NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bringing the body down
From: Gary LaPook
Date: 2017 Oct 28, 17:29 +0000
From: Bill Morris <NoReply_Morris@fer3.com>
To: garylapook@pacbell.net
Sent: Friday, October 27, 2017 11:40 PM
Subject: [NavList] Bringing the body down
From: Gary LaPook
Date: 2017 Oct 28, 17:29 +0000
Or, simply precompute the sight and pre-set the sextant. Get the almanac data, just for the nearest hour, and hit HO 249 or calculator with even number of degrees, don;t need the minutes, and the resulting Hc should be close enough.
gl
From: Bill Morris <NoReply_Morris@fer3.com>
To: garylapook@pacbell.net
Sent: Friday, October 27, 2017 11:40 PM
Subject: [NavList] Bringing the body down
I was hoping that someone more experienced than I at celestial navigation at sea might reply to Frank's trenchant criticism of a perfectly good and useful technique, but no one has, so I will offer my relatively inexperienced point of view.
But first <<you can use a sextant just fine that has no horizon filters at all>>. I hope Frank simply forgot to qualify this statement. My own experience with sea horizons is that the glare on the sea beneath the sun makes the use of a horizon shade necessary in full sunshine. Tha is why two or three are provided in most sextants.
Bringing the sun down to the horizon in the manner that Frank disparages is not necessary on land or at sea on a stable platform using a sextant that has a telescope with a decent field of view through the mirrors (the Davis Mk3 sighting tube has a large field of view but tiny mirrors and index shades). However, in a rolling, pitching vessel it is a useful technique, as it is easier to find the sun by pointing the instrument at it than to find it by pointing the sextant at the horizon beneath the sun and trying to find the sun as the horizon bobs in and out of view. It is a particularly useful technique for twilight star shots, as the body is first seen in company with its neighbours if one is lucky. A useful addition to the technique in heavy weather is to remove the telescope, place it in one's pocket and bring the body down with the naked eye, when the telescope can be replaced if desired and the horizon swept to bring the body into view. Incidentally, Captain Cooke took most of his shots on his first voyage of exploration without a telescope.
Bringing the sun down is a technique that is not difficult to acquire. Like any technique it needs a little practice, but once acquired, it is a useful addition to one's observing armementarium, less necessary on land than at sea. ...peddling (sic) this pointlessly tricky (sic) methodology to beginners.. by showoffs...? Perhaps not. Maybe on day two, while Frank shows off his makeshift kamal...?
By the way, getting an accidental blast of the sun while observing with a sextant does not result in immediate eye damage. Rather, one blinks and shifts the sextant a little to the left to get the view of the sun where it belongs.
Bill Morris
Pukenui
New Zealand
