About
This is a growing collection of illustrated essays about how the sky is worked out. Each one takes a single idea and draws it until the argument is visible.
What this is not
It is not a textbook, not a reference and not a stargazing guide. There is no syllabus, no problem sets, no obligation to cover anything in particular, and no catalogue of objects. An essay exists because there is a figure that explains something better than a paragraph can — and if there is no such figure, there is no essay.
The figures are computed, not drawn
Every illustration is generated as SVG at build time by code in this repository. None are photographs, none are exported from a drawing program, and none are bitmaps. The consequence that matters is not that they are sharp or small, though they are both:
- They are correct by construction. An orbit drawn at eccentricity 0.6 really is a conic with that eccentricity, with the primary at a focus and not at the centre. A body sweeping equal areas is stepped through Kepler's second law rather than spaced to look convincing.
- They are parameterised. The same generator draws the orbit at any eccentricity, the parallax at any distance, the transfer between any two radii, and the essay picks.
- They respond to the light and dark themes, because they reference colour roles rather than colours. Gold always means light; indigo always means something moving.
The full set of generators is listed on the figure library page.
Nothing here can be visited
Astronomy is a science of inference at a distance. Almost every number in it was obtained from an angle, a brightness, or the timing of something repeating — never by picking the object up. This site tries to keep that visible: where a quantity is measured, the essay says what was actually observed and what had to be assumed to get from the observation to the number. The distance ladder is not a detail of technique, it is the reason the numbers exist.
Where the model stops
Celestial mechanics is a stack of approximations, each excellent inside a range: two bodies and no others, point masses, no relativity, circular and coplanar orbits. Every one of them fails somewhere interesting, and the failures are where the subject got its best results — Neptune came out of a residual, and so did the first evidence that Newton was not quite right.
On being wrong
Corrections are welcome and will be made. A figure that is beautiful and wrong is worse than no figure, because it is more persuasive.