# NavList:

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

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Re: Cosine formula to solve a right spherical triangle?
From: Fred Hebard
Date: 2020 Jan 22, 11:35 -0500
I have a vaguely related question.

If you are computing the great circle distance between two points, why is the point where your latitude line meets their longitude line not a right angle?  I have difficulty envisioning this.

Fred Hebard

On Jan 21, 2020, at 16:16, Alexandre Eremenko <NoReply_Eremenko@fer3.com> wrote:

```Cosine formula has one drawback: when you want to determine an angle or a side from its cosine,
and this angle or side is small, the error is magnified.

The reason is that when the angle is close to zero, cosine varies very little, so small errors in cosine
can lead to big errors in the angle.

Alex.
________________________________________
Sent: Tuesday, January 21, 2020 1:01 PM
To: eremenko---.edu
Subject: [NavList] Cosine formula to solve a right spherical triangle?

Can Cosine rule be used to solve a right angled triangle . this is with reference to Napiers Analogies, and is it possible to use Cosine formula instead of the Napiers rules .

Thanks

Rajeev

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