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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars -Venus!
From: Frank Reed CT
Date: 2004 Apr 23, 19:14 EDT
From: Frank Reed CT
Date: 2004 Apr 23, 19:14 EDT
Mike B wrote:
"Speaking of Lunars Venus is"in distance" at least it was Wed night! The Moon was virtually the " horizon" I.e. no odd sextant angles required! "
When the two objects are nearly in a vertical line like this, you can drop the "quadratic correction" from many of the lunars methods. In other words, clearing a lunar becomes identical to clearing a star-star sight.
Venus is rapidly approaching its June 8th transit of the Sun. That means it has a significant angular diameter (0.5 minutes of arc) and also a significant parallax (0.3 minutes of arc). The parallax correction isn't tough. You handle with it the same way you do in ordinary cel nav altitude sights. Dealing with the angular diameter is the tricky part. Normally with planets that have large angular diameters, like Jupiter, you shoot a lunar by carefully centering the small disk of the planet on the Moon's limb. Venus, however, has a large phase to go along with its size. Probably the safest bet would be to make a best guess at the location of the planet's true center in the visible image and center that on the Moon's limb, but this would require a sextant with a powerful telescope. Alternatively, you could bring the limb in contact and then subtract out the semidiameter. Both options are open to error. Some lunarians, including Letcher, recommend ignoring Venus when it comes to lunars because of this problem.
Getting ready for the transit...
You'll be seeing this quotable quotation again many times in the next six weeks:
William Harkness of the US Naval Observatory after the transit of Venus in 1882 wrote,
>>>> There will be no other till the twenty-first century of our era has dawned upon the earth, and the June flowers are blooming in 2004. When the last transit occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows. <<<<
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"Speaking of Lunars Venus is"in distance" at least it was Wed night! The Moon was virtually the " horizon" I.e. no odd sextant angles required! "
When the two objects are nearly in a vertical line like this, you can drop the "quadratic correction" from many of the lunars methods. In other words, clearing a lunar becomes identical to clearing a star-star sight.
Venus is rapidly approaching its June 8th transit of the Sun. That means it has a significant angular diameter (0.5 minutes of arc) and also a significant parallax (0.3 minutes of arc). The parallax correction isn't tough. You handle with it the same way you do in ordinary cel nav altitude sights. Dealing with the angular diameter is the tricky part. Normally with planets that have large angular diameters, like Jupiter, you shoot a lunar by carefully centering the small disk of the planet on the Moon's limb. Venus, however, has a large phase to go along with its size. Probably the safest bet would be to make a best guess at the location of the planet's true center in the visible image and center that on the Moon's limb, but this would require a sextant with a powerful telescope. Alternatively, you could bring the limb in contact and then subtract out the semidiameter. Both options are open to error. Some lunarians, including Letcher, recommend ignoring Venus when it comes to lunars because of this problem.
Getting ready for the transit...
You'll be seeing this quotable quotation again many times in the next six weeks:
William Harkness of the US Naval Observatory after the transit of Venus in 1882 wrote,
>>>> There will be no other till the twenty-first century of our era has dawned upon the earth, and the June flowers are blooming in 2004. When the last transit occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows. <<<<
Frank E. Reed
[ ] Mystic, Connecticut
[X] Chicago, Illinois
