Astronomy

Why the planets go round the way they do: where they are and how far apart, the shape of an orbit and the speed along it, why the outer planets crawl, resonance, the seasons, the backward loops of Mars, and what counts as a planet at all. The module the rest of the Solar System content hangs off. Complete: nine chapters of nine.

How the planets differ, and why.

What a moon is, where one can exist, and where one cannot.

What a star is, what powers it, and why its mass decides everything that happens to it.

What a black hole is, what it is not, and why the two largest misconceptions about them are both consequences of one scaling law. Complete: seven chapters of seven — `bh-shadow` closes the story with the Schwarzschild visualiser, D2's fragment shader over an offline geodesic-integration lookup table.

What the expansion is, what it is not, and what four independent measurements say about the matter nobody can see. Complete: all eight chapters.

How planets around other stars are found, what each method is blind to, and why the known population says more about our instruments than about the galaxy. Complete: all seven chapters.

Seventy years of spaceflight on one axis, with each date drawn as precisely as it is actually known. Who was first at what, why the failures are here in the same detail as the successes, and how to tell a first that still stands from one that has been taken. Complete: all six chapters.

Why a 2.4-metre mirror above the air beats a 10-metre one under it, how Hubble's was ground two micrometres wrong by an instrument nobody doubted, why JWST's much larger mirror produces the coarser picture at the wavelengths it was built for, and how the colour pictures are made — from grey ones, by people, out of light no eye can see. All five chapters. The colour chapter works on a model nebula through the real Hubble and JWST filters rather than on the published frames, which are not cleared for this platform, and the widget says so on its face.

Where the speed came from, where the Voyagers actually are, why crossing the heliopause is not leaving the solar system, and why nothing on the distance tracker is a stored number. All five of the ledger's chapters, including the gravity-assist chapter, which watches one flyby from two frames and finds that a slingshot creates no energy at all.

Why Mars is the neighbour worth the trouble, why a launch to it cannot be rescheduled by a fortnight, where the rovers actually drove, what it costs a mission team to live on a day that is 39 minutes too long, and what is sitting in tubes at Jezero waiting for a ride home. All five of the ledger's chapters; the traverses are drawn from the missions' own localization records rather than over an orbital basemap, and the map says why.

Why the Moon was the destination, what a free-return trajectory is and how narrow the corridor of them is, why Apollo 11 and Apollo 8 both left theirs, why Apollo 13 did not have to find a way home, and what changed in April 2026. Complete: all five chapters. The descent to the surface is described from the mission archive rather than modelled — widget 54 stops at the translunar trajectory, and the chapter says so.

What it means to orbit something, read off a tracker rather than asserted: the Earth still pulls with 88% of its surface gravity where the station is, the ground track slides 23.6° west every ninety-three minutes, and a visible pass needs the station in sunlight while you are in the dark. Complete: all five chapters.

Why reaching orbit is a speed problem wearing an altitude problem's clothes. The Tsiolkovsky equation with the payload drawn to scale, the structural ceiling that makes staging an impossibility argument rather than an efficiency one, and a launch you can fly straight up to watch it land back on its own pad. Complete: all five chapters.

Our own galaxy from inside and as a model from outside, why it is a band across the sky, how galaxies are classified and what the classes mean, the rotation problem, the Local Group and beyond, and what happens when galaxies collide. Complete: all six chapters.

The four kinds of nebula and the wreck of an exploded star, why some glow and others only reflect, what colour a nebula really is, and why a magnitude 3.4 galaxy can be invisible while a magnitude 8.8 nebula is not. Complete: all four chapters.

The night sky as something you can go outside and use: why it turns, why it changes through the year, why a constellation is a line of sight, how the pole star gives your latitude, why the pole star is temporary, and what light pollution takes away. Complete: all seven chapters.

From a slowly turning cloud of gas to a flat disc, the frost line that sorted rock from ice, the climb from dust to planets, and the asteroid belt that never became one. Complete: all five chapters.

How empty the asteroid belt really is and why Jupiter carved gaps in it, how a comet works and why its tail can go first, where comets come from, why meteor showers seem to come from one point, and what decides whether an impact digs a crater or bursts in the air. Complete: all six chapters.