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An apsis is one of two extreme points in an orbit around something.

  • The furthest point is the apoapsis or apocentre. The minimum orbital speed occurs here.
  • The closest point is the periapsis or pericentre. The maximum orbital speed occurs here.

The plural is apsides.2 English has a lot of -apsis words, which refer to distances in the orbit around some body, though most of them aren’t commonly used. These include:

apoapsis periapsis orbiting
apogee perigee the Earth
aphelion perihelion the Sun
apocynthion pericynthion the Moon
apolune perilune the Moon
apohermion perihermion Mercury
apocythe pericythe Venus
apoareion periareion Mars
apojove perijove Jupiter
apochron perichron Saturn
apodemeter peridemeter Ceres
apastron periastron any star other than the Sun
apogalacticon perigalacticon the centre of a galaxy

Attention Conservation Notice

…we will begin each Viridian Note with a useful set of its key concepts. With some practice, we hope that you will be able to reject a Viridian Note, confidently and without a pang, within two or three seconds.

This effort, however, may not be enough. You may still find yourself painfully tempted to actually read the Note. We therefore offer a backup safety system, our unique “Attention Conservation Notice.” This will begin each Note by explaining to you, in some brief detail, why you should NOT read it.

This has never been done before in print-based publishing, but in the text-glutted electronic context, we feel this practice makes a lot of sense…


  • TLA+ Helped Me Count to Six (Computer Things #86, Hillel Wayne, Oct 2020)

    The term for this is subitizing, which is the worst-sounding word I learned all year. Subitization is the process of exactly determining the number of objects in a collection without having to explicitly count them. I looked for resources on improving my subitization but all the resources are for teaching children. Children can barely subitize two, and then gradually get better until they can subitize around four, where it stabilizes. If I want to improve my adult subitization then I’m on my own.

    This isn’t the first time I’ve tried to learn a weird mental skill. I call them “knacks” for some reason. Not complete mental disciplines, not party tricks, just small things that come often. I’ve been fascinated with them ever since I learned about compass belts.

  • How to Memorize a Larger Multiplication Table (Computer Things #86, Hillel Wayne, May 2022)

Programming dictionary

Describes a procedure which can be repeated without changing the initial result or causing unintended side effects. Without this property, the caller may need to track whether the procedure has been run.

Semantic line breaks

A name for the practice of putting line breaks between clauses in markup. Buckminster Fuller called it ventilated prose. (Found via Gwern.net.)

I encourage students to treat the files as private “source code” that they are free to format semantically. Instead of fussing with the lines of each paragraph so that they all end near the right margin, they can add linefeeds anywhere that there is a break between ideas.

The reason I am writing this is that I find Ventilated Prose a valuable aid to composition of text intended for presentation in conventional, unventilated format. In this essay I first present an example of Ventilated Prose as an aid to reading and then describe my use of it in composition.

Style Guide for online hypertext

  1. Due to this regular transformation by T. nutricula, it is thought to have an indefinite lifespan. 

  2. Pronounced /ˈæpsɪdiːz/