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For the sake of clarity, I mean a fantasy world rather than a science fictional one. It has a moon, a sun and stars, though not stars as we would know of them. Rather, saints who have ascended into the heavens and glow there, which has happened now and again over the course of thousands of years.

Edited to provide more info:

  • I would like years to exist, for narrative reasons, so readers (and I) have some context and things seem less abstract
  • Day and night still happens
  • No seasons
  • When you say a year do you mean a period of roughly 525,600 minutes, or do you mean one orbit of your fictional planet around its fictional sun? – sphennings Dec 18 '23 at 20:07
  • Well, given that Earth has only begun to revolve around the Sun in the 16th century, I would say that there are some three millennia of precedent of how to measure a year on a world which does not revolve. Quiz: how did the ancient Babylonian astronomers measure a year? The Earth was not revolving and it was not rotating in those days, and yet they were perfectly able to measure the duration of a year with great accuracy. – AlexP Dec 18 '23 at 20:51
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    what do its sun and moon do? do they have cycles? What is the purpose of your year measurement? – John Dec 18 '23 at 22:00
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    You're basically talking about Minecraft. That has days, but does not have years, since all the days are exactly the same. Years are a consequence of there being day-to-day differences that occur in cycles. Perhaps use something else that takes more than a day. – Monty Wild Dec 19 '23 at 00:14

2 Answers2

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Given the fantasy setting, I'm going to assume that you can hand-wave all of the physics involved in this, e.g. how and why anything feels a gravitational pull.

Unless it's permanently daytime/nighttime depending where you are on the disc, the sun and moon must move around in some fashion. Assuming they form a day/night cycle (even if that does not approximate Earth's) then it's possible to measure the passing of days, and after that it's really up to you to decide what a year is or whether there is any reason to define a 'year' at all.

On Earth a year basically measures a seasonal cycle, but there is no reason to assume that regular seasons exist on a magical flat disc planet. The passing of a 'year' may well be a social construct with entirely arbitrary rules: 200 days, or 100π days, or whatever you like. If seasons exist then: how long are they? That's the answer.

K. Morgan
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    Yeah, there is no reason they would have a concept of 'year' at all. They might have something based on agricultural cycles or similar, but not a year in our terms. – Kilisi Dec 18 '23 at 20:39
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Your disc probably has day and night cycles. If the sun always follows the same path over and over, then seasons don't exist, and therefore neither do solar years. If the moon is also unchanging then there are no lunar years either. If so, years will be completely arbitrary and based on whatever people had on their minds when coming up with stuff, just like the imperial system.

Some people might count years in groups of a hundred days, because a hundred were the sneezes that saint Incognito let put before he died. But some people will count the year as 314.16 days, because the world is a disc and the number π is holy. Also leap days in this latter system are a mess.

If the sun's path does tilt one way or another, ciclically, you have seasons. A solar year will be completed when the sun is done.

Similarly, if the moon has phases or a tilting path, you have lunar years.

You could also get some inspiration from the Discworld, but in that case the disc does revolve. On top of that most people living on the disc hate astrologers and astrologers hate the gods due to the inherent chaos that is the motion of their stars.

The Square-Cube Law
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