NavList:

John,

Columbus and Dias would have been very secretive of their respective discoveries and techniques. Nation states at the time would have been interested in imperialistic/military benefit and privately sponsored exploration would have been very protective of trade routes and fishing grounds. I wonder how this is factored into analysis of old logs and charts available to the public at that time.

Greg Rudzinski

[NavList] Re: I couldn't resist!
From: John H
Date: 19 Oct 2012 12:26
All Polaris or eclipse, according to Morison. I'm attaching a few pages I put together summarizing his measurements. Again, this is all from Morison's biography, I just pulled it all together.

What's strange about Polaris is that he has to do a 4 degree correction to get latitude depending on where it was in the sky (e.g. relative position to Cassiopeia and the Big Dipper).

As far as I can tell he only used the Regiomontanus information for eclipses and did the other sightings on Polaris because he memorized this 4 degree trick with Polaris.

Bartholomew Dias, on the other hand, didn't have the luxury of this Polaris trick, and I think he used a sun-shot for the latitude of the Cape of Good Hope (and was off by nearly 10 degrees, too).

On Fri, Oct 19, 2012 at 11:53 AM, Brad Morris <bradley.r.morris---com> wrote:
No noon sunshots? Okay, there's the end of my wild a**ed guess (wag).

I had assumed he did, based upon my memory of the following items. (1) The ephemeris compiled by Regiomontanus included tables of solar declination (2) That the ephemeris was of the period (3) That Columbus had instruments capable of the measurement of the sun's altitude at meridian crossing.

Odd to think that he did not put these items together to sail a latitude line.

Regards
Brad

On Oct 19, 2012 10:33 AM, "Apache Runner" <apacherunner---com> wrote:
I don't have all of Samuel Eliot Morison's bio of Columbus memorized, but I can try to summarize from memory.

Western celestial nav was still in it's birthing stages when Columbus made his voyages. I forget the precise year, but Don Joao II sent an astrologer on a Portuguese vessel down the coast of Africa in 1480. He took a sighting of the sun and was off in latitude by about 7 degrees or so.

Bartholomew Dias took a shot at the Cape of Good Hope and sighted 42 degrees when it should've been 33 or so.

Columbus knew some of these shots, and took an ephemeris with him, quadrant and astrolabe. On his first voyage, he took a shot of Polaris, which required a 4 degree correction back then. In Cuba he got a latitude of 42 degrees (i.e. Boston's latitude). Morison speculates that he misidentified Polaris. Later in Haiti, he got a better shot, but was still pretty far off. On his return, he tried a shot on approaching the Azores, but the wind affected his instruments too much.

On later voyages he did somewhat better. On his 4th, he got a pretty good value of latitude for Saint Anne's Bay in Jamaica. He also did a longitude measurement by using Regiomontanus' tables for an eclipse, but was off by a huge amount.

On his 3rd voyage, he took three shots of Polaris while in the doldrums, but one of them was way off on one of them and concluded that there was a large bump in the ocean.

So, very much experimental - he wasn't really using the stars to navigate, but he was playing around with it.

Most of the navigation was dead reckoning using magnetic compass and log line. Only later in the 1500's, however, did people seriously try to be more systematic in correcting for magnetic variation.

On Thu, Oct 18, 2012 at 8:24 PM, Brad Morris <bradley.r.morris---com> wrote:
(Okay,no list comments on the fact that Columbus didn't practice celestial,I only learned that later)

Didn't he cross the ocean on a latitude line? Most likely by using the noon altitude of the sun? That's celestial!

On Oct 18, 2012 7:20 PM, "Lu Abel" <luabel---com> wrote:

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