Model scope and corrections — read first

Seasons are driven mainly by Earth's axial tilt relative to the orbital-plane normal, which changes solar angle and day length; Earth–Sun distance still has a smaller effect on incoming radiation. Dates such as 3/21, 6/22, 9/23, and 12/22 are approximate equinox/solstice markers and vary by year. At an equinox the subsolar point is on the equator, with nearly equal day and night in an ideal geometric model; refraction and the Sun's apparent disk mean observed daylight need not be exactly 12 hours. The sinusoidal solar-declination formula using day of year (doy) is an educational approximation, not a precise ephemeris.

← Back to contents

Geographic Time · Module 02

Earth Revolution

Earth's revolution · Obliquity · Equinoxes and solstices · Causes of the seasons

Key concepts

Orbital date

March equinox 3/21 doy 80

Obliquity (explore a counterintuitive scenario)

Axial tilt 23.4°

Drag to 0°: seasons disappear; the subsolar point remains on the equator
Drag to 45°: extreme seasonal contrasts; the tropics expand

Animation controls

Orbital speed 20×

View presets

Visualization layers

Concept explanations

geo · revolution · seasons

Earth's axis / Subsolar parallel

Equator

Circle of illumination

Orbital plane

Left-drag to rotate · Scroll to zoom · Right-drag to pan

Live data

Date—

Obliquity23.4°

Subsolar latitude+0.0°

Sun–Earth distance (approx.)1.000 AU

Orbital speed (approx.)29.8 km/s

Equinox and solstice comparison (Northern Hemisphere)

March equinox 3/21

δ = 0°
Circle of illumination passes through both poles
Day and night are equal worldwide

June solstice 6/22

δ = +23.4°
Polar day in the Arctic
Longest daylight in the Northern Hemisphere

September equinox 9/23

δ = 0°
Circle of illumination passes through both poles
Day and night are equal worldwide

December solstice 12/22

δ = −23.4°
Polar day in Antarctica
Shortest daylight in the Northern Hemisphere

Subsolar latitude δ (year-round)