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Q&A

Must calendars be based on solar systems?

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Must calendars be based on solar systems (Must calendars be "relational")?

Is it plausible to desire a "universal" calendar applicable everywhere in our universe?

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I'm not sure this is really a question about physics - it seems more like a cultural question. (2 comments)

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It depends on what you want from your calendar. If you simply want a way to keep track of time, then you can base it on anything you like. The earth spinning on its axis, the moon orbiting the earth, and the earth orbiting the sun are irrelevant.

However, most calendar systems were developed to help synchronize human activity here on earth to the seasons. Those seasons repeat in a cycle synchronized with the earth orbiting the sun. For this reason, the concept of a year was ultimately important in early calendar systems. For example, when it had been winter for a while and then got warmer, you wanted to know whether it's really spring and time to plant crops, or was it just a fluke temporary warmup and winter wasn't over yet. A year-synched calendar can answer that question.

Note that ultimately keeping track of where you are within a year doesn't mean you have to measure that directly. Years aren't so easy to measure accurately by lay people. Days and lunar cycles are. You can have a time-measuring system that is based on days and/or lunar cycles, that ultimately advances you thru the yearly calendar.

Unfortunately, there aren't integer days or lunar cycles in a year, so you have to fudge something occasionally. The usual way is to add or drop whole cycles of whatever you are measuring so that you stay synchronous to the yearly cycle in the long term. This is the whole point of our modern leap years. If I remember right, the jewish calendar is based on measuring lunar cycles, and uses leap months to stay in sync.

Nowadays, we have defined the second as the standard time measure, independent of earth's rotation (which changes over time). A minute is defined as 60 seconds, an hour as 60 minutes, and a day as 24 hours. This is very close to the earth's rotation time, but not exactly. We add or drop a second at the end of the year as needed to re-sync to the earth's actual rotation.

If you don't care about easily knowing where you are within earth's seasons, then you don't need to base your calendar on the earth's orbit around the sun. A colony on Mars, for example, might use a different calendar for every day use.

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No.

A calendar or, more generally, a time measurement system, can be based on anything. While human calendars have (generally) been based on:

  • Day = One cycle of the Earth's rotation
  • Month = One cycle of the moon's revolution around the Earth
  • Year = One cycle of the Earth's revolution around the sun

those are not universal (e.g., some calendars use 12 lunar months and "slide" ~ 11 days per year; most use months that are longer (except February) than lunar months; etc.).

As with most other measurements (length, weight, etc.) the calendar has been redefined to be based on physical principles that are independent of the Earth, sun and moon. The second is defined as:

the time duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental unperturbed ground-state of the caesium-133 atom.

and everything else flows from there - e.g., 86,400 seconds in day, 365 days in a standard year, etc.

In practice, due to the affected population all (except ISS, and they're orbiting Earth) residing on Earth and the physical aspects of the solar system therefore still relevant, there is an adjustment system of leap seconds to keep our calendar synchronized with the solar system (or at least with the sun). But it is all arbitrary. A Mars colony might choose to make a calendar based on the average local day and year, but they would likely have to coordinate with an Earth-centric calendar as well. Long-term deep space missions might stick with an Earth-centric calendar even though the original basis - Earth, moon, sun - would not be relevant at all. In the end - it is all just math.

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I dont think anyone can make a universal calendar because time flows more slowly or more fast between different regions in the universe or it can even go backwards if you come close to a rotating black hole.

And a planck unit of time is not the smallest amount of time ,the planck time is defined as the planck length/c and the planck length is a scale at which quantum gravitational effects become important.

So again since the rate of flow of time isnt universal we create a calendar based on the measurement of the clock of 1 second at the surface of Earth.

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