# Year, Day, Hour?, Minute? Second?

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#### rogerinnh

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On any given planet orbiting its sun we define its year as being the amount of time it takes to orbit its sun, and we define its day as the amount of time it take to rotate once around its axis (well, more precisely, the amount of time it takes any point on the planet that's pointing directly at its sun to rotate arond and again be pointing directly at its sun). But is there a standard way to define an hour, minute, or second on a given planet? Do we just take the earthly convention that an hour is 1/24th of its day, a minute 1/60th of its hour, and a second 1/60th of its minute? Or do astronomers not deal with "planet-local" hours, minutes and seconds?

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#### thalion

##### Guest
There are standard, highly-technical and specified international agreements on the "second", based on atomic oscillations. The other units are also agreed upon internationally. So a second, minute, or hour on Earth is a second anywhere else--not taking relativity into account, of course.

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#### heyscottie

##### Guest
On a given planet, we may very well change the local definition of "day" or "year". For instance, a "local day" might equal XX.YY hours, and a "local year" might equal VV.WW local days.<br /><br />We would be very unlikely, however, to change the definition of seconds, minutes, or hours. In particular, seconds are just about the very basis of our measurement system:<br /><br />* Meter: Defined as the distance light travels in some particular fraction of a second<br /><br />* Kilogram: Defined as the mass of a cube of water 10 cm on a side at 4 degrees C and standard atmospheric pressure. (Depends on meter, which depends on second)<br /><br />* Newton: 1 kilogram*meter/(second^2)<br /><br />* Ampere: Coulumbs/second<br /><br />* Joule: Newton*meter<br /><br />* Watt : Joules/second<br /><br />The only units I can think of that are independent of the definition of the second are temperature.<br /><br />Scott

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##### Guest
There's no <i>one measurement</i> of a second it's the agreed average between many atomic clocks. These would, of course, vary with position, gravity or speed within the universe. The speed of light being constant is no help either as we can't determine our speed relative to it. So it does appear, to me, there's no universal second.

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#### rogerinnh

##### Guest
Yes, I'm familiar with the second being a standard for science. My question was more about dividing planetary days into smaller, more convenient increments. If you were living on Mars, for example, the Martian "day" is called a "sol" and is a bit longer than Earth's 24 hour day. So what would we divide the Sol into? 24 Sub-Sols, or Martian-Hours? Would 24 be the standard number of divisions within a planetary day? But why 24? That's merely what we on Earth have adopted. Maybe a different number would be more reasonable for Mars or some other planet. A hundred, perhaps? Or maybe make it binary, so that the Martian Day (sol) is divided into 2 or 4 or 8 or 16 increments.

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##### Guest
The metric system divided the day into 20 hours & the week into 10 days. However the peasants revolted over working a longer week & having less hours off a day. I think the French still keep to 400 degrees in a circle though.

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#### heyscottie

##### Guest
Yes, and they had 100 seconds in a minute and 100 minutes in an hour, but that didn't fly either.<br /><br />A second IS rigorously defined: It is defined as the duration between 9 192 631 770 radiation periods of cesium 133.<br /><br />I see no reason that a sol would be divided into exactly 24 of anything. I would just let it be divided into some not necessarily integral number of "standard" hours.<br />

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#### CalliArcale

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>So what would we divide the Sol into? 24 Sub-Sols, or Martian-Hours? Would 24 be the standard number of divisions within a planetary day? But why 24? That's merely what we on Earth have adopted.<p><hr /></p></p></blockquote><br /><br />Well, the Mars Exploration Rovers team have adopted a 24-"hour" Martian clock. They divide each sol into 24 Martian hours. They did this because the Martian hours are only slightly longer than Earth hours, and the team was already used to those. <img src="/images/icons/wink.gif" /> There's a Java program that'll tell you the current Mars time, and MER teammembers all wear two watches -- a normal one to tell them the current time in southern California, and a specially-designed Mars watch to tell them the time on Mars. They're actually living on Mars time to maximize use of the rovers. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### thalion

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I don't dividing a planetary rotation into convenient units would be worthwhile unless the day were very different from Earth's. For a planet with a rotation nearly equal to or much shorter than Earth's, hours would be fine, IMO. Only on planets with rotations much longer than Earth's would someone probably want to change the time units, but even then I think they'd keep the convenient units of hours, minutes, and days for reference to any "new" time units.

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#### igorsboss

##### Guest
A second is defined as <font color="yellow">9 192 631 770 radiation periods of cesium 133.</font><br /><br /><font color="yellow">there is no universal second.</font><br /><br />Relativity says that you are both right!<br /><br />The second is defined relative to an observer at rest with respect to the average position of the cesium 133 atom being observed. Since relative motion results in time dilation, two atomic clocks moving relative to each other may record different results.<br /><br />TRUE STORY:<br />While browsing a "Sharper Image" store several years ago, I happened upon a display which touted "The World's Most Accurate Wristwatch". A few moments later, a salesman asked me if I had any questions.<br /><br />"Are these the most accurate wristwatches in the world?", I asked.<br /><br />"Yes, they are.", he replied, confidently.<br /><br />"Then why are these two watches here in this case twenty seconds off from each other?"<br /><br />He was completely stumped! I've never seen a salseman so utterly at a loss for words! I just kept looking at him, quietly awaiting his reply, as I savored the moment.<br /><br />I just kept thinking of one of my favorite fortune cookie lines: <font color="yellow">A man with one watch knows what time it is. A man with two never does.</font><br />

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#### heyscottie

##### Guest
You're right, of course. I should have mentioned that we need to be relatively at rest with respect to our clock, or we will think the clock is running slowly.<br /><br />So an update:<br /><br />...... of a cesium 133 atom in a given Galilean reference frame.<br /><br />So there IS a standard definition of a second!

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#### unnikms

##### Guest
Comment to<br /><br />The metric system divided the day into 20 hours & the week into 10 days. However the peasants revolted over working a longer week & having less hours off a day. I think the French still keep to 400 degrees in a circle though.<br /><br /><br />The reason for 7 days a week is from Bible. It says that in the first day God created Light, second day ...., 6th day he created Man and 7th day he took rest. This may be the reason for 7 days in a week and Sunday to be holiday.<br /><br /><br />I have one more doubt. I read somewhare that one day is not exactly 24 hours, it is 23 hours 59 minutes 57 seconds. If it is so how the clocks are adjusted for this.<br /><br />Unni

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#### Saiph

##### Guest
The french probably reverted to 360 degrees in a circle, as its used everywhere, and is ubiquitous. <br /><br />Anyway, as for the 23 hr, 56 minute and some odd seconds per day, as opposed to 24...<br /><br />both are right. It just depends on what you use as your standard. If you use when the sun crosses the meridian (a line straight from north to south, passing overhead), its 24 hours. If you use when the earth faces the same point in the sky (a very distant star) you get the shorter version.<br /><br />This is caused by fact that the earth is rotating <i>and</i> orbiting the sun. As the earth goes around the sun, it has to rotate just a little bit more to face the sun than a full circle. To see this, take the extremely exagerated picture of: The earth rotates twice a year. It's facing the sun now, and goes half way around the sun. If it rotates only to face the same distant point (say any landmark in your room, besides the "sun"), the same point that faced the sun in the beginning...now faces directly away. <br /><br />In order to face the sun, the earth has to rotate a little bit more (in our exagerated version, a full 180 degrees more) which takes more time.<br /><br />In reality, the earth has to rotate for only 4 more minutes to completely face the sun again (instead of just next to it), creating the problem.<br /><br />This is corrected for, by the use of leap years in our calendar. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### harmonicaman

##### Guest
I think what member <b>RogerInNH</b> is asking is; "Apart from our completely arbitrary system of measuring time based on the rotation of the Earth, does the universe have a built in clock mechanism?"<br /><br />The universe does not really have an obvious ticking clock, but there are several phenomenon in the universe which occur at very precise intervals and we can utilize these to calibrate our completely subjective definition of time. <br /><br />We currently measure a second as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition, or change in energy level, of the cesium-133 atom.<br /><br />Another way to measure time would be by the Planck time unit. This is the amount of time it takes a photon travelling at "c" (the speed of light) to cross a distance equal to a 'quantum of length' (the Planck length). This is a ‘quantum of time’, the smallest measurement of time that has any meaning, and it is equal to about 10<sup>-43</sup> seconds. Unfortunately, this is a theoretical estimate and we do not have the technology to use this extremely tiny interval for accurate time keeping...<br /><br />On the macro scale, Pulsars have been observed to rotate at very precise intervals, up to 619 times per second, but this can change over the eons as their mass changes.<br /><br />I think an alien civilization would also base time keeping on their planets rotation because this is such an obvious measurable interval; but they would eventually rely on other observable phenomenon to maintain accuracy and precision.<br />

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