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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Venus transit June 8 and sextants
From: Frank Reed CT
Date: 2004 May 10, 23:47 EDT
From: Frank Reed CT
Date: 2004 May 10, 23:47 EDT
Jim T wrote:
"At very least it is a way of testing our ability to view Venus' disk (1/30 the size of the sun's on that day) with our sextants' telescopes, for example. That's all I plan to do. Do you have other suggestions?"
You could try to measure the semidiameter of Venus. It's big enough for a good sextant. The technique would be the same as for the Sun (discussed recently on this list).
And wrote:
"I plan to view the transit using the 6x30 telescope on my sextant, with the appropriate horizon and index mirror shades. That shouldn't be a problem, should it? Even if I watch for long periods of time? The shades are in front of the mirrors and telescope, so they are not exposed to concentrated rays of sunlight as they would be if the observer makes the terrible mistake of putting a shade between his eye and the aperture of a telescope. The shade could heat up and shatter if it is placed between eye and aperture, suddenly flashing concentrated light into the observer's eye with terrible consequences."
Ouch!
When I want to look at the Sun through a sextant --maybe to see sunspots, for example-- I usually set the arm to the Sun's approximate altitude and then hold the instrument in the usual orientation for altitude sights. This is convenient ergonomically (no neck strain) and it also means that you're not looking towards the Sun so there's less glare. But there's nothing more dangerous here than the limb-to-limb procedure for measuring index error, so if you want to set the instrument to zero, swing in the horizon shades, and aim it up into the sky, there's no particular problem. We know sextant shades are safe for that purpose. The transit's in progress for eastern Canada, so there's probably little practical difference for you.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"At very least it is a way of testing our ability to view Venus' disk (1/30 the size of the sun's on that day) with our sextants' telescopes, for example. That's all I plan to do. Do you have other suggestions?"
You could try to measure the semidiameter of Venus. It's big enough for a good sextant. The technique would be the same as for the Sun (discussed recently on this list).
And wrote:
"I plan to view the transit using the 6x30 telescope on my sextant, with the appropriate horizon and index mirror shades. That shouldn't be a problem, should it? Even if I watch for long periods of time? The shades are in front of the mirrors and telescope, so they are not exposed to concentrated rays of sunlight as they would be if the observer makes the terrible mistake of putting a shade between his eye and the aperture of a telescope. The shade could heat up and shatter if it is placed between eye and aperture, suddenly flashing concentrated light into the observer's eye with terrible consequences."
Ouch!
When I want to look at the Sun through a sextant --maybe to see sunspots, for example-- I usually set the arm to the Sun's approximate altitude and then hold the instrument in the usual orientation for altitude sights. This is convenient ergonomically (no neck strain) and it also means that you're not looking towards the Sun so there's less glare. But there's nothing more dangerous here than the limb-to-limb procedure for measuring index error, so if you want to set the instrument to zero, swing in the horizon shades, and aim it up into the sky, there's no particular problem. We know sextant shades are safe for that purpose. The transit's in progress for eastern Canada, so there's probably little practical difference for you.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
