| MadSci Network: Astronomy |
Hi,
A colleague remembers reading this question in a magazine or newspaper recently - probably the New Scientist or Guardian. I've had a look through the back numbers of New Scientist in the library, but can't find it; unfortunately we don't keep back copies of the Guardian.
So, here's my guess...
You seem to expect that the sunrise and sunset times will be the same, ie. that `daylight' lasts exactly 12 hours. Unfortunately the definition of sunset and sunrise aren't what you might expect(!) - check this information from the dominican observatory for an official definition which uses the upper limb of the sun.
Of course, you really want times that use the centre of the sun, rather than the upper limb. You could calculate the correction if you know the angular size of the sun (from memory it's half a degree across but you might want to check that!).
The best idea would probably have been to measure the times yourselves. Even then, when you know the exact definition of `sunset' and `sunrise' you may have to worry about diffraction through the atmosphere making the sun dwell on the horizon (and indeed, the tables in the link above include a correction for refraction).
One other point that may be relevant if you are wondering why the times are not symmetric (ie. at 6am and 6pm exactly) is that your local time zone isn't exactly aligned with astronomical time.
Your apparent equinox, from the figures in the paper, was earlier (ie. more into `winter') than expected. That is in the sense expected for sunrise/set times based on the upper edge of the sun (which overestimate the time for which the centre of the sun is visible). It is also the sense expected for diffraction , which, since the air column is greater nearer the horizon, tends to let you see `over' the horizon a few degrees. Maybe you could compare the magnitude of the two effects, given the correction for the edge/centre times, and check that with the variation in refractive index you would expect through the atmosphere (sounds like quite a tricky problem!).
I hope that helps. I've concentrated on the basic ideas rather than the numbers since I reckon you can walk to walk to the library as easily as I can! If I find out any more I'll pass it on by email - please tell me if you think of a different explanation. And feel free to ask for more information/explanation if I have been too brief above.