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FIELD STATION · EARTH IN SPACE

Seasons & Sunlight Lab

Practice this: Seasons Studio
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CHOOSE YOUR INVESTIGATION LEVEL

Why does Earth have seasons?

Both levels test the same big idea with your own measurements. AP adds insolation by latitude, surface albedo and the ice–albedo feedback.

About this model

Orbit and axis. Earth's orbit has eccentricity 0.0167: about 147.1 million km at perihelion (early January) and 152.1 million km at aphelion (early July). The axis stays tilted 23.44° and points the same way in space (toward Polaris); precession is ignored. The space view is not to scale. The what-if tilt (0–90°) and eccentricity (0–0.4) sliders change the model, and every readout and graph is labelled “what-if” while they are not real Earth values.

Sun position. Solar declination and Earth–Sun distance follow the USNO approximate solar coordinates algorithm. Noon altitude = 90° − |latitude − declination|. Day length uses the sunrise equation cos H = −tan φ · tan δ for the Sun's centre with no refraction, so real days are a few minutes longer than shown. Times are local solar time.

Sunlight. Daily top-of-atmosphere insolation uses Berger (1978) with solar constant 1361 W m⁻². Sunlight on the ground uses a clear-sky model: air mass from Kasten & Young (1989) and the Meinel & Meinel (1976) beam formula 1361 × 0.7^(AM^0.678), plus about 10 % diffuse sky light (estimate). Clouds, haze and elevation are ignored.

Schoolyard instruments. The black plate's temperature above the air is 0.95 × sunlight ÷ 20 W m⁻² K⁻¹ (estimate for a light breeze). Air temperatures are monthly normals where we have them. The panel is a 0.1 m², 18 %-efficient module lying flat (estimate).

Heating bench. The plate absorbs lamp power × sin(angle) × (1 − albedo) and loses heat in proportion to how much warmer it is than the 22 °C room (a lumped heat model with tuned constants: 120 W effective lamp, 420 J/K plate, 3 W/K loss). The PV cell gives 2.4 W × sin(angle) (tuned). Probe noise is σ = 0.15 °C and meter noise 1 %, drawn from a seeded random generator: the same seed repeats the same readings. Lab 3's painted plates all have the same size and heat capacity; only the paint's albedo changes.

Albedo (AP). Fresh snow 0.85, desert sand 0.35, grass 0.25, forest canopy 0.15, asphalt 0.08, open ocean 0.06, sea ice 0.7: representative values from ranges in the literature, not fixed constants. The sea-ice patch is at 75° N on June 21 with clear skies.

Real data. Monthly mean temperatures are 1991–2020 normals: NOAA NCEI station USC00049367 (Visalia) and Dirección Meteorológica de Chile, Quinta Normal (Santiago). Temperature also depends on oceans, elevation, clouds and air masses, not on sunlight alone.

Earth images. Day, night-lights and cloud maps: Solar System Scope (based on NASA imagery), CC BY 4.0.

USNO solar coordinates · NASA Earth fact sheet · NASA Space Place: seasons · PVEducation: air mass · NOAA climate normals · Dirección Meteorológica de Chile · NASA: measuring Earth's albedo · Solar System Scope textures

What am I looking at?

How the lab works. Pick Explore or one lab at the top. Each lab has five tabs: Pre-lab, Bench, Data & graph, Claim · Evidence · Reasoning, and Test. On the Bench, the coach on the right lists your steps; they tick themselves as you work. Press What do I do now? when you're stuck.

① Earth in space. Left: Earth's orbit seen from above at an angle. Earth's axis (N at the top end) always points the same way, toward Polaris. Drag Earth, use the Date slider, or zoom (＋ / －, Z / X, scroll or pinch). Right: a close-up of Earth on that date with sunlight from the left. The red strip shows the noon sunlight at your place, and the ray count shows how many rays land on it compared with the spot where the Sun is overhead.

② Schoolyard. You are standing at the chosen place, facing the equator. The dashed curve is the Sun's path for the whole day; the 1 m stick casts a shadow away from the Sun. The black plate's thermometer, the solar panel's meter and the light meter all follow the Sun. Use Time of day, or press Play to run the day.

③ Heating bench. A heat lamp on an arm that keeps the same distance from the target. Change the angle (or, in Lab 3, the paint), run a 5-minute trial, and record the probe and meter readings.

Advanced (under the scene) has the what-if tilt and orbit shape, extra readouts and the instrument seed.

Keys: Space play / pause · → step · R reset · Z / X zoom (space view).