It's the Spring Equinox — But Are Day and Night Really Equal?
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It's the Spring Equinox — But Are Day and Night Really Equal?
Solar Term No. 4: The quietly misleading science behind the equinox
Today, March 20, is the Spring Equinox — ์ถ๋ถ (Chunbun) in Korean, the fourth of the 24 traditional solar terms. You've probably heard it described as "the day when day and night are equal." That's not quite right. In Seoul today, daylight runs roughly 8 to 16 minutes longer than the night.
The reason comes down to the size of the sun itself. Sunrise is defined as the moment the sun's leading edge first peeks above the horizon — not when its center crosses. Sunset, meanwhile, doesn't happen until the sun's trailing edge fully disappears. Since the sun is a disc rather than a point of light, those extra minutes of visibility add up on both ends of the day. On top of that, Earth's atmosphere bends sunlight, making the sun appear above the horizon even when it's technically still below it. Both effects push daylight a little longer than midnight math would suggest.
So when are day and night actually equal? Typically 3 to 4 days before the equinox — around March 16 or 17. It already passed. The "equal day and night" description is really an astronomical shorthand for the moment the sun's center aligns with the celestial equator. What we experience on the ground is a little different.
The word equinox itself tells the story: it comes from the Latin aequus (equal) and nox (night). In Korean, ์ถ๋ถ (ๆฅๅ) means "the division of spring." Historically, the equinox carried real weight — the Joseon royal court held a formal ceremony on this day, and ordinary families gathered to make naittteok, little rice cakes each person ate one by one for every year of their age. In Iran, the spring equinox is still celebrated as Nowruz, the Persian New Year. In Japan, it's a national holiday.
One more thread worth pulling: the date of Easter in the Catholic calendar is anchored to this very day. Easter falls on the first Sunday after the first full moon following the spring equinox — which means the equinox has been quietly setting the rhythm of religious calendars for over a millennium. If the evening sky looks a little brighter on your walk home today, it's not your imagination.
๐ก Today's Takeaway
"Equal day and night" is only half true — daylight is already longer than darkness today, and the day they were actually equal slipped by a few days ago without anyone marking it.
Sources & References
- Source ↗Namu Wiki — ์ถ๋ถ (Spring Equinox)
- Source ↗Wikipedia (KO) — ์ถ๋ถ
- Source ↗Korea.net — Chunbun Cultural Overview
- Source ↗Korea Astronomy and Space Science Institute — Equinox Day Length FAQ(URL unverified)
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