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Accuracy of sun position

I was doing some calculations using exact sunset and sunrise times which I got from SkySafari Pro Android, by looking at the exact time the center of the sun passed the horizon (altitude 0). However, the times differed by some minutes from what I was expecting. I checked on my PC using Stellarium, and that gave me values more close to prediction. For example, for my location (51d27m0sN, 5d28m40sE), I find times using SkySafari of 7:25:57 and 19:34:27 for yesterday (22-9), while Stellarium gives me 7:29:42 and 19:30:51 for the same location. Given that it was the autumnal equinox yesterday, I expect a duration between sunrise and sunset close to 12 hours, but with the times from SkySafari, I find a duration of approximately 9 minutes more, which seems too much.

Does anybody know an explanation for these results, or if I might be doing something wrong? 

7 comments

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    Christopher K Erickson

    What is your altitude?

    When I am on top of Mauna Kea (13,800') I have noticed that SSP5 shows the sunset to be way earlier than the actual sunset.

     

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    Ronald

    I thought of that, but already ruled that out. I'm in the Netherlands, so just a  few meters above sea level ;-) 

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    Bill Tschumy

    Recall that due to refraction the Sun appears about 30' higher in the sky at the horizon than it really is.  Are you taking that into account?  You can disable this in the Precession settings if you want but then it won't match the actual sunset.  

    I'd be very surprised if SkySafari was incorrect.  I've compared it many times to other sources and it is always correct.

    Christopher, as I'm sure you are aware, if you are on a mountain top, your horizon will not be at 0º altitude.  If your horizon is at (say) -10º, you can correct for this in the Horizon & Sky settings (oops, you can't since we don't allow a negative horizon value.  I need to fix that some day).

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    Ronald

    Thanks! Disabling the refraction setting indeed did the trick. I know the effect of course, but didn't see it in the settings, but I didn't look in the precession menu. In Stellarium I had  the refraction setting disabled. 

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    Bill Tschumy

    You probably want refraction enabled in both apps.  It does reflect reality.

     

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    Ronald

    Yes, I know. But currently I'm doing some calculations on celestial mechanics, and don't want to bother with the atmosphere, so I'll keep it disabled for now.

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    Christopher K Erickson

    Except at high elevations...

     

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