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

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From: Paul Hirose
Date: 2020 Nov 1, 14:05 -0800

```The ′ gauge mark on scale ST coincides with the number of minutes per
radian (ca. 3440) on scale C. Thus, it helps you convert between minutes
of arc and radians. And since the measure of a small angle in radians is
also its sine and tangent at slide rule accuracy, you can get trig
functions and their inverses.

For example, Moon horizontal parallax is 59′. What is its distance?
(Assume Earth radius = 6400 km.)

1. Set cursor to 59 on D.
2. Set the ′ mark to cursor.
3. Set cursor to 6400 km on D.
4. Read 373 000 km at cursor on C.

Step 2 puts the C index at sin 59′ on D (about .01715). Step 3
calculates 6400 km / sin 59′.

--
Paul Hirose
http://sofajpl.com/
```
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