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Accuracy of sun position

Ronald

September 23, 2017 09:25

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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? 

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7 comments

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  Christopher K Erickson September 23, 2017 18:51

  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 September 23, 2017 19:08

  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 September 23, 2017 21:15

  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 September 24, 2017 08:45

  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 September 24, 2017 15:56

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

   

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  Ronald September 24, 2017 18:46

  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 September 24, 2017 19:38

  Except at high elevations...