# NavList:

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

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Brown-Nassau slide rule for clearing lunars
From: Francis Upchurch
Date: 2015 Oct 28, 00:48 -0700

The BN was originally designed for rapid reduction of Hc and Az for aviators. I have found it very quick and easy,(under 2 minutes) with av. accuracy of about +/-5', but unfortunately with the occasional outlier of 10' or so. I guess ok for 1944 wartime pilots?

Rather by accident (or sub conscious design!), I have found I can do very simple and rapid lunar clearance.( again about 2 minutes.)

I've used Karl page 93 as an example, but will try it out with my own lunars in the next few weeks. For LOPs you start with dec, lat and LHA. But here we do not have RBA (similar to LHA on the nav triangle diagrams) we want that.

1) Find Relative bearing angle (RBA) or difference in Az.

dec use moon alt corrected for IC, dip + SD (hsd)

Lat, use sun alt likewise corrected (Hsd)

Instead of LHA (we want that) use observed lunar distance corrected for IC,+SD (LDsd)

Once you have these, it is just 1 movement of the rotor to set dec (hsd) .

Then very fine  red marker pen dot on the rotor at intersection of Lat (Hsd) on baseplate and LDsd on rotor.

Follow this down to baseplate gives "LHA" = RBA=Diff Az =64° 13'

2) Find corrected and cleared LD (LDo).

This time, the standard process (as per LOPs).But we now use fully corrected Ho and ho

Pen dot intersection LHA (=RBA) and Lat (Ho) on baseplate.

Move rotor to Dec (ho)

Follow dot down blue rotor , gives "Hc" of 37° 58'.

90° - 37° 58' =52° 2' = LDo which nearly spot on.

Once you have the starting numbers for Hsd, hsd, Ho and ho and LDsd, all of which you need for calculator or slide rule clearance, the whole process only involves 2 movements of rotor, 2 rep pen dot marks, no writing down of intermediate values etc.

I have found this quicker than calculator and this first attempt amazingly accurate. (using magnifiying glass + vernia calipers to improve accuracy)

I suspect with repeats, the accuracy may drop to +/ 5'. which is ok for LOPs but no good for useful  lunars.

If the BN had been available in the 18th century, perhaps a bigger version, say the size of a chart table, which would be ok for a ship, this could have made life easier for the poor navigator spending hours clearing by Maskelyne.

I may get round to building a large prototype, probably the size of an admiralty chart, probably 2 required  (each side fixed to a separate table) and see what sort of accuracy I get.In theory, if I can keep the precision up on the expanded scales etc, this should give very rapid and accuate LOPs and lunar clearance.

I still have not figured how to do this with the Bygrave, probably not possible with 2 intermediate values W and Y which you do not need with the BN

Watch this space. Long live the resuscitated BN! I think I have the only full sized working replica, the only original in the Smithsonian nobody has seen. They did not reply to several email enquiries so I bet they have lost it!

Oh well.

Francis

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