Hi Michael, i just checked SkySafari Plus and the reported sunset time given in the Sun's Object Info window matches the time in the sky chart when the Sun's upper limb goes below the horizon line. Make sure you set the Horizon to "Transparent with line" to see this clearly.
This is the actual shot I took on Feb 25, 2020 at 7:32pm.
I was expecting the lower limb to touch the horizon at 7:29pm as simulated by SkySafari but it was not.
I have checked my clock on the spot to make sure it's not off.
I'm yet to have another chance to confirm the discrepancy, as the evening weather are always bad. Such clear sunset only happen about 10 days a year :(
This discrepancy is probably explained by refraction. A long time ago I learned that the Sun has set some time before we see it, because of the bending of the light in the atmosphere, called refraction. The exact degree of refraction is impossible to calculate because it depends local weather conditions.
I found an article on the subject here: http://www.icoproject.org/pdf/saifee_2106.pdf The value of the refraction is very dependent on observer height above the sea. In order to observe a delay of 2-3 minutes you would probably have to be about 100m above the sea.
Steenkh is correct. I didn't realize you took an actual photo :) There are a number of factors that can explain the discrepancy.
Hi Steenkh, the photo was indeed taken from 327m above sea level.
Thanks for the explanation.
