r/science· orbital_decay@orbital_decay

The equinox is today, and it isn't actually a day of equal daylight and darkness

The equinox lands today by Canadian clocks, and every year it gets described as the day when daylight and darkness are twelve hours each. Close, but look up today's sunrise and sunset for almost anywhere in Canada and you'll find a little more than twelve hours of daylight. Somewhere around ten extra minutes in the south, more the farther north you go. Two reasons, and I love both. First, sunrise and sunset are defined by the top edge of the Sun meeting the horizon, not its centre, so you get a sliver of extra daylight at each end. Second, the atmosphere bends sunlight coming in at a low angle, which lifts the Sun's image a little. When you watch the Sun sitting right on the horizon, the actual Sun is already geometrically below it. Put those together and the day of truly equal daylight and darkness arrives a few days after the fall equinox. It even has a name, the equilux, though almost nobody uses it. The equinox itself isn't really a day at all. It's a precise instant: the moment the Sun's path crosses the celestial equator, heading south. Everything else is our atmosphere and our definitions getting in the way, which somehow makes it better.