The Vernal Equinox and Spherical Trigonometry

The spring equinox occurs around March 20 when the sun crosses the celestial equator, giving equal day and night. Computing its exact date requires spherical trigonometry: the geometry of angles and distances on a sphere rather than a flat plane. The length of daylight follows D(t) = 12 + A × sin(2π(t − t₀)/365), where A depends on latitude. At the Arctic Circle, A = 12 (midnight sun in summer, no sun in winter). Ancient megalithic monuments like Stonehenge were aligned to the equinox — the first astronomical instruments used spherical geometry before it had a name.