Celestial mechanics - spring equinox v spring equilux
today is the spring equinox (Friday, March 20, 2026), but the equilux - the day of equal light and dark - actually passed a few days ago for most of the Northern Hemisphere
"Equinox" means "equal night” and occurs at the moment the Sun is directly above the equator (which only happens twice a year during the Earth’s orbit around the Sun) but the actual 12-hour day happens earlier in the spring (and later in autumn):
atmospheric refraction makes the Earth's atmosphere act like a lens, bending sunlight so that we see the Sun "rising" before it actually crosses the horizon, and we see it "setting" after it has actually dipped below the horizon
the Sun is a disk, not a point, so sunrise and sunset are measured by the top edge of the Sun, but the equinox is measured from the Sun's centre, which adds a few extra minutes of light onto either end of the day
For most mid-latitude locations (like the UK and parts of North America), the equilux occurred around March 17 or 18 - so by today's equinox we already have roughly 8 to 10 minutes more daylight than darkness