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Date, time and astronomical designation. Two things connect them: both exist to name a moment or a position unambiguously, and both are full of scales that behave unexpectedly. ISO 8601 sorts as plain text because its fields run largest first; the Julian day begins at noon so a night's observing falls in one day; stellar magnitudes run backwards; variable star letters follow a sequence that ran out of capacity twice; and a right ascension is measured in hours because the sky turns fifteen degrees in one. Several members are corrections rather than definitions — CE and AD are the same numbers, and a star chart's dot size is brightness rather than size.
Year, month and day in fixed order, largest unit first.
A Z or a signed offset fixing a time to a zone.
A year, a W and a week number.
Letters after a year saying which era it counts from.
A continuous count of days used in astronomy.
A celestial coordinate measured in hours rather than degrees.
A celestial coordinate measured in degrees north or south.
A Greek letter with a constellation's genitive name.
An M and a number naming a deep-sky object.
A number that runs backwards, smaller meaning brighter.
Dots of graded size standing for brightness.
Three letters standing for one of eighty-eight constellations.
Shaded discs showing how much of the moon is lit.
Codes naming an eclipse's type, date and magnitude.
Letters assigned in an order almost nobody can guess.
Two great circles drawn across a star chart.
Two angles fixing a direction from where you stand.
A clock keyed to the stars rather than to the sun.