADLINK-com/lists/navigation/0408/0118.html
Here's the relevant section (my notes in parentheses, the rest is directly quoted):
"May 12, 1823
(some comments about depression and boredom and poor health followed by...)
The Sun and Moon are not in distance. No Occultations are visible where we are so that we have no means of obtaining the correct Longitude. The observations taken in the preceding days were obtained under the disadvantage of squally weather. (...) Roast Chicken and Eggs for Dinner.
Lat DR: 32 20N Lat Obs: 32 32N Lon by Lunar: 37 53W (again, by D Lon this is almost certainly a "lunar account" longitude)"
And Alex wrote:
"I sort of remember someone saying that Lunars give bad results when the distance is too small... In the case of occultation the distance is zero."
The calculation is much longer, but it can be done. In practice historically, it was rarely done.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
