Anatomy Light

Why the north has winter and summer

Two independent instruments, built to answer one question. They were drawn separately and they disagree about nothing: the north has winter because the Earth is tilted, not because it is far away.

What it explains

The tempting answer is distance, and it is the wrong one.The Earth is actually closer to the Sun in January — in the middle of the northern winter — and the whole variation over a year is about 3%. Distance moves the wrong way and not far enough.

What changes is the tilt.The axis leans 23.44° and keeps pointing at the same star while the planet goes round. For half the orbit the northern hemisphere leans toward the Sun and for the other half away from it. That does two things at once: it changes the angle the rays strike the ground at, and it changes how long the Sun stays up. A high Sun lays the same light on less ground; a low Sun spreads it over more.

And one fact settles it: the north's summer is the south's winter.Follow both curves and they cross exactly on the equinoxes, twice a year, and never touch again. Distance cannot do that. Nothing about how far away the Sun is can make June and December fall in opposite seasons on opposite sides of the planet.

40° N, measured at the groundsun 73.4° up at midsummer and 26.6° at midwinter — 16.6° off vertical against 63.4°. The patch of light stretches more than three times as long, and any one square metre of it receives 501 W/m² in June and 152 in December.

One knob, the whole yeardrag either figure, or use the arrow keys. The year runs on its own when you leave it alone, and freezes for anyone who asks their system for reduced motion.

Open one

MiniMax version · darkThe goniometer benchA fixed-radius rail proves distance cannot be the cause; a two-pen recorder traces the northern and southern years on one panel. MiniMax version · lightThe goniometer benchThe same two figures on warm paper, for daylight. Mimo version · darkThe seasonsAn orrery: the Earth on its fixed axis, a month ring for the orbit, and a gauge reading noon Sun height and daylight hours. Mimo version · lightThe seasonsThe same orrery on warm paper, for daylight.

How it is made

The object is the explanation.A goniometer bench, a chart recorder, an orrery — real machines with real parts. No boxes with arrows and no floating labels: you read the mechanism off the object itself.
One model drives every number and every motion.The module that computes the physics is the same one that moves the parts. The two channels cut into the recorder's panel are not drawn by hand — they are the model's own curves, so a pen cannot mark anything the physics has not already said.
Four tones, correct painter's order, one accent.Flat shading from four greys, drawn back to front so nothing crosses wrongly, with a single lit accent per figure: the beam in one, the northern channel in the other.
Nothing floats and nothing sinks.Every solid reaches the stand through real face-to-face contacts. A geometry audit runs at build time — clearance, cable terminals, support, depth order — and the build fails if it finds anything. A second gate proves every tick and graduation lands on an edge rather than in mid air.
Calm, and usable.Eased motion, no bounce. Pointer drag and keyboard on both figures, a live readout in words and numbers, reduced-motion honoured, and the whole thing pauses when it scrolls out of sight. Every page ships dark and light from the same build.

Both pages were built with the Anatomy skill — an open skill for building explanatory figures as real objects, in isometric, with the physics in the drawing rather than beside it.