< Calendar

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I think a lot of misconceptions about calendars arise from the idea that a calendar measures time, like a clock or like a ruler measures distance.

All known calendars work by naming days (This is the definition in the Nupedia article). As such, a calendar can not measure time more accurately than 1 day. Furthermore, the duration of the day is known to vary in time owing to change in Earth's rotation rate, so is not a good standard in time measurement.

A solar calendar may measure years, by virtue of the fact that its year runs close to the cycle of seasons, but more accurate measurement can be got from an ephemeris.

The year even when measured in SI seconds, varies in duration and so can not be considered a unit of the same time that measured by SI seconds.

That's not a misconception at all--a calendar does indeed measure time just like a clock does, just not very precisely. That doesn't mean it's not a measurement. A measurement simply answers the question "how much"; one can answer precisely or vaguely. If you ask "How much sand in this bucket?", One can measure in "handfuls" just as easily as kilograms, and one is performing essentially the same function. Likewise, "How much time since I was born?" can be answered roughly in years with the help of a calendar, and in fact is a much more useful measurement for most purposes than an exact measurement in seconds: I'm over 1.2 billion seconds old; quick-- can I go into a bar? Run for president? --Lee Daniel Crocker

For more discussion on Calendars and Time measurement, you may join the E-mail list CALNDR-L at

This terminology from the existing article seems less than ideal. "A pragmatic calendar is one that is based on observation; an example is the religious Islamic calendar."

How about calling it "observation based calendar" and get rid of the pragmatic terminology.

"A theoretical calendar is one that is based on a strict set of rules; an example is the Jewish calendar. Such a calendar is also referred to a rule-based or arithmetical calendar."

I thought a "theoretical calendar" was one proposed bu never used.

Let's just move away from those two term and go with Rule based and observation based.

-Paul Hill

I've never seen the term pragmatic calendar used to mean observation-based calendar outside the Nupedia article in which it appears. It may be the author's invention. The terms astronomical and observation-based have been used elsewhere and I have added them to the article.

The same applies to the addition of arithmetical and rule-based for theoretical calendars.

Karl Palmen