So we know that humanity has a pretty unified understanding of time at this point. There's been various calendars throughout time but the general trend is using the sun, moon, and stars because we can see them and they're regularly recurring. Of course one day is the rising and the setting of the sun, that's when you can see. A subdivision is months for convenience but also because it roughly lines up with moon phases and when certain constellations are in the sky. (Speaking really generally here.)
Eridians, though, cannot see their star for a few reasons: one, they're BLIND, two their atmosphere is too dense for light to penetrate it (hence the blindness). We know they have a clock and roughly day periods but it's unclear WHY the days are the length they are because they are MUCH shorter than human days, don't seem to line up with eating or sleeping schedules (Like a "Day" could've been the average time between sleeping or eating). Heck, we don't even know if their "days" line up AT ALL with our concept of a day and that might just be the word Grace and Rocky settled on (or just Grace) to define some period shorter than a year. I think its reasonable to conclude that if you've made it to space you did at some point figure out how long it takes for one's planet to finish an orbit around their sun AND figure out how to get to a different star. Therefore the human concept of a "Year" is probably defined similarly but may not have the same significance due to possibly not having that awareness previously and depending on how recently that was discovered PLUS how many people care. So Eridian years may not be based on solar revolutions. Which could be fun to play around with.
Any form of light based clocks are obviously out and based on general consensus plus canon evidence fire based clocks are also unlikely.
So a few possible options for time division are thus:
-Hunting seasons or the rest/active periods of prey.
-A full body cycle of some kind. How long it takes to digest something, how long it takes a worker cell to reproduce, the cycle of some other important function like ovulation, the half life of some important mineral etc.
-On that note it could also be the half life of an important mineral NOT in the body as well. Perhaps one of religious/superstitious relevance.
-Perhaps Erid is very seismically active and has regular quakes/volcanic eruptions(of low to high severity) that can be tracked and predicted with regularity. Maybe it's a specific region that's convenient to refer back to for political/social/traditional reasons.
- Or Erid could have predictable weather patterns like unto El Niño or La Niña which would probably be VERY apparent with an atmospheric pressure of 49x Earth. Or some version of Earth's water cycle.
-Magnetic poles as well! We know Eridians can sense their magnetic sphere. Earth's poles switch around every now and again (really cool stuff but takes AGES) what if Erid's poles swap around MUCH more frequently than Earth's for geological reasons that would be beyond my expertise to speculate about but probably something to do with also having a liquid iron (or other conductive material??) core.
-Speaking of tradition, could be divided like that arbitrarily due to old myths and traditions. Perhaps there are certain holidays like independence days or unification days. Birthdays of important historical figures or discoveries.
-One I think is quite likely and fun would be the pendulum clock using the harmonic frequency similar to how Grace figures out the gravity but used as a time piece instead like how we did historically.
-Same idea for water(or other convenient liquid) clocks or some equivalent to the hourglass where a certain amount of some solid substance is put in an hourglass esc contraption with a fixed volume and rate of falling due to gravity and friction.
Something of a flaw with a fair number of these is the possible universality of them. It took humans a long time to unify our measurements and when they stopped/started and a lot of it was synching up with the astral bodies (sun moon stars) which then begs the question of how Eridians perceive the usage of time. Is Eridian time is perceived the same across the planet? Is it only used for matters of science or other matters where precision and timing would be important to have measured and comparable results? Is it regional like Time Zones but perhaps less organized where you just gotta adapt to the local time when you travel city to city? Would historically close cities that traded/interacted often have linked time? How do they ensure they stay linked? Local geological activity??? Many many interesting questions.
