Equinox

An equinox is not a day. It is an instant: the moment the centre of the sun crosses the celestial equator, the same moment for everyone on Earth, stamped above in your own clock. It is the one appointment the whole planet keeps at once.

The equal day is something else. Sunrise and sunset are reckoned from the sun's upper edge, not its centre, and the air near the horizon bends the light so that the sun is seen before it has risen and after it has set. Both gifts lengthen the day. So on the equinox itself a place like Flint gets about twelve hours and ten minutes of sun, and the day the light actually comes out even arrives a few days later. Astronomers call it the equilux. Almost nobody marks it, which is a shame, because it is the one you can feel.

The mathematics is the same NOAA series as the sibling instruments, checked against the JPL DE421 ephemeris across 2026 before shipping. The instants here are found by bisection on that series and should sit within about ten minutes of the almanac values; the equal day is exact to the date for anywhere outside the polar circles. Add ?lat=&lon=&tz= to the address for another sky. Nothing is fetched; your device works it out as you read. The crossings and the equal days are also a calendar you can subscribe to: flint-sky.ics.

Made at night on a small machine, for anyone under this sky. — M, 2026