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    Re: Star - Star Observations
    From: Frank Reed
    Date: 2010 Mar 13, 10:01 -0800

    Brad, you wrote:
    "A search through the archives provided this clearing method.

    Ah yes! A reference to one of my star-star posts from six long years ago.

    You asked:
    "Would this still be considered a good general case solution?"

    Oh sure. Definitely. Note that you can often make a good estimate of the "corner cosines" (that's a trademarked "Frank Reed" school term, by the way) just by eyeball. If the corner angles (the angles between the vertical and the star-star arc) are within 10 degrees of 0 or 180, then the corner cosines are very nearly unity (0.98 and -0.98 respectively) so the effect of refraction would be very close to the values for vertical alignment which are just the usual refraction corrections from the standard tables in the Nautical Almanac.

    Brad, you wrote:
    "The minimum power I use for this exercise is 10x."

    Sounds good. Greater magnification is the key to this procedure.

    And you wrote:
    "Perhaps this is backwards, but I calculate what the observed distance should be and then set the sextant to that distance."

    Also sounds good. Not only is that not "backwards", it's recommended --both today and historically. If you look at that book implementing Lord Ellenborough's method, you will find just that advice recommended. They picked it up presumably from earlier works on lunars which almost always recommended pre-setting to the correct distance. Contrary to what you might think, 19th century navigators did not know the stars very well. But lining up the right star for a lunar was usually relatively easy: preset the angle, aim at the Moon, and then swing until you see the star.

    And you wrote:
    "That is difficult enough, with the contorted positions and my bad eyesight"

    Re eyesight, do you wear glasses or contacts when you do this? You may want to try that. Also, as some of us discovered a few years ago, try these sights without dark adapting. Or, set the angles up nearly right, then turn on a flashlight for a few minutes to de-adapt. Do the final adjustment.

    Re contorted positions, yeah, that can be very uncomfortable and it's an important reason for preferring sights when the objects are in the same vertical circle as in Ellenborough's old method.

    Brad, of the Skyscout's data, you wrote:
    "The star data, again provided in right ascension, has been converted to arc and then into SHA. I have spot checked star data against the Nautical Almanac day page and have seen consistently good results."

    Just out of curiosity, could you post some sample RAs and Decs for today from the Skyscout? Five or ten first magnitude stars would be enough to get a sense of the position quality.

    You concluded:
    "However, using the Skyscout has made this a fun exercise. Point at object one, get RA and declination. Enter into spreadsheet. Point at object two, get RA and declination. Enter into spreadsheet. Read observed distance from spreadsheet, set the sextant to that distance and perform all kinds of contortions in a hysterical attempt to observe the bodies pass through one another. Twist and bend like a crazy maniac!! All the tabular lookups are removed, none of the long reductions which are prone to human error are present. I just point and shoot."

    Yes, it does sound fun. This connects with another thread. Somewhere along here in the past week the issue of space sextants has come up. As I am sure you know, there was a sextant built into the hull of the Apollo spacecraft. This instrument was more like an electronic theodolite, but it did have many features in common with a sextant. While not used for position navigation directly (except in practice, and they did practice on the way to the Moon), the angles from the sextant were used to correct the drift of the inertial navigation system. The astronaut would ask the system to point at a known star. The computer would drive the sextant to given angular coordinates relative to the spacecraft axes. Then the astronaut would look through the eyepiece and see the star more or less in the center of the field of view. He would then adjust the sextant for perfect alignment on the crosshairs, typically a couple of minutes of arc from where the computer had pointed. Then he would press a button, and the angular coordinates relative to the spacecraft axes would be reported electronically back to the computer (or entered by hand). He would repeat this for one or two other stars. It was "point and shoot" just like your sights. After the sights were taken, the computer would then report back the estimated error of the orientation of the inertial system and it would ALSO report back the star-star distance error. There's no refraction in space, of course, so this was simply a check on the quality of the observations. Typically the observed angular distances were within half a minute of arc of those in the computer's memory. If there was an unacceptably large discrepancy, it probably meant that the astronaut had sighted on the wrong star.

    Funny story: on Apollo 10, they lost the sextant's eyepiece for a while. Apparently the designers forgot about zero-g and didn't include any restraining system for the eyepieces. The sextant eyepiece floated off and ended up behind one of the astronauts' sleeping bags. By the way, they also had a telescope, distinct from the sextant, with a magnification of 1.0.


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