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    Re: Pointer star question
    From: Frank Reed
    Date: 2013 Feb 3, 12:42 -0800

    Sean, you wrote:
    "Speaking of Polaris and Stellarium: I was recently using said program to explore Polaris' apparent position over tens of thousands of years."

    I don't know how much you can trust any software like this for very long periods of time. Most depictions of the long-term motion of the celestial sphere include ONLY precession and not the other long-term changes in the Earth's orbit and especially its inclination. These precession-only simulations are valid for a few thousand years, but they are only rough approximations when you get to the 10,000 year range.

    You added:
    "I concluded that all of us are very fortunate to have been born during the current time period because it was not so long ago (relatively) that Polaris was much farther away from the celestial pole. For example, in 1492 Polaris was almost three and a half degrees away."

    Very true! As it fades into history, celestial navigation just keeps getting easier and easier. ;)

    You wrote:
    "I would focus on the star, enable the angle measurement tool, left-click and drag a small distance, move the view down and right-click the horizon, completing the angle measurement. Zoom out to see the displayed angle and voila! One simulated sight."

    Nice. At some point soon, it should be possible to do something like this using the motion and orientation sensors in smartphones. The whole sight-taking process including swinging the arc, even the pitching and rolling of a ship at sea, could be simulated without the annoying inconveniences of the real world. Oh the horror... ;)

    You also wrote:
    "I must say the only other program which offers a similar capability is Ilan Papini's 'Virtual Sailor', and that program has some very serios known flaws with its model that the designer has no plans to fix. >:("

    Yes, I've played with that. It's fun but more like a video game than a virtual simulation.

    And:
    "If you like Stellarium, you might want to check out another free program called "Space Engine". It's like Stellarium, except you can travel to any point in the Universe and see what the view would be like from that spot. It was an awesome sight to be able to zoom into the middle of the Pleiades and see how the stars are positioned relative to each other in full 3D! Really an awesome program!"

    Yes (also try "Celestia"). I'm surprised Stellarium doesn't have this feature since it's almost required in planetarium programming today. You can at least go to any planet, moon, or minor planet within the Solar System in Stellarium and see how the sky looks from there. These views can be very accurate since distances within the Solar System are tightly constrained by dynamical laws. It's very different outside the Solar System, and that view from within the Pleiades that you're describing is smoke and mirrors. The distances to the individual stars of the Pleiades can only be roughly estimated. The uncertainty in the individual distances of the stars, even after careful refinement and re-analysis of the Hipparcos data, is about 2% and it could be more. That's 2% of some 400 lightyears or +/-8 ly, but the cluster's core diameter is about 5 lightyears so the positions of stars within the cluster along the line of sight are dominated by residual observational noise. For more distant clusters, the uncertainty can be ten times the estimated diameter of the cluster. Simulations often deal with this by forcing the cluster members into a roughly spherical distribution at the right distance from the Earth for the cluster as a whole with RANDOM relative distances within the sphere. The only star cluster for which the 3D view is considered genuine is the Hyades.

    -FER


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