# NavList:

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

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Re: Inverting Scope
From: Bruce Stark
Date: 2004 Oct 10, 11:44 EDT
Alex,

All I know about optics comes from having read an article on the selection of binoculars. And that was half a century ago. But I suspect your scope has been damaged. Perhaps, as Fred suggested, there's a microscope expert at your university who can repair it.

The wires in my scope don't show up well against a dark sky. But if the moon is in the picture they show clearly.

That you had no trouble learning to use the two types of scope surprises me. I found it very difficult. You must be right: The problem is in getting over old habits.

The purpose of the wires is to help you keep your line of sight parallel to the frame of the sextant. That is, perpendicular to the axis the index turns on. If the scope is parallel to the frame, and what you see is midway between the vertical wires, the line of sight will be parallel to the frame.

To see why this is important, imagine you are at the center of the earth, and the index arm of your sextant is pivoted on the north-south pole.

Bring the reflected image of one meridian to the direct image of another. For example, bring 120 degrees east to the prime meridian. Suppose there are visible spots on the meridians at every degree of latitude. Every spot on 120 east will overlap a spot at the same latitude on the prime meridian.

Measured from the axis of the earth, the angle between each set of spots is 120 degrees. But that's only if measured in a plane perpendicular to the axis. That is, in a plane parallel to the circles of latitude. Unfortunately you are stuck at the center of the earth, and only the pair of spots at the equator are a full 120 degrees apart from your perspective. Still, if you could see them in the mirrors, you'd measure the matched spots as 120 degrees apart all the way to the north or south pole.

Bruce

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