Spring Equinox

From South Dublin Astronomical Society

The Spring Equinox 2006 will be 20 March at 18h 26m, when the Sun will cross the equator going Northwards. From then on, we are told, days will be longer than nights, in the N Hemisphere.

But even on 19 March the Sun will be above our horizon for more than 12 hours! Tables of sunrise and sunset usually give sunrise as the moment when the top of the Sun's disc first appears above the horizon, and sunset as when the top of the disc disappears below the horizon. But even if one takes the rising and setting of the centre of the Sun's disc, it still appears above our horizon for over 12h 2 minutes on 19 March! How come? Because of the refraction effects of the Earth's atmosphere. When we see the centre of the Sun on the horizon, it has actually already 'set', but the atmosphere bends its light just enough for us to still see it.

In fact, under most conditions, when the bottom of the Sun's disc appears to touch the horizon, the whole of the Sun has actually set!

It's the refraction effect which often makes the Sun's disc appear noticeably flattened when it's near the horizon.

So the 'observed' Spring equinox actually occurs a few days before the theoretical date. And of course the observed Autumn Equinox occurs a few days late for the same reason.

--Terry Mosley