Equinox
What is an equinox?
An equinox is the moment the Sun crosses Earth’s equator, around March and September. Day and night are then close to equal in length across much of the world.
The one day the map is symmetric
Jump to an equinox on the day/night map and the shadow’s edge stands nearly vertical — every latitude cut in half. It is the only date that is true. The poles split the difference most strangely of all: there the Sun rolls along the horizon, taking days to fully rise or fully set. An equinox at the pole is not a day — it is the hinge between a six-month day and a six-month night.
“Equal” is approximate
Check your own sunrise table on the equinox and you will find slightly more than twelve hours of daylight. Two honest reasons: sunrise and sunset are counted from the Sun’s top edge rather than its centre, and the air bends the low Sun upward into view, stretching both ends of the day by a few minutes. The date when day and night truly measure equal — the equilux — falls a few days toward the winter side.
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Why the date wobbles between the 19th and the 21st
The equinox is an instant, not a date — the moment the overhead Sun crosses the equator. A real year of seasons is about a quarter of a day longer than 365 days, so that instant lands about six hours later each calendar year, then jumps back a day when the leap day is inserted. That is the same stray quarter-day the leap year exists to absorb, showing up as a wobbling date.