When you are on the phone with someone, are you talking to them at the same time or at different times? 5 minutes is always 5 minutes no matter where you are. Time zones are set by humans to match daylight hours with time in the day- more or less so noon is when the sun is directly overhead. We have to maintain consistently so we can’t say it’s 12 here and next door it’s 12:01, no one would know what time it is. We group them into zones into where it makes the most sense. It literally is an imaginary line where you have to set the clock forward, so you can match everyone else in the area.

Timezones are used to track keep places/people aware of the same time for societal reason. They are split about roughly around how much daylight there is.

Okay so what you’re saying is there are indeed subtle differences and Northern California would have a tiny bit different time, but we just don’t acknowledge it?

Tracking time is entirely a human construct

There's no reason we can use a Universal clock for the whole planet. Technically we have that UTC or Universal Time, also called Greenwich Mean Time or GMT.

The problem is that people base their day on the time. Noon is when the sun is the highest, you go to work for 8am, etc

If you use Universal time noon would be in the evening in North America and people don't like that. Which is why timezones exist.

Timezones are kinda, not really, but mostly just lines drawn on a map that says "for this area the clock will be X hours ahead or behind GMT so that noon is when the sun is directly overhead"

Each location has its own local time. This didn't matter for most of history.

Once we had trains there was a need to ensure that clocks were synchronised to ensure that trains departed at the same time for everyone concerned. These times meant there was a need to have standardised time over a large area.

WWI in Germany saw the need to make maximum use of available daylight in factories and this practice of daylight saving was quickly adopted in high latitude locations. In equatorial locations there is no point in daylight saving as sunrise and sunset don't vary much throughout the year.

Yes, because Time Zones basically came around with the advent of the railroad. It was important that everybody agree on what time it is, otherwise it would be impossible to plan anything. The earth spins at a constant rate, meaning that the sun goes across the sky at a constant rate. It would just be completely impossible AND useless to split the entire world into, say, 1-minute time zones.

Time, as we track it, is our own invention. We get to say what time it is somewhere. Is the time different place to place when we compare where we are to the sun and moon? Yes. But we set the time zones, which we do hour to hour, so differences of minutes don't make a difference.

Heres a map of sunset times throughout the timezones of the US. you'll notice that the closer you get to the "border" of the next timezone west, without changing your clock, the later sunset it.

https://cdn.theconversation.com/infographics/120/12cc5a0adf037d84383fbe28a54dc8a1e726dbac/site/images/status_quo.png

https://theconversation.com/the-hazards-of-living-on-the-right-side-of-a-time-zone-border-116630

What? How did you come up with that?

No, the only difference is the position of the sun at any given instance in time. Google midnight sun or 24 hour darkness.

When it’s noon in one time zone, it’s noon everywhere in that time zone. Latitude has little bearing on the time. Our timekeeping is just to maintain a standard. Call someone in NorCal at noon and they’ll say it’s noon. Not 12:01. The sun would be lower in the sky compared to socal due to earth’s curvature, but “standard time” is just an agreed upon construct so we can all be harmonized.

Would the different position of the sun not indicate a slight, but unacknowledged difference of time?

Time is a construct we use to allow us to manage our lives and communicate.

It is not created by the universe, time is clothing we put on the universe for decency to our small minds.

So to your question, no, there's not minor drift in times within timezones, because the timezones are an artificial construct that does not do so.

Some countries do not use time zones at all at that.

> so what you’re saying is there are indeed subtle differences and Northern California would have a tiny bit different time, but we just don’t acknowledge it?

Not really. The subtle difference between nearby places is in when the sun (and other things in the sky) rises and sets. There is no objective "time" at which these events occur. It's determined by the local horizon.

Muslims need to keep track of the sunset for fasting reasons. They are advised that if they happen to be at the top of the Burj Khalifa, they should remember that the sun sets three minutes later at the top of the tower than at the bottom. That doesn't mean it's "actually three minutes earlier" at the top of the tower. It's the same time as it is on the ground, it's just high up so the sun is above the horizon longer.

It takes the Earth 24 hours to turn on its axis once. Or, as it seems to us, for the Sun to go across the sky and come back around. So at points 15 degrees apart in longitude, solar noon will occur an hour apart.

On the equator, the Earth is about 40,000 km around. So at points 1 km apart, solar noon will occur 2.16 seconds apart.

It's not that we don't acknowledge it, it's that we don't acknowledge it in our day-to-day lives because the difference is so small. Your talking femto-seconds or less between Northern and Southern California (due to acceleration and gravity and all that jazz).

If you're thinking time zones (which it seems like you are), those are artificial constructs designed to keep people in different parts of the world talking about time of day in the same terms (If I say it's 4pm, you have abpretty good idea that the sun is up and bright and there's yay-and-so-many hours until dusk, regardless of where I am).

You keep saying things like "tiny bit different time." No, the clock on the eastern edge of the time zone is the same as the clock on the western edge. If it is 12:30 PM in the Eastern US time zone, it's not 12:05 in Indiana, 12:30 in Pennsylvania, and 12:55 in Maine. It's 12:30 across the whole time zone. That's sort of the whole point of having the time zones in the first place: to sync everyone up in that area, while still allowing everyone to hit "noon" roughly when the sun is overhead in their own area.

When you cross into Central, the clock will read 11:30. So in that sense, yes, the time is an hour different just because you cross that magic border line.

I think what you are really after though is whether or not the sun is in a different position, and the answer to that is yes. Lookup sunrise/sunset times in various cities in the same time zone, and you'll see they vary slightly. So again, in the eastern time zone, sunrise today in Indianapolis was 7:30 am, in Pittsburgh it was 7:05 am, and in Boston it was 6:28 am. All of them hit 7:30 AM at the same time, but the sun position in the sky can vary across the same time zone.

Only East to West. Not North to South. North to South only changes the height of the sun in the sky, not the amount of time it takes to pass through its path.

> The earth spins at a constant rate,

Constant-ish rate.

Great American Gladys West developed her model of the world to take into account variations in the earth's spin to ensure that sat nav is accurate.

With Daylight Savings, noon is now at 1:00pm, since we all decided to call what’s actually 11:00am 12.

I know that muslim scholars decided that muslims in space can track their day/night for fasting based on the time in Mecca.

How much does the rotation of the earth vary, and is it significant enough to warrant this detour in an explanation to a 5-year-old?

[removed]

Timezones are artificial constructs that people have made up, that is why they are just a line you cross.

If you move from one country to the next you suddenly are subject to all sorts of different man made laws and sometimes even manmade ideas of what time it is.

Because that is all timezones are a government declaring what time it is inside their borders.

Timezones are artificial but they are based on a natural things that is more real.

You know how the sun rises in the east and sets in the west?

If you move westwards you experience dawn and noon and sunset slightly earlier than someone east of you. (Like seeing a cars headlights sooner if you are further in the direction it is coming from.)

when we first came up with ways to tell the time we were pretty simple about it we had dawn and dusk when the sun rises and sets and we had midnight in the middle of the night halfway between sunset and dawn and noon halfway between dawn and dusk.

You didn't need a clock to tell the time just your eyes.

Dawn was at a slightly different time everywhere, but people would have to travel quite a bit to notice the difference and they traveled so slow that nobody really felt the difference.

Later people separated the time when the sun was up into 12 equal part called hours and the time when in wasn't into another 12 hours.

This mean that how long an hour was differed from day to day and location to location , but it was just a convenient way to split up the time between dawn and noon into 6 parts and so on.

Later the whole day night cycle was split into 24 equal parts this meant that dawn did always happen at the same hour but all hours were the same length.

than at some point when timekepeing got good enough we split the hour into 60 minute parts and got minutes and later still split those into 60 second minute part and got seconds.

At that point we basically had the time we have today.

The difference being that each place had its own time.

Noon was always halfway between dawn and dusk when the sun was highest in the sky.

Each town with it own church tower clock had its own local time based on the sun.

If you were rich enough to have an accurate clock or watch and set it based on the official time in one town and then traveled east or west to another town your clock would be off.

Of course clocks weren't very accurate or and travel was slow for most people.

The invention of a very accurate clock that could be compared to the time as seen from the sun was actually what enabled sailors to tell how far west or east they had travailed and thus tell where they were.

This all was very well until railroads came along.

Steam engines can move people very fast over great distances, fast enough that the time difference between towns mattered.

Keeping an accurate schedule is very hard when each town you stop has its own timezone.

Railroad companies made things easier by creating a unified time for their company.

At first this made things more complicated because each company decided on a different time, so you essentially could move between timezones by going from one platform to the next.

Eventually this shook out to the system we have today though were each country or in large countries each state within the country chooses a timezone based roughly on what the time would be based on the sun somewhere nearby. These timezones are mostly offset from each other by a full hour with a few exception being 30 or 15 minutes of.

These timezones are mostly the same north and south of where you are and different if you go far enough east or west.

In a few place you can go to a different timezone by going north or south though.

Noon and midnight will always happen simultaneously in a single line north to south from pole to pole (ignoring midnight sun phenomena etc) The line of when dawn and dusk happens is not quite north south but somewhat angled at times. So it can happen sooner or later north and south of you.

The legal time however isn't bound by that natural time. it follow the lines drawn by man

La of California has the same timezone, but if you go from Baja California to Baja California South in Mexico you move between timezone even if the dividing line between the two states is east to west.

If everyone set their watches by the sun, local time would vary by 1 hour for every 15 degree of longitude (east/west position). Latitude (north/south position) does not affect local time.

> How much does the rotation of the earth vary, and is it significant enough to warrant this detour in an explanation to a 5-year-old?

ELI5 is not for actual 5 year olds. It's for an intelligent layperson.

Day length fluctuations explains all in a depth that is beyond me.

When I went with my son to Greenwich observatory on the River Thames, London.

We looked at our GPS co-ordinates we found that the zero meridian was 100m or so further east than the observatory. This was explained as being caused by the earth slowing more than anticipate and so the 0° meridian moves further east.

Every so often there is a leap second when this occurs the GPS data resets the zero meridian back to the observatory

So answer the question I was obviously asking.

Not sure exactly what you are asking - could you rephrase it for me?

You explained that ELI5 isn't literal. But I was clearly asking if the clarification you're making is necessary within this kind of explanation. Was my technical inaccuracy negatively impacting the explanation I gave in any way?

We're that far down in the weeds nobody is interested in our conversation

I think the idea that the time the earth takes to rotate 360° varies and is thus not constant is important and it has consequences that are verifiable to anyone with a mobile phone and GPS.

In terms of science etc I think the idea of a constant should only be used for things that do not change. IE π e and the speed of light in a vacuum.

Obviously I thought it was important or I wouldn't have mentioned it.

As a distraction here's a man in the 1970s with a GPS wondering about the implications of a better organised society

Your response was to my TOP LEVEL COMMENT.

But I also want to make it clear that I am also part of the nobody that is interested in this conversation.

ETA: actually, that wasn't clear at all. I am not interested in this conversation and I think your correction is absolutely unnecessary.

I thought my comment was warranted. You didn't. Fine. Good people may agree to differ.