NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2014 Oct 11, 13:40 -0700
Frank,
Did whaler captains have optics on board ? If so then would the extintion altitude of Polaris be lower ? Are there log records for MP observations of Dubhe from south latitude ?
Greg Rudzinski
From: Frank Reed
Date: 2014 Oct 11, 11:31 -0700
The north star, Polaris, has an apparent magnitude of just about 2.0, not very bright. Mariners enjoyed watching for its visibility as they entered the northern hemisphere from the south and also observed its disappearance when they headed south. How close to the equator could it be seen? The science of extinction is fairly well-established. Given its apparent magnitude and the ICQ extinction tables, all we have to decide is how faint a star a navigator at sea under very dark skies might be able to see at the zenith. That gives us a solid theoretical answer. But what about historical observational evidence from logbooks? Do observations agree with the extinction models?
In the central Pacific between Tahiti and Hawaii in March of 1845, a journal from the ship Morrison records:
"the north star has been visible for two or three nights in succession - when first seen we were between two & three degrees North of the Line "
In the eastern Pacific, just after passing the Galapagos in 1882, in the book "A Landlubber's Log of his Voyage around Cape Horn" Morton MacMichael wrote in the evening of October 12:
"As I am finishing this the mate calls down that the north star is in sight."
He records the noon latitude on the previous day as 0° 52' N and on the following day it's 2° 43' N. Splitting the difference, they saw the north star near or maybe a dozen miles south of 2° north latitude.
There is another from the Charles W. Morgan c.1850 which I cannot find right now. They first saw the north star, heading northbound at 3° N latitude.
At considerably higher latitude, Joshua Slocum (first solo circum-navigator) first saw the north star when returning towards the USA in May 1898 at latitude 7° 13' N. He does not note whether he had clear weather on earlier dates.
Note that in the 19th century the north star was further from the celestial pole, so it traced out a small circle with variable altitude every night in the sky. All other things being equal (are they?), the northern limit of visibility ought to be about a degree further north today since that circle is correspondingly smaller.
Does anyone have any other examples of minimum northern latitude of visibility for the north star?
-FER
