I will try to explain this as simple as possible, as I understand it. The rotation of the earth around the sun takes approx 365 1/4 days. This is why every 4th year, except for turn of century years, we have an extra day, hence leap year.

The bit I dont understand is that, say we are at mid day, thefirst/ second year that midday would have moved to 6pm, the second/third year midnight the third/fourth year 6pm bringing us back to normal mid day 4 years later. How can you just place a day on and it works out OK?

So why is it always light at midday, surely this should only happen every leap year.

oh gawd...

please please tell me I didnt just read this...

do you know how many arguements I had with my A level physics teacher over this subject? (and many others cus I love to argue!)

his explanation, as far as I could fathom out as it changed all the time... was that every day isn't actually 24hrs, its something like 23hrs 59mins and some seconds... and thats how everything manages to stay light the same...

its stupidly hard to understand... from my logical POV I would say on the 2nd year day should be night and night should be day... but boy oh boy did he have a paddy when i said that...

Mr Beadman, if your reading this... your appalling at explaining things :P

anyways, thanks for ruining my day... now I'm going to be sat pondering about this all day again... or is it night time? oh crap! :(

My astronomy is pretty limited (is this even astronomy? :)), but surely night/day is about when the earth rotates around its own axis, and it also moves around the sun (almost) once a year. So you just have to adjust the number of days in a year to make it "line" up with reality.

Hmm, this may be why I'm not cut out to be a teacher! ;)

I shall try to explain.

The clock from which we get hours and days is based on the rotation of the earth.

The calender from which we get years is based on the rotation of the earth about the sun.

The two rotations are independent.

It would be some coincidence if there were exactly x number of rotations of the earth in each year.

There are EXACTLY 24 hours per day as we defined the hours from the rotation of the earth.

If we didn't have leap years, the seasons would gradually shift by a quarter of a day a year, not the clock.

Hope this makes sense.

who decided that there was 365 days in a year anyway???

The length of the day and the length of the year may appear to be connected, but this is not true. The year is timed by the distance travelled around the sun, wheras the day is governed by the rotation of the earth.
I believe that the two are independent of each other.

Taking the earth's journey around the sun as being about 578 million miles (assuming a circular orbit for the purposes of the discussion) this is covered in a constant time of the equivalent of 365.25 days - roughly speaking 1.58 million miles per day to travel from point A on the orbit back to point A.

Under new metrication rules shortly to be announced by the EU, the length of the day is being modified by altering the speed of rotation of the earth. By slowing the rotation of the earth on its axis the new day will be twice as long as before.

The distance around the sun remains the same, the speed of rotation around the sun remains the same, all we have done is to slow the rotation of the earth on its axis.

Originally posted by scottf
who decided that there was 365 days in a year anyway???

Thats the whole point I was trying to make. There was never a decision in this case. Thats just the way it is! The earth rotates about its axis about 365 times in the time it takes to go round the sun once.

What we did decide was that there were 12 months in the year, 24 hours in the days, 60 minutes/seconds etc. Thats all historic and makes no logical sense. Why not 100 hours in the day, each with 10 mins each min with 10 secs...

Yes, like xafier and rich said - you have two rotations to work with - the orbit of the earth round the sun, and the axial spin of the earth.

this is further complicated by the axial tilt of the earth and the elliptical orbit of the earth around the sun.

What it basically boils down to is that the earth is not an accurate clock, but our atomic clocks are.

We use our atomic clocks to measure time and adjust them periodically to account for a 'slower' earth clock. This means inserting leap seconds every so often, and delaying midnight (UTC (universal time code)) by a second or two every year.

This keeps the UTC in time with the rotation of the planet.

Leap years are calendrical adjustments of a whole day, and keep the calendar in time with the orbit of the earth round the sun in the same way leap seconds keep the clocks in time with the rotation of the earth.

[QUOTE]Originally posted by Hopman
[the length of the day is being modified by altering the speed of rotation of the earth. By slowing the rotation of the earth on its axis the new day will be twice as long as before.

doesn't makes sense to me :-S

surely speed = time x distance covered which shouldn't affect the earth's day? I mean, the metric system doesn't affect time does it?

Maybe I'm getting things confused but further explanation may be required......

Originally posted by tim_rutter
What we did decide was that there were 12 months in the year, 24 hours in the days, 60 minutes/seconds etc. Thats all historic and makes no logical sense. Why not 100 hours in the day, each with 10 mins each min with 10 secs...
Isn't common thinking that numbers like 60 and 360 (degrees) are useful because of their many factors? Anything divisible by the first three prime numbers gives you a good chance of being able to work things out in your head :)

Now that's been sorted, you can start whittling about lunar and calender months and the timing of Easter :D

And May Day being anything other than May 1st

Originally posted by rich951
Isn't common thinking that numbers like 60 and 360 (degrees) are useful because of their many factors? Anything divisible by the first three prime numbers gives you a good chance of being able to work things out in your head :)

Yeah but we've got ten fingers. Makes counting easier :D

Originally posted by tim_rutter
Yeah but we've got ten fingers. Makes counting easier :D
Counting yes, but if you were counting the number of degrees in half a circle would you really count up to 180 on your fingers? Arithmetic is pretty handy sometimes! :)

Ok, rubbish example, but say a circle was 100 Somethings. Split that into three, then divide that section into 5 and tell me how many Somethings that is. Using only your fingers! ;)

Originally posted by rich951
Counting yes, but if you were counting the number of degrees in half a circle would you really count up to 180 on your fingers? Arithmetic is pretty handy sometimes! :)

Ok, rubbish example, but say a circle was 100 Somethings. Split that into three, then divide that section into 5 and tell me how many Somethings that is. Using only your fingers! ;)

How about we split it into two pi. Then we'll have easy maths!

After reading this thread I have decided that I am not a clever clogs!! I kind of understand the bit about the 2 rotations being seperate but the rest just went straight over my head!!

And I got an A in my physics GCSE!!!!

Originally posted by scottf
who decided that there was 365 days in a year anyway???


More importantly, who decided there was only going to be 2 days to a weekend! :suspect:

Originally posted by tim_rutter
There are EXACTLY 24 hours per day as we defined the hours from the rotation of the earth.
.

Not quite true - A day (as defined by a rotation of the earth) is not exactly 24 hours, it is 24 hrs +/- a few seconds, due to the tilt of the axis.
This is why 'leap' seconds are added or subtracted to the atomic clock, these decisions are made by a group of scientists in a darkened room somewhere whos jobs are to decide when to add and subtract seconds. Sounds like a fun job.:suspect:

Hey, not all science is boring - I consider myself a scientist and have a PhD in tennis balls! :D (and I never did any work in a darkened room - I tried when we had a power cut but couldn't find any volunteers for the cycle electricity generator...)

Originally posted by rich951
Isn't common thinking that numbers like 60 and 360 (degrees) are useful because of their many factors? Anything divisible by the first three prime numbers gives you a good chance of being able to work things out in your head :)

12 hours in a day (and 12 in a night) is easy.

Try dividing a circle into 10 segments with string and chalk

Now try dividing it into 12. Much easier.

Now go and live near the equator (early human civilization) and make a sundial. How many segments are you going to split it into? 12. And since on average the day is as long as the night, that gives you 24 hours a day.

Also -
Babylonians (who are responsible for much of time as we know it) used base 60. (3*4*5), counted to 12 on one hand and generally were into their numbers.

Interesting thing about the clock - have you noticed how lots of primes are on top of one another
1/13
5/17
7/19
11/23

Originally posted by tim_rutter
Yeah but we've got ten fingers. Makes counting easier :D

Each finger has four points. (base joint, 1st joint, 2nd joint, tip). Using your thumb as a marker you can count to 256 on two hands or 16 on one...

The way I understand it is that it takes 365¼ days for the earth to travel round the sun. Each day is 24 hours which is the time it takes for the earth to spin on its axis.

Therefore, the time it takes for the earth to travel round the sun has nothing to do with the number of hours in the day, which nearly always remain the same (every so many years I think a second has to be added because I think the number of hours in a day is actually 24 plus a fraction of a second).

:cool:

Originally posted by rich951

Ok, rubbish example, but say a circle was 100 Somethings. Split that into three, then divide that section into 5 and tell me how many Somethings that is. Using only your fingers! ;)

That's the beauty of a 12 base system, you can easily have a third of many things, - but with 10 base a third is rarely a whole number.

The groat is still a coin of the realm :)

PLEASE STOP! You're making Bob's head hurt!

Originally posted by Greybeard
Now that's been sorted, you can start whittling about lunar and calender months and the timing of Easter :D

The mystery of the timing of Easter is easily explained......

Easter Sunday is calculated to be on the first Sunday after the first official full moon following the official vernal equinox.

Simple wasn't it! :?:

Apparently the EU are going to decimalise time: http://www.therockalltimes.co.uk/2002/05/13/decimal-time.html

And yes everybody would be better at mathematics if we worked in a base of 12, instead of only being good at working in things divisible by 10,2 and 5, we would easily be able to work in 2,3,4,6 and 12, all we need is three new symbols to represent the figures 10,11 and 12 and Bobs your mothers brother...............

Dont think I've explained that as well as I thought it....

Never mind.

Originally posted by floyd77
Not quite true - A day (as defined by a rotation of the earth) is not exactly 24 hours, it is 24 hrs +/- a few seconds, due to the tilt of the axis.
This is why 'leap' seconds are added or subtracted to the atomic clock, these decisions are made by a group of scientists in a darkened room somewhere whos jobs are to decide when to add and subtract seconds. Sounds like a fun job.:suspect:

It must be ! There is a page here that describes the process and lets you know when they insert the positive leap seconds. There isnt one in June 2005. You can get email updates to let you know when they are on their way if you're that way inclined :P or of course you can listen out for 7 pips on the hour instead of the normal 6 :thumbsup:

http://www.npl.co.uk/time/leap_second.html

Forgot to say - you can find out who settles on 365.???? days per year here :

http://www.npl.co.uk/time/time_scales.html

:thumbsup:

Gotta love the National Physical Lab :thumbsup: :thumbsup:

[QUOTE]Originally posted by Hopman
[B]Under new metrication rules shortly to be announced by the EU, the length of the day is being modified by altering the speed of rotation of the earth. By slowing the rotation of the earth on its axis the new day will be twice as long as before.

Tee Hee, and by slowing the eath on it's axis (as if it could ever be done :heyhey: ) we'll lose the gravity and all fly off into oblivion Hmmmm :heyhey:

How about metric time?

http://zapatopi.net/metrictime.html

Originally posted by SHarper
.... all we need is three new symbols to represent the figures 10,11 and 12 and Bobs your mothers brother

Er, don't start counting in Base 13. That is going to cause you no end of headaches.

You can easily use your fingers to count in lots of bases including 4, 5,8,10,12 and 16

Originally posted by rtapper
It must be ! There is a page here that describes the process and lets you know when they insert the positive leap seconds. There isnt one in June 2005. You can get email updates to let you know when they are on their way if you're that way inclined :P or of course you can listen out for 7 pips on the hour instead of the normal 6 :thumbsup:

http://www.npl.co.uk/time/leap_second.html

I stand corrected! :thumbsup:

Originally posted by Hels
[QUOTE]Originally posted by Hopman
[B]Under new metrication rules shortly to be announced by the EU, the length of the day is being modified by altering the speed of rotation of the earth. By slowing the rotation of the earth on its axis the new day will be twice as long as before.

Tee Hee, and by slowing the eath on it's axis (as if it could ever be done :heyhey: ) we'll lose the gravity and all fly off into oblivion Hmmmm :heyhey:

gravity is nothing to do with rotation, gravity is a function of mass (of 2 bodies).

The centripetal acceleration imparted to you by the rotation would be lessened if the rotational velocity were reduced, so you might feel slightly heavier. But i doubt it would amount to much.

Originally posted by Phanerothyme
Now go and live near the equator (early human civilization) and make a sundial.

Slightly north of the equator to ensure the shadow alwzys goes clockwise.

If you put a sundail on the equator for half the year the sun is in the southern sky and the shadow goes clockwise, for the other half of the year the sun is in the northern sky and the shadow goes anticlockwise.

On the two equinoxes the shadow shrinks from the west towards the gnomen then extends out to the east after midday.

(I may have got my directions wrong, but that's the idea).

--
JGH

A long time? ago, I was taught field astronomy in the Army.
"Theory of Time", and the relationship of "Time" with Longtitude.
Greenwich Mean Time, Siderial Time, and Universal time all came into these calculations. Local Mean Time being wherever you were at that time.
One hour in time being equal to 15 degrees in Arc, very useful when navigating by the "Sun Compass" method.
It all ties in nicely with the sexagisimal system.
24 Hrs x 15 Deg's = 360 etc.
This was in the days of Nato.
The French used a Metric System, with 400 Grad's in a circle, and based on the Meridian of Paris.
The Americans used a mixed system of Greenwich Meridian, and 6,400 Mills in a circle.
The Astro' observations were used to provide Bearings and positions would could then be used to site and fire missiles, Artillery pieces etc.
No mean feat in the days of Function tables, and hand cranked "Twin banked" mechanical calculators.
Coupled with Grids and Projections, which produce a great variety of Maps. These resulting in a great deal of further calculations of Grid to Grid, Grid to Geographic, and involving a mystical thing called a "Nomogram".
The whole system being linked to a mean sun, rising at 06:00am on the Mean Equator, and setting at 18:00pm.
Don't get me going on Latitude North or South, which results in the length or shortness of the mean Day.
We did sometimes manage to hit things with the aforemention projectiles, but the whole experience provided me with some wonderful stories of trying to achieve an integrated Nato Unit.