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    Re: Pointer star question
    From: Frank Reed
    Date: 2013 Feb 2, 14:50 -0800

    Alex, you wrote:
    "I discovered this when I tried to see the recent event of Jupiter passing the Moon. Stellarium gave a totally wrong picture. I suppose because it had wrong Moon diameter, but did not investigate this further."

    Stellarium definitely has issues. It's a collaborative open-source project, and that can be problematic. For stellar coordinates, it's quite reliable simply because that's the most basic functionality out there. On the other hand, the refraction model appeared to be flawed last time I checked. Another bug: the planet Mercury does not go through its phases properly in the version that I have (it's never a crescent).

    As for the size of the Moon, that may be a "feature" rather than a "bug" (this is only one possible explanation for what you saw). Stellarium is intended as display software for small planetariums, and it is, in fact, used in many planetariums. It is an interesting aspect of the planetarium illusion that the Moon really has to be displayed several times larger than its correct angular size. Stellarium includes an option in the settings page (I couldn't find it just now, but it's in there somewhere!) which allows you to display the Moon magnified in this fashion. Obviously this will foul up exact display of occultations involving the Moon (and this is true of EVERY planetarium that any of us have ever visited), but at least in Stellarium it is optional.

    On a more general level, Alex, I try to point out fun tools like Stellarium because many NavList members have no idea that such things are available --there are plenty of folks reading these messages who could be charitably described as "completely, totally clueless" about computers and similar technology :). So I count Stellarium as an "appetizer" for more sophisticated astronomical software. And it's great for all sorts of interesting experiments and demos. Here's a fun game you can play with Stellarium that has some navigational interest: Use the time setting control panel (from the left edge pop-out menu) to set the local clock time to 12:00:00 noon exactly at your location. Now turn on the altitude-azimuth coordinate grid (hit "Z" for a quick toggle of this grid). You may want to turn off the atmosphere ("A" toggles this). Look towards the South. You'll find the Sun near 180 degrees azimuth. Now jump ahead a day at a time and watch how the Sun's position at noon changes (keyboard shortcuts: "-" to decrement the date, "=" to increment). You can hold down the "=" key and the Sun will fly through the sky making a tall thin figure-eight path during the course of each year, the so-called "analemma" come to life. For your location, on what dates is it really "noon at noon", meaning the Sun is on the meridian at 12:00 o'clock standard time? Are there any such dates? For my location here in southern Rhode Island, the best date is coming up in a week. On "Sun Slow Day" the Sun is on the meridian near here right at noon by the clock.


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