# When is a day not a day?



## cornflakegirl (Nov 17, 2010)

This question is the fault of my work colleague. It was messing with his head, so he started talking to the rest of us about it, and now it's messing with our heads too...

Question

If the earth didn't spin then there would be no days, but there would be one period of night time and one period of day time, right?

Bit it does spin ... and I know that it takes 365.24 days for the earth to go around the sun AND there are 365 periods of day time and night time in a year and 366 in a leap year.

Where did my extra period of day time and night time go?

Experiment

We tested this with a mug and a pencil and if the mug span twice in the time it took to go round the pencil then there would either be 3 periods of daytime and night time, or 1, depending on which way we span the mug. So 2 days, but 1 or 3 periods of day and night time.

I don't get it.

Please help


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## Greg Truby (Nov 17, 2010)

Well the thing is that we're clocking it at 24 hours/rotation as we revolve around the sun.  So every 24 hours we complete the rotation.  The thing is that after 365 days we're _almost_ back to where we started in our orbit - we've only six hours worth of orbit left to go before getting back to where we were midnight, January 1st of last year.  But we don't stop the clocks and wait, we just start January 1st of this year a little be earlier in the orbit.  After four orbits, we're a day in back of where we were four years ago and so we toss another rotation in the calendar to get us back to our orbital starting point.

At least that' how I'm reasoning it out.  Does that answer the question you're asking?  Or did I misunderstand your query?


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## Atroxell (Nov 17, 2010)

I am no astronomer, but here's my take:

I think your assumption that there would still be a day period and a night period if the Earth did not spin is incorrect. If the Earth were gravitationally locked in its orbit then there would only be one side exposed to the sun at all times, and the only way to get a "day period" would be to travel to the lit side, and vice versa for a "night period". 

Therefore, in your example a "non-rotating" Earth having a day period and a night period would imply some some rotation to the Earth, albeit very slow. The Earth would just take one annular orbit of the Sun to attain 1 terran day of revolution.

Use the moon as an example: Since it is gravitionally locked in its orbit, the same side always faces the Earth. It does not rotate. Hence the "bright side" of the moon, and the "dark side".

Of course, someone on this forum will likely have the answer before I do, but just for science's sake (and my own curiousity), I am going to forward your question to a friend of my daughter who has worked at NASA and is currently working on her PhD in astrophysics.


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## cornflakegirl (Nov 17, 2010)

Greg - I don't *think* the leap year thing is it - I think it's just another annoyance to throw into the mix (along with the elliptical orbit and the slant of the Earth). But I'm not certain!

Atroxell - interesting idea. Our mug and pencil experiment involved a felt-tip mark on the side of the mug, and we kept the mark always pointing in the same absolute direction, rather than always pointing towards the sun (pencil). Which made sense to us as "not-revolving". But may be a fundamental error.

Would very much like to hear what your astrophysicist has to say on the matter (even if it's just that we're stupid!  )


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## sous2817 (Nov 17, 2010)

Here's my take:

The vernal equinox is the time when the sun is directly above the Earth's equator, moving from the southern to the northern hemisphere.

The mean time between two successive vernal equinoxes is called a tropical year–also known as a solar year–and is about 365.2422 days long.

Using a calendar with 365 days every year would result in a loss of 0.2422 days, or almost six hours per year. After 100 years, this calendar would be more than 24 days ahead of the season (tropical year), which is not desirable or accurate. It is desirable to align the calendar with the seasons and to make any difference as insignificant as possible.

By adding a leap year approximately every fourth year, the difference between the calendar and the seasons can be reduced significantly, and the calendar will align with the seasons much more accurately.

(The term "day" is used to mean "solar day"–which is the mean time between two transits of the sun across the meridian of the observer.)


Just kidding, I stole it from here: http://www.timeanddate.com/date/leapyear.html)


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## Atroxell (Nov 17, 2010)

Well, I copied the link to my email and sent it home since I do not have her email handy. So it will be this evening before I can send it--through my daughter, since I do not have her email address. The response may be slow in coming, but I will post it here when I get it. 

Or I may just send the link and suggest that she join the forum to post a response...


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## Greg Truby (Nov 17, 2010)

sous2817 said:


> The mean time between two successive vernal equinoxes is called a tropical year–also known as a solar year–and is about 365.2422 days long.


 
See, I think this is where we go awry. If we're going to define a year as going from one vernal equinox (or equilux) to the next -- and I agree this is the most logical definition -- then also by definition, it has to be an integer -- 365 days *exactly*. <s>The problem is that we're defining "days" as 24-hour periods instead of full rotations. So the error comes in that although the time from one (vernal) equilux to the next is completed solar days, 365 days isn't 8,760 hours, but rather 8765.8 hours. Or at least I _think_ that's it...</s>  [See below]


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## Taul (Nov 17, 2010)

deleted - more homework needed


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## Jonmo1 (Nov 17, 2010)

I'm with Atrox

The flaw is your inital assumption here



> If the earth didn't spin then there would be no days, *but there would be one period of night time and one period of day time, right?*


That is an incorrect assumption.
Like the Moon, the same side of the Earth would face the sun throught the entire 365.24 days it takes to orbit the sun.
So there is no "lost" day.


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## alansidman (Nov 17, 2010)

I think that the 6 hours per year is approximately 2.5 seconds per day.  Hardly noticeable.


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## Jonmo1 (Nov 17, 2010)

This brings up something I've always wondered.
If it's actually 365.24 days per year, and we just do 366 every four years to make up the difference...

Why then is it not Dark outside at 1AM this year, then Daylight at 1AM 2 years later, then back to dark 2 years after that, and so on??


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## Greg Truby (Nov 17, 2010)

jonmo1 said:


> ...Why then is it not Dark outside at 1AM this year, then Daylight at 1AM 2 years later, then back to dark 2 years after that, and so on??


 
That's more or less what I was trying to figure out in my post #7 here.  I think I may be completely missing Emma's question, though.


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## cornflakegirl (Nov 18, 2010)

My colleague found this.

jonmo - my brain hurts even more now. Greg - apologies for completely missing what you were getting at!


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## snowblizz (Nov 18, 2010)

jonmo1 said:


> This brings up something I've always wondered.
> If it's actually 365.24 days per year, and we just do 366 every four years to make up the difference...
> 
> Why then is it not Dark outside at 1AM this year, then Daylight at 1AM 2  years later, then back to dark 2 years after that, and so on??


I don't know if it matters, but it's not only +1 day every four years. There are a couple of other "rules" to more finely adjust it. I forget them exactly, but even centuries and those divisible with 4 or something like that are/are not leap years to get it all adjusted correctly.

There's also leap seconds inserted to keep the day at the "correct" length apparently.
Don't know if it helps but this seems to be explaining a lot:
http://en.wikipedia.org/wiki/Day


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## malteaser (Nov 18, 2010)

I'm the confused (and now less confused) colleague!!

I think I've figured this out. If you were looking from the sun then you'd see the earth rotate 365.24 times per year.

If you were floating on Jupiter then you'd actually see the earth rotate 366.24 days per year.

To look at it another way; it depends on you definition of rotate. If the earth behaved like the moon did around the earth then looking down at the North pole, it would rotate once, but looking from the sun it wouldn't rotate at all.

So it's a matter of your position (or your opinion) that decides whether the earth rotates an extra time.

Somoene else said this:
If it's actually 365.24 days per year, and we just do 366 every four years to make up the difference...

Why then is it not Dark outside at 1AM this year, then Daylight at 1AM 2 years later, then back to dark 2 years after that, and so on?? 

If I'm understanding the question correctly then if we used a solar year, this *would* happen. i.e. the year after a leap year should end at 6am on 1/1/xx. the next year it would end at mid-day on 1/1/xx and so on. Therefore we cheat the Science and restart our year 6 hours early (to coincide with the spinning of the earth or the definition of a day), then make up for it every 4 years on Feb 29th.

My Conculsion:
Secondary school Science wasn't that simple after all!! 
(and perhaps this year I'll celebrate the new year a few hours later!!!)


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## Greg Truby (Nov 18, 2010)

Correct Malteaser.  Or to answer Jonmo's questions another way - it doesn't happen because a year doesn't equal a year:

1 vernal equinox to the next <> 1 complete orbit.

Because our calendars use the former, we must use the leap years to resync them to the latter.


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## Wayne Duncan (Nov 18, 2010)

Strange how the mind makes connections.  For some reason this thread brought to mind to old puzzle; " if you walk around a tree and a squirrel keeps himself on the opposite side of the tree (as squirrels are wont to do), after you complete a complete 360 degree revolution around the tree, have you walked around the squirrel?"


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## sous2817 (Nov 18, 2010)

Greg Truby said:


> Correct Malteaser.  Or to answer Jonmo's questions another way - it doesn't happen because a year doesn't equal a year:
> 
> 1 vernal equinox to the next <> 1 complete orbit.
> 
> Because our calendars use the former, we must use the leap years to resync them to the latter.



What I can't wrap my head around is....We don't make up time over the course of the year(s), we do it in one lump some on Feb 29th. So 2 years in to a leap year cycle, when we've presumably lost 12 hours (or there about) of time, why isn't night and day reversed?


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## Jonmo1 (Nov 18, 2010)

sous2817 said:


> What I can't wrap my head around is....We don't make up time over the course of the year(s), we do it in one lump some on Feb 29th. So 2 years in to a leap year cycle, when we've presumably lost 12 hours (or there about) of time, why isn't night and day reversed?


 
I think I get it...

These are 2 different measurments of time.

1. How long it takes the earth to spin on it's Axis = 1 Day
2. How long it takes the earth to orbit the Sun = 1 Year

Those are 2 completely different and unrelated measurements of time..(i said it again because it's worth repeating).
They ARE NOT directly related to each other.
We Humans as a species over time have decided to correleate those 2 things together.
We collectively decided that 365 Days = 1 year.
But technically that is not correct.

The "extra day" is related to how long it takes to orbit the sun, it has nothing to do with how long it takes the earth to spin around on it's Axis.

Year 1, we spin on our axis 365 times, but we have not completely orbited the sun. There is a small gap left in that complete circle
Year 2, we spin on our axis 365 more times, and that small gap got a little bigger
Year 3, we spin on our axis 365 more times, small gap gets even bigger
Year 4, we spin on our axis 365 more times, small gap got big enough to = 1 day, so we put an extra day in the year.



Clear as Mud right?


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## cornflakegirl (Nov 18, 2010)

Because the correction for the length of a year is about keeping the seasons in sync (because of the tilt of the earth), not about day and night. Consider the Islamic or Jewish calendars - the number of days in a year doesn't equal a full solar year. And so the months (and festivals) move about (and I think from time to time an extra month gets slotted in to restore balance). Makes no difference to daytime (when we are facing the sun) and nighttime (when we are pointing away).

[EDIT: or what Jonmo said]


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## HalfAce (Nov 18, 2010)

> we've presumably lost 12 hours (or there about) of time, why isn't night and day reversed?


Depending on the season of the year, for us it _is_! 
(You ever read a newspaper (in _natural_ light) outside at 2AM? - Or had to use your headlights to go home for lunch at noon?)


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## sous2817 (Nov 18, 2010)

HalfAce said:


> Depending on the season of the year, for us it _is_!
> (You ever read a newspaper (in _natural_ light) outside at 2AM? - Or had to use your headlights to go home for lunch at noon?)



But isn't that more due to the tilt of the earth either towards the sun or away from the sun rather than the rotation point Alaska is in at the time (if that makes any sense).  In the summer your tilted towards the sun so you have more daylight and in the Winter you're tilted away so you have less sun.  That's what I always thought at least.


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## Greg Truby (Nov 18, 2010)

sous2817 said:


> But isn't that more due to the tilt of the earth either towards the sun or away from the sun rather than the rotation point ...


 
Yes, Dan's just cracking wise.


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## HalfAce (Nov 18, 2010)

> due to the tilt of the earth either towards the sun or away from the sun  rather than the rotation point Alaska is in at the time (if that makes  any sense).  In the summer your tilted towards the sun so you have more  daylight and in the Winter you're tilted away so you have less sun.   That's what I always thought at least.



Yes, you're absolutely right. - Both you and Greg.
(How often do you hear _that_ Greg???) 

See, I'm just being a smart a$$...


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## cornflakegirl (Nov 18, 2010)

HalfAce said:


> Depending on the season of the year, for us it _is_!
> (You ever read a newspaper (in _natural_ light) outside at 2AM? - Or had to use your headlights to go home for lunch at noon?)



Yeah, that kind of puts my current whinging that it's pitch black here at 5pm into context!


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## arkusM (Nov 18, 2010)

This is a great conversation. 
What messes with my head is trying to put an analog process- rotations into a "digital" arbitrary framework - time. Sounds simple but wow the details are crazy. We humans wanted a nice calender with seasons repeating on predictable intervals, but our Gregorian calender has some months with 31 days, other with 30 and one with 28/29?? We want 24hrs in a day yet that does not work either... 
 We humans are so smart... we land a man on the moon but have complicated work arounds to get our clocks and calenders to work. LOL
"I am so smart, I am so smart, S-M-R-T.... Doh!" 

Oh by the way the moon does rotate. It has to in order to stay facing the earth the whole time and to have gravity. It is a synchronous rotation.


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## Atroxell (Nov 18, 2010)

arkus--you're right. I had this rolling through my head all day yesterday and last night, and something was trying to tell me that I missed something. It came to me about 3 AM (when thinking occurs for me) that I had botched my own attempt at explanation...

I kept mentally picturing the moon orbiting the Earth and trying to decipher why it looked wrong with respect to my post. Then it finally hit me that it _has_ to rotate to keep the same face towards the Earth. It just does not _visually_ rotate from our perspective. Ah, well. It sounded right when I wrote it...  

Luckily for myself and everyone else, my business obligations do not include any Astronomy.

There are obviously many people out there who are much S-M-R-T-R than I. lol


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## cornflakegirl (Nov 19, 2010)

Atroxell - don't apologise - it was your insight that made us realise we were talking rubbish!

The link that I posted said what you said - that the Earth rotates 365.25 + 1 times in a year - 365 relative to the sun + 1 in actually going round the sun. But we only see 365 of them as days.


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