# How would timezones work on a planet 100 times the size of our Earth

I thought the answer would be simple and maybe it is, but it's just rattling my brain.

If there was a planet 100 times the circumference of Earth, assuming everything is similar to Earth, 1 day (1 rotation) is 24 hours (therefore the planet will be spinning faster than Earth?) so I was wondering:

Would there still be 24 time zones, one for each hour? As the planet is just a sphere and the sun would just simply hit the areas it can see regardless of the larger size. So I'm assuming there would only 24 zones.

But trying to imagine someone getting up in China the same time someone gets up on the west coast of America is getting to me, but since the planet must be spinning faster to compensate, it should make sense?

• On the planet wich is 1 000 000 (one million) times more massive than Earth, no one would ever get up. It would be a star, not a planet. Aug 12, 2019 at 12:14
• BTW, Earth has 37 time zones -- but that's because there are a number of off-longitude and half-hour zones. Aug 12, 2019 at 12:14
• While a rotation may take 24 hours, is that what the natives consider "a day"? Do they divide a day into 24 "hours"? 30 "reps"? 18 "toqs"? Aug 12, 2019 at 12:33
• Your "America vs China" comparison falls down on the fact that the curvature would be so much less: while the physical distance is large, the time difference between noon in each would be much less. On the other hand, "Flat Earth" might get more traction! Aug 12, 2019 at 14:05
• Big problems here. 100x the size = 1 million x the mass. Your "planet" is 3 solar masses (before counting degeneracy effects), it has no fusion to support it. An object of three solar masses and no support promptly becomes a black hole. Even if it doesn't collapse you're looking at a surface gravity of 100g. Life is going to be awfully flat! Aug 13, 2019 at 0:50

You don't need to consider another planet, Earth is sufficient for this question.

The real problem is in thinking of time zones as a natural phenomenon. They aren't.

Take where I am right now. I'm located somewhat west of the center of my timezeone, so my clock is 20 minutes faster than a sun-based clock. And then there is daylight saving time, which adds another hour. Does it bother me (and others) that our clocks are "wrong" by an hour and twenty minutes? Not at all. Most people aren't even aware of the concept.

Take China as a larger example. By the sun, one end of the country is 4 hours different from the other end. But unlike other countries, China has only 1 official time zone, not 5. When some people get up and have breakfast their clocks will say 6:00, while for other people it will say 10:00. But in both cases, the sun has just risen. For some people "noon" is at 10:00, for others it is at 14:00. The official clock time doesn't match the sun's clock, but people get used to it.

And as for making a planet larger, it makes no difference. We can already see that situation here on Earth.

At the equator, 1-hour time zones are about 1000 miles across, but farther north, the lengths of the latitudes get smaller.

Iceland's time zone is only 440 miles across.

Even farther north, at Alert, Nunavut, Canada, the timezone is only 135 miles across.

Stand near the North Pole (or South Pole), and walk around it. You'll have to change your watch by an hour after each step.

Obviously time zones can become confusing and inconvenient when they are too small, but in such situations (remember, the zones are an entirely artificial human invention) it's common to designate the whole area as a single time zone (typically UTC) even though it spans many, or even all, actual time zones.

But in the OP situation, everything is larger, not smaller, so, except near the poles, time zones would be even less of a problem than they are on Earth.

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– L.Dutch
Aug 15, 2019 at 2:58

I sense some confusion in your ideas, let's try to set things straight.

a planet 100 times the size of earth, assuming everything is similar to earth, 1 day (1 rotation) is 24 hours (therefore the planet will be spinning faster than earth?)

Setting aside the plausibility of such a planet, Earth does a complete rotation around itself in 24 hours. This planet does a complete rotation around itself in 24 hours. The rotational velocity is the same for both.

Would there still be 24 time zones, one for each hour?

In principle yes, but it can also be that, due to the larger linear distances between the zone extremes, half hour time zones can be used. Don't forget time zones are just a convention, in the past each city had its own time.

But trying to imagine someone getting up in China the same time someone gets up on the west coast of America is getting to me,

This is blatantly false: if you look at the official time zones on Earth,

You see that while China is +8 UTC, America west coast is -8 UTC. This means 16 hours difference.

• The comment about China and America was due to the distance, not the actual time zone difference, as the average timezone distance I found was around 1035 miles, while a world, where that's scaled by 100, would mean I assumed the sizes of the timezone would expand as well. The distance between China and California is around 6687 miles. Aug 12, 2019 at 12:25
• Actually it's 8 hours difference the other way around. Aug 12, 2019 at 23:08
• @PaŭloEbermann except the internal dateline would make that 8 hours fall on two different days. Aug 13, 2019 at 12:41
• @JosephWebber Basically, if the planet revolves once in 24 "Earth hours" as we use them, then yeah, it would make sense to put China and California into one time zone. Of course the surface of that planet at the equator will move much quicker than Earth's so you'd feel much less gravity there, weather might be different etc
– smcs
Aug 13, 2019 at 14:48

If the planet rotates at the same rate as Earth, it'll have the same difference in sunrise, sunset, noon and midnight times for a given number of degrees of longitude, so will have the same "number of time zones" as Earth. These zones will naturally be much wider than those on Earth, but they'll work just the same way -- including needing a "date line" so you don't lose a day if you circumnavigate to the west (as Magellan's crew did).

However: The contiguous United States has four time zones for a width of around 3000 miles. If the United States were, instead, 300,000 miles wide, there might well be three hundred time zones. Why?

Time zones originally came from railroads. Before the 1860s, each town would set its clocks based on (usually) local noon, because it's easy to measure. But with railroads and telegraphs, it was necessary to know what time the train would arrive and depart -- which meant it was necessary, as well, to know what time it was in Tempe when you were leaving Kansas City. Having only four zones for the whole nation meant you could know that Tempe, Sheridan, and Butte had their clocks set the same -- and telegraphy made this actually practical, by allowing near-instantaneous transmission of time synchronizing signals.

But trains can only travel a few hundred miles in a day (at least with early steam technology), so there's no need to have time zones as coarse as 75,000 miles across -- there might well be "minute" zones, and they'd still be wider than the "hour" zones we have on Earth.

• What if they decide to make time zones half-hourly, and have 48 that are 50-times as wide as ours? Or minutely, and have 1440 time zones that are 5/3 as wide as ours? Aug 12, 2019 at 12:44
• That's up to your railroaders and telegraphers. But why would you need a time zone to be 35,000+ miles across? It'd take a full day to cross it, even in a 747. Aug 12, 2019 at 12:46
• @Chronocidal Then again, minute zones have the issue of requiring a table to tell what time it is virtually anywhere. I can remember the zones of most of Earth's major cities, but with 1440 zones (and ten thousand times as many cities as Earth) there's just no way... Aug 12, 2019 at 12:48
• As you say, 10,000 times as many cities. By comparison, Earth has less than 5,000 cities, so you're looking at more than 2 cities for every city on Earth per city on Earth! I think you'd be hard pressed just to remember the major Countries, let alone Cities Aug 12, 2019 at 13:03
• The idea of 300 time zones is intriguing, but there's not really a need to have time zones that are less coarse than 75,000 miles across, either. The idea is to have standardized clock times that roughly correspond to solar times. Geographically larger time zones would correspond equally well to solar times, so chopping them into more, smaller time zones gets you a system that's more complex but ultimately not much more useful. I don't care if solar noon comes at 12:15 or 12:30. Aug 12, 2019 at 21:04

Another way to approach the problem:

"Suppose humans were 1/10th their current size. Would that require any change to time zones?"

No, not really.

As others have said here, time zones aren't a natural phenomenon, so there's no reason to make a change unless we wanted one.

"Timezone" are all "subjective"! It was first created by Scottish-born Canadian Sir Sandford Fleming in 1876. It's all for us-human-being's convenience!

The very fact that the timezone lines on Earth are all crooked at many places prove it!

The main part of Greenland lay across 4 timezones! But they don't like the idea and make the whole chunk of land under 1 timezone!

And if you across the sea from the Northwest coast to Quttinirpaaq National Park, you had "jumped" across "TWO" timezones!

And certain countries have "Daylight Saving Time"! Which means in the same timezone, the time is different in Summer and Winter time!

That further proved that "time and timezone" are all for "OUR" convenience!

So to come back to your question, you could have "as many timezones as you want"!

You can even divide the globe into "15-minute-timezones" if you so wish!

And that might be a better idea since your planet is SO~ big!

• I realise that it's pedantry in the context of your answer but Greenland has four timezones. There are two pockets on the east coast each with their own timezone, there's the bulk on Nuuk time and then there's Thule. In fact, at I guess around 10,000 people per timzeone, I think Greenland may be a leader in timezonyness. Aug 13, 2019 at 19:43

It depends what you want out of your timezones. If we consider an idealized Earth time-zone, it is 1 hour 'wide', meaning that at each edge of the zone, sundials will disagree by a difference of 1 hour.

If then we consider what it means for the sundials to differ by that amount, we can realize that it means they are resting at different angles (since the Earth's surface is roughly spherical). The exact angle just so happens to be 1/24th of the way around the Earth, or 360 * 1/24 = 15 degrees. So, our ideal time zone is 15 degrees wide.

Now, let us change the size of the planet. Since a circle of any size still encompasses 360 degrees, a 1-hour time zone would still cover 15 degrees.

So, in conclusion, the width of the time zones are determined more by the time difference than anything else, and as other answers have suggested, the time difference is a purely social/political/economical decision.

There is no reason to have 12 (24) hours per day other than "because".

In ancient Babylonia, people used a sexagesimal (base-6, not base-10) counting system. For... whatever reason. Which means that everything, length, weight, and time was measured and accounted for in multiples of 6.

So for example, you would have us, which was 60 ells, and nus and sar, which were multiples of that (sar would be 1800 ells). Very queer, but I guess if you count base-6 then it feels quite normal.

It so happens that 10 is not a multiple of 6, but 12 is. Hence the day (the complete day, including night!) had 12 danna. This, somehow, nobody knows how, remained until some 2,000-3,000 years later, the Italians started counting not at midnight, but when the sun rises, and they thought that a two-hour period was too coarse over the (half) day, so they split the danna in two, giving 24 hours on what's called the "big clock".

Some hundred years later, people found it easier, and cheaper, to use the "little clock" where you no longer have 24 hours, but 12 + 12 hours instead, reusing the same dial on the exceedingly difficult to manufacture and extremely expensive single mechanical clock they could (maybe, possibly, if lucky) afford.

So, it all boils down to what you count in, and what you are comfortable with. Your planet is twice or twenty times or a hundred times the size? Can still count 12 hours, or 120 hours. Or, while you are at it, why not precisely 1,000 hours which would work much better with SI units.
I'd consider "What about gravity?" a much more challenging question if your planet is that big. A hundred times the circumference means it has ten thousand times the gravity (give or take some, might be somewhat more or less, depending on density, the general figure remains though). Does it rotate so fast that centrifugal force at least partly compensates for stuff weighting 10,000 times as much? That'd be fun watching stars in the sky :-)