The Mudcat Café TM
Thread #123689   Message #4120597
Posted By: Steve Shaw
22-Sep-21 - 07:35 AM
Thread Name: BS: Equinox Greetings
Subject: RE: BS: Equinox Greetings
It's a bit of a conundrum that, on the day on which the equinox occurs, most places on Earth get more than 12 hours' sunlight. The extra minutes get more as you move closer to the poles, as much as 15 minutes or more extra in the Arctic, for example. It's to do with the fact that, at sunrise and sunset, the sun's light is diffracted (bent) by the atmosphere, meaning that we actually see the sun "above the horizon" a little while before astronomical sunrise and a little while after sunset. The lower the angle at which the sun rises or sets, which is what happens the further from the equator you go, the greater the effect. At the equator, the sun on this day plunges vertically down below the horizon at sunset, so there's little or no discernible effect. Here in Cornwall, at 50 degrees north, today's sun reaches a maximum of just 40 degrees above the horizon, and at sunset it will slide gently down at a low angle, making the diffraction effect last longer. Etcetera!

It's another lovely day here, by the way. The shirt is off, the shorts are rolled up and I'll be cutting the grass this afternoon.