# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Bubble Sextant as Dip Meter
From: Gary LaPook
Date: 2015 Oct 23, 23:11 +0000
Well, yes, you can use a bubble sextant to determine the dip of the horizon as influenced by refraction but you have to be standing on dry land. If you could use the bubble sextant to determine the dip while on shipboard then you could just use the bubble sextant to take the celestial observations. The problem is with the accelerations on a ship that make the bubble dance around so much that it has a large uncertainty in any reading taken on a ship (unless beset by ice.) One of the ways you check the index error on a bubble sextant is to stand on the beach with your feet in the water and measure the altitude of the horizon, it should be minus 2 minutes so the index error  is the difference between the reading and minus 2 minutes. This, of course, assumes normal refraction and your not being a midget or a giant. IF you already know the index error and you are standing with your feet wet, or you know accurately your height of eye, then you could determine the actual dip as affected by the existing refraction  refraction.

gl

From: Brad Morris <NoReply_Morris@fer3.com>
To: garylapook---.net
Sent: Friday, October 23, 2015 2:17 PM
Subject: [NavList] Bubble Sextant as Dip Meter

I was reading HO551, Manual of Ice Seamanship.
After a discussion of abnormal refraction and the care a navigator should take in regards to it, the following interesting statement is made
"If the motion of the ship permits use of a bubble sextant, it can quickly be determined whether the apparent horizon is appreciably displaced"
The manual gives no further information.
Am I to infer that a bubble sextant can be used as a dip meter.  Or is it merely a gross check, as in 'the horizon is far from normal'.
Has anyone tried this?  Can a bubble sextant be used as a dip meter?
Brad

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