View Full Version : daytime 7 min longer than nightime on the day of vernal equinox.why?
2006-Mar-27, 09:13 PM
On the day of vernal equinox March 20 2006 in my location N.35,20 E 25.08
the sunrise occured at 06.24 and the sunset at 18.31.
Why this difference of 7 minutes between day and nightime ?
Does not equinox mean equal daytime and night time?
I would appreciate your help
2006-Mar-27, 09:22 PM
In part the answer is that we measure sunrise by when the first edge of the Sun tips over the horizon, and sunset when the trailing edge sinks below the horizon. This is a theoretical horizon. So, in essence, this gives the Sun an extra half a degree on the daytime side of the journey. This doesn't take into account the bending of the light from the Sun making the daylight even longer than listed.
2006-Mar-27, 09:25 PM
Also, the exact moment of the equinox was not in the middle of any particular calendar day. I'm not sure when the equinox occured for you at 25E, but I would expect there to be websites where one could figure this out.
2006-Mar-27, 09:26 PM
The sunlight refracts through the atmosphere, so the sun is actually below the horizon at sunrise and sunset. Also, the sun is not a point, and these times are measured from the edge of the sun being visible and not the center.
From the US Naval Observatory (http://aa.usno.navy.mil/faq/docs/equinoxes.html): The times of sunrise and sunset in almanacs are calculated for the normal atmospheric refraction of 34 minutes of arc and a semidiameter of 16 minutes of arc for the disk. Therefore, at the tabulated time the geometric center of the Sun is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level region.
2006-Mar-27, 09:54 PM
Thank you all for the explanation and the references
The final word:
2006-Mar-28, 11:34 AM
The final word:
deja vu... :)
2006-Mar-29, 05:47 PM
deja vu... :)That'd be ToSeeked, Bob :)
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