NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Longitude by lunars during solar eclipse
From: Peter Hakel
Date: 2015 May 9, 20:08 +0000
Sent: Tuesday, April 21, 2015 12:36 PM
Subject: [NavList] Re: Longitude by lunars during solar eclipse
From: Peter Hakel
Date: 2015 May 9, 20:08 +0000
Hello Piero,
I finally had some time to look into this and I don’t understand the following detail: the Sun and Moon semidiameters are both about quarter of a degree, so if the disks overlap their center-to-center distance cannot exceed (roughly) half a degree. How did you come up with a whole degree of centered lunar distance? Also, during a solar eclipse the near-limb distance is in fact negative.
I’m also not sure how you used the lunar_distance.xls spreadsheet because it does not calculate longitude - it calculates Universal Time for checking or resetting one’s chronometer. If I wanted to extract longitude with available Navigation Spreadsheets, I would use the other lunar-distance spreadsheet, ld_prec.xls, in which I’d have to manually iterate on the longitude input (all other inputs are known) until the topocentric distance matches the observation (i.e., an exercise in modeling the Moon’s parallax effect). Such a result, however, would not have the highest accuracy, because this particular spreadsheet does not account for refraction.
Peter Hakel
http://www.navigation-spreadsheets.com/lunar.html
I finally had some time to look into this and I don’t understand the following detail: the Sun and Moon semidiameters are both about quarter of a degree, so if the disks overlap their center-to-center distance cannot exceed (roughly) half a degree. How did you come up with a whole degree of centered lunar distance? Also, during a solar eclipse the near-limb distance is in fact negative.
I’m also not sure how you used the lunar_distance.xls spreadsheet because it does not calculate longitude - it calculates Universal Time for checking or resetting one’s chronometer. If I wanted to extract longitude with available Navigation Spreadsheets, I would use the other lunar-distance spreadsheet, ld_prec.xls, in which I’d have to manually iterate on the longitude input (all other inputs are known) until the topocentric distance matches the observation (i.e., an exercise in modeling the Moon’s parallax effect). Such a result, however, would not have the highest accuracy, because this particular spreadsheet does not account for refraction.
Peter Hakel
http://www.navigation-spreadsheets.com/lunar.html
Sent: Tuesday, April 21, 2015 12:36 PM
Subject: [NavList] Re: Longitude by lunars during solar eclipse
Hi Frank ,
actually I used the excel file lunar_distance.xls as mentioned in the attached manual [*], after usual corrections due to T and pressure. My data :
date = 3/20/2015 , 9h 2' GMT ; T=14° , p = 1021.4 mb ; elevation = 18. m ; sun low-limb ( sextant measurement ) = 38.° . GHA and declination from your nautical almanac . As the moon covered a large upper portion of the sun I assumed a lunar distance ( center to center ) of 1° .
date = 3/20/2015 , 9h 2' GMT ; T=14° , p = 1021.4 mb ; elevation = 18. m ; sun low-limb ( sextant measurement ) = 38.° . GHA and declination from your nautical almanac . As the moon covered a large upper portion of the sun I assumed a lunar distance ( center to center ) of 1° .
Since the excel wants far limb or near limb lunar distance I put in input , as near limb , 30' . Anyway I point out that "Easy lunars" ( in your web ) writes "This method is also prone to error when the lunar distance itself is below 15 degrees" .
Thanks a lot for your helping .
[*note from FER: the "attached manual" mentioned above was from Peter Hakel's navigation spreadsheets. Since it was irrelevant to this question and also carries a copyright notice, I have removed it]
