When is a day not a day?

[cornflakegirl]

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

 

[Greg Truby]

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?

 

[Atroxell]

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.

 

[cornflakegirl]

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! )

 

[sous2817]

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)

 

[Atroxell]

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...

 

[Greg Truby]

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]

 

[Taul]

deleted - more homework needed

 

[Jonmo1]

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.

 

[alansidman]

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