In the world I'm creating, I have merfolk who have human-level intelligence, live in pods/tribes of 10-50 (but are connected to each other and trade and exchange information frequently). Most worship a goddess of the sea who created them, and some have magic (mostly power over tides and currents).

They would all use the same calendar, which needs several seasons, a few months, and maybe even a zodiac system. Time would be tracked based on the movements of ocean currents or something similar, or even an internal clock. They wouldn't look at the sky or go above the surface to keep track of time, though most live at a depth where they could see the sun filtering through the water, and therefore sunset.

I read a bit about seasons in the oceans, but I'm unsure how I would turn that into individual months. I have no idea how I would do underwater zodiacs, but I like the idea and would prefer to keep it if possible.

  • $\begingroup$ Whale or salmon migrations could be kept track of I suppose... not sure how reliable that would be though. Should be good-ish for keeping track of years. $\endgroup$
    – Lemming
    Nov 12, 2021 at 14:21
  • $\begingroup$ If you want help refining your questions (referring to the one you have self-deleted), you can use the Sandbox in meta. $\endgroup$
    – L.Dutch
    Nov 12, 2021 at 15:39
  • $\begingroup$ So reading though the posts, one question that needs answered is, how accurate you need the calendar to be. If you rely on environmental cycles. Tides tempters, fauna flora cycles, You cant be very accurate. The only way to get any true accuracy is some sort of astronomical observation. I would welcome being corrected however. $\endgroup$
    – Gillgamesh
    Nov 12, 2021 at 20:49

5 Answers 5


What a Calendar Does

First thing's first: calendars were originally made to measure time and therefore predict the cyclical nature of events in time. Most Earth calendars measure months (cyclic Moon phases) and seasons within the year; and they are used to predict important agricultural events like planting and harvesting.

A perusal of an almanac is instructive at this point!

What a Calendar Might Do

In order to apply this to your merfolk, we need to analyse the data you've given.

Merpeople themselves are intelligent and have religion, magic & trade. A calendar might be useful in determining festivals and fairs. So far so good.

Merpeople are aware of several natural phenomena: day/night, tides & currents, and storms, particularly hurricanes.

Merpeople are not aware of stars and lunar phases, though they may be aware of "really bright nights" when the full Moon is overhead and "really dark nights" when this crescents or the new Moon is overhead. They don't go to the surface, so don't observe these things.

Is a Calendar Practical?

Given what we know of your particular merfolk, I think it is safe to say that there are not a lot of assuredly & predictably cyclical events that they can observe.

Earth has four tides a day, making them useless for a calendar but if they had clocks, an almanac might be more useful. Hurricanes are seasonal, but not predictable in either place or time. A calendar might be useful to set polar points around the "height of hurricane season" and its nadir. In some regions, monsoons are more predictable, so calendars there might reflect monsoon variations. Storm avoidance is apparently very important for marine life, merfolk included, and being able to predict the large seasonal storms with some accuracy would be a good use for a calendar.

Day/night length cycles would be known to merfolk, so their calendar might reflect seasons of longer days and seasons of shorter days.

Moon phases, motions of stars and planets, terrestrial seasons would largely be unknowns to them and thus they'd have no zodiac, no Moon phases or any astronomical stuff in their calendar.

What Does it Amount to?

So I think your merfolk could have a calendar, but I think it would be rather different than a human calendar in the same region.

I think it would likely incorporate the Day/Night as a basic measure of time, and I think its principle focus would be seasonal storm tracking and thus something like a "year" might evolve as a secondary measure of time.

Months, fortnights, weeks and astronomical seasons I think would not be on their calendar. Longer cycles like cometary years and planetary cycles would be of no importance at all.

  • $\begingroup$ Wouldn't they want to predict higher/lower tides? $\endgroup$ Nov 14, 2021 at 9:46
  • $\begingroup$ @PaŭloEbermann -- They probably will. On Earth, we get four tides a day and the times vary by location. This is not what a calendar will be useful for -- most almanacs, at least in the US, have tide tables for coastal regions that will note when the tides are and how high they'll be. Probably merfolk won't care about how high the tides will be, but they will want to know when they happen so they won't be stranded on the beach as the waters recede. $\endgroup$
    – elemtilas
    Nov 15, 2021 at 0:07

The zodiac makes sense only if they look at the sky with systematic observation,and you say they don't look at the sky.

The closest periodic phenomena I can think of which can be perceived underwater is the tidal flow, if their planet has a significant moon: it changes with the relative position of sun and moon, paired with the duration of the day and the night light cast by the moon it would make a decent basis for a periodicity.


So, one earth we have two high tides with low tides in between roughly every day. They track with the moon, so as the moon goes through its phases it affects the tides. Thus the tides are only roughly twice a day, but precess once a lunar month, adding or removing a tide cycle depending on the moon in your world.

Multiple moons would have multiple surges each tracking one moon. The sun also affects the tides but little enough that they only strengthen the tides at new and full moons and weaken them at first the last quarter.

Throughout the year, the obvious affect is that the polar caps freeze more in the winter. The north pole freezes around December and the south pole freezes around June.

Then there are El Niño/La Niña cycles at the equator, alternating between El Niño and La Niña every six months. However, climate conditions change the intensity of these and whether they happen at all.

Winds have an impact on water currents. The tilting of the planet's axis affects the latitude of the persistent winds throughout the year.

Seasons might affect growth of algae and other sea plants.

Melting snow affects the volume of river water at certain times of year (Spring). This can affect the mix of estuaries.


The sun is still the source of energy of all life. I realize could probably make a case for and a feisty debate among xenologists/zoologists if life in deep ocean thermal vents could exist in a vacuum absent any sunlight in the biosphere at all, that aside. Taking into account, and I'm assuming, the merfolk are the classical type, or near to it, humanoid. I see no reason they would completely ignore the sun or moon. As you said they spend much of their time in the top 200m of the ocean. Within that depth I could easily see them constructing stone... coral(?) calendars or observatories in the vein of Stonehenge or that could track the course of the sun overhead indicating seasons. They don't need to see the sun set or sun rise. There is no reason the day cant be marked along its course from its zenith. They just need to modify their construction to "think" in a vertical plane which Id imagine they would anyway.


Different arrangement of moons

This is feasible if your planet has a different arrangement of moons than the Earth. On earth, the strength of the tides varies monthly. When the moon is in line with the sun, tides are strongest ("spring tides"), and when it is at right angle to the sun-earth line, the tides are smaller ("neap tides"). This is probably already detectable to sensitive aquatic creatures and gives you a natural "month".

If the moon's orbit were highly elliptical, you would also introduce a small annual modulation to the tidal strength, as the major axis of the moon's orbit is parallel or at right-angles to the earth-sun line. Multiple moons with different orbital periods and resonances could give even longer cycles.


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