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Re: ? ? ? Sextant Scope Par allelism ? ? ?? (was Re : SNO-T Sextant)
From: Bruce Stark
Date: 2004 Aug 12, 14:06 EDT
From: Bruce Stark
Date: 2004 Aug 12, 14:06 EDT
Frank,
Here's another way of looking at the problem.
The sextant is designed to measure an arc of a great circle. That circle is in a plane perpendicular to the axis of the instrument. The axis is a supposed line drawn lengthwise through the center of the pivot behind the index mirror.
Your eye and two stars form another plane. As long as this second plane is perpendicular to the axis of the sextant, the sextant can accurately measure the great circle distance between the stars. If it is not perpendicular to the axis the angle will measure too great.
This is a serious problem only with large angles. You might experiment by measuring the distance between two stars 110 or 120 degrees apart. Move them back and forth across the field of view and see what happens.
Bruce
Here's another way of looking at the problem.
The sextant is designed to measure an arc of a great circle. That circle is in a plane perpendicular to the axis of the instrument. The axis is a supposed line drawn lengthwise through the center of the pivot behind the index mirror.
Your eye and two stars form another plane. As long as this second plane is perpendicular to the axis of the sextant, the sextant can accurately measure the great circle distance between the stars. If it is not perpendicular to the axis the angle will measure too great.
This is a serious problem only with large angles. You might experiment by measuring the distance between two stars 110 or 120 degrees apart. Move them back and forth across the field of view and see what happens.
Bruce
