<<Could this be that in theory the sun sets due west on the equinox (as it would if the earth had no atmosphere) but in reality is sets slightly south of due west due to refraction, and would this explain the time delay? This has really bothered me, because I want my dials to be correct.(my dials are corrected for longitude and I corected the observatory times with the EOT)
Please comment on this somebody!>> _______________ Hi John, Hmm, not sure I know what I am talking about - but when did that stop me? :-) It really is quite complicated and I don't pretend to be an Astronomer but astronomical sunset (that given in almanacks) is when the sun's upper limb is on the true horizon of an observer at sea level. So, assuming you are at sea level, in addition to refraction you have to consider the effect of the sun's semi-diameter and data calculated from an Almanack will need this correction to be made before you know the time of sunset of the 'true sun. Of course if the data given is that for sunset and sunrise then maybe it has already been taken into account. My 1997 copy of 'Whitaker's Almanack' seems to indicate that the combined effect of refraction and sun semi-diameter is (always) an increase in the hour angle at rising and setting. At a Latitude of 52 degs the combined corrections in 1997 for various declinations of the sun were: Decl Mins correction 0 5 10 6 20 7 However I suspect the biggest cause of 'error' is the fact that you are not at sea level. At a height of some 100 feet above sea level (say) the angle of depression of the perceived horizon is some 5 minutes (of arc) and this must delay sunset a bit too. There will be another small adjustment because the equinox is a point in time on one day and in general the sun will not actually be rising or setting just at that time. That error will probably be less than a minute though... I guess (but am still not entirely sure!) that all these will contribute to the differences you see. However, in reading your e-mail again maybe these shift things the other way!! perhaps we have some astronomers reading this list who can comment!! Patrick
