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Q: Equinox ( Answered ,   1 Comment )
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 Subject: Equinox Category: Science > Astronomy Asked by: gregorydove-ga List Price: \$20.00 Posted: 23 Jul 2006 05:05 PDT Expires: 22 Aug 2006 05:05 PDT Question ID: 748707
 Take the spring equinox (spring in the Northern Hemisphere, of course); it is usually on the 21st of March. I used to think that this was the day when day and night were equal [from the derivation of 'equi' and 'nox' (for night)]. But I now know that on the 21st of March, as given by sun-rise and sun-set tables, the day is something like 25 minutes longer than the night. [London sunrise 21 March 2005 was 6:00 a.m. but sunset was 6:13 p.m.] I've been offered two explanations for this. 1. that sunrise is the leading edge of the sun coming up over the horizen and sunset is the trailing edge of the sun dropping below the horizon, and that if you measured the time from the diameter of the sun at each end of the day the time would be close to equal. 2. that the light of the sun is bent by going through the atmosphere at sunrise and sunset and so one is actually seeing the sun before it actually would be above the horizon by measure of a straight line. How much do each of these factors effect the timing of sunrise and sunset at the equinox? I've seen some explanations of this (NY Times Science column a few years ago) where #2 is the only factor mentioned, and others where only factor #1 is mentioned. Is one the predominant factor and the other only a second-order effect? Clarification of Question by gregorydove-ga on 31 Jul 2006 11:40 PDT I the two answers given you exactly pinpoint my question but not the answer. First of all, the discrepency is not 'a few minutes': the day is 26 minutes longer than the night! And my question remains, which of these two factors accounts for the lion's share of the discrepancy. My 'gut-feel' is that the disc vs. point of light would account for 90 to 95% of the discrepancy, but I have someone betting with me that each factor [disc. vs. point and bending of the light] account for about 50% each. So I'm looking for the definitive answer.