Equinox & Solar Daylight Simulator
Analyze how Earth's 23.44° axial tilt governs the terminator line, seasonal equinoxes, exact daylight duration, and solar insolation across every latitude.
Earth Axial Illumination & Terminator
Interactive 2D Cross-Section. Drag observer ring or tap any latitude.Equinox Science & Astronomical Mechanics
An equinox occurs twice each year when Earth's orbital plane aligns such that the planetary axis of rotation neither tilts toward nor away from the Sun. The subsolar point crosses the celestial equator directly at 0° latitude.
Why isn't day length exactly 12 hours everywhere on the equinox? Standard civil day length accounts for atmospheric refraction (bending solar rays upward by ~34 arcminutes) and the finite apparent size of the solar disc (~16 arcminutes semi-diameter). Thus, sunrise begins when the Sun's upper limb touches the horizon (zenith angle 90.833°), granting several extra minutes of daylight even at the equator.
Calculated Equations & Physical Constants
Calculated via standard celestial mechanics: sin(δ) = sin(23.44°) · sin(λ), where λ is the Sun's ecliptic longitude computed from Day of Year (DoY).
Computed from spherical trigonometry: cos(ω₀) = (sin(-0.833°) - sin(φ)·sin(δ)) / (cos(φ)·cos(δ)), yielding day length 2 · ω₀ / 15° hours.
Integrated daily flux above the atmosphere using solar constant I_sc = 1361 W/m² and Earth-Sun orbital eccentricity adjustment.