Seasons: Why Tilt Matters
Close to ready: the day-night boundary is fixed and the page works on a phone. Still to come: announcing results to screen readers, and the direction Earth is drawn orbiting.
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Predict
Predict
If Earth’s axis were not tilted, what would happen to seasons?
Play
Play
- Compare tilt = $0^\circ$ vs tilt = $23.5^\circ$ and note what changes.
- Change date and observe day length changes between hemispheres.
- Relate sunlight angle to ‘how concentrated’ the energy is.
Explain
Explain
Use day length and sun angle to explain how tilt produces seasonal temperature changes.
Learning goals
- Explain why axial tilt causes seasons.
- Relate sun angle/day length to heating.
- Distinguish tilt-driven seasons from distance myths.
Misconceptions targeted
- Seasons are caused by Earth being closer/farther from the Sun.
Model notes
- Declination uses a simplified geometry (toy model): $\delta = \sin^{-1}(\sin\varepsilon\,\sin L)$, where $\varepsilon$ is axial tilt and $L$ is treated as uniform in time (about $1^\circ$ accuracy vs ephemeris).
- Day length uses a standard sunrise/sunset hour-angle relation; polar day/night appear naturally when geometry demands it.
- Earth–Sun distance uses a first-order eccentric model (not a Kepler solver): $r \approx 1 - e\cos\theta$; perihelion is anchored near day 3 (Jan 3) with an uncertainty of about $\pm 2$ days.
- Key idea: opposite hemispheres have opposite seasons; distance variations are small and not the main cause.
About this demo
This demo lets you change day-of-year, axial tilt, and latitude to explore how sun angle and day length change through the year in each hemisphere.