# NavList:

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

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Re: Floating mirror
Date: 2015 Apr 20, 15:00 -0400

Paul & Alex
That is true only if you don't rotate the mirror and average

Assume the mirror has a pitch forward of 5°, but only in that angle, side to side is perfect.

The first reading will be off in one direction.

Now rotate the mirror 180°.
The second reading will be off in the other direction.

But when the two readings are averaged, the error is nulled.

That is the whole trick

On Apr 20, 2015 2:52 PM, "Paul Dolkas" <NoReply_Dolkas@fer3.com> wrote:

The main problem with all these floating mirror schemes is that there’s no guarantee that the reflecting surface is going to be parallel to the surface of the water. In theory it is, if the mirror is perfectly made, which means it almost assuredly is not. Any variation in thickness,  density, etc. will produce a slight list to one side, and unless you have a CG adjustment gizmo attached - or are very lucky - I’d be surprised if it works.* Remember – it needs to be level within a hundredth of a degree or so.

That’s the beauty of liquid mirrors – they are always perfectly level.

Paul Dolkas

*That being said, at \$3 per mirror, I’d be very happy to be surprised.

From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Dave Walden
Sent: Monday, April 20, 2015 6:12 AM
To: paul{at}dolkas.net
Subject: [NavList] Floating mirror

A \$3 plastic camping mirror from REI.  Floats in a bucket of water.  (backing may have issues after a while)

Not suprisingly, clouds all day, so no testing.

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