The Leap Second

Is this article incorrect to attribute the leap second to a slowing in the earth's orbit?I thought I understood this issue already, but as I tried thinking through this to prove to myself how the article was wrong I suddenly realized that I wasn't able to resolve the two degrees of freedom: the earth's revolution around the sun, and the earth's rotation. Leap days are designed to bring the year to an end at the same place in the earth's orbit. Leap seconds are intended to keep midnight at the point in the earth's rotation. Or at least that's what I thought. But how do leap days and leap seconds work together to make the year end happen at the same time of day every year (within the accuracy of the gross adjustments of leap days)? Am I just confused, or am I right in seeing the issue is a little more involved than I originally thought. And is the earth's orbit really slowing? I was only aware of the slowing rotation.--Percy

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I'll go off the top of my head for a first pass at this. The article is wrong.The leap second is associated with the slowing rotation of the earth on it's axis NOT it's orbital motion through space.I also think that the leap second is NOT added during the last day of the year but rather is when needed.this site has reasonable details
http://tycho.usno.navy.mil/leapsec.htmlAs best as I can tell, all that has happened is that the earth has been "sped up" by a millisec or so a day for a bit. Tides etc. I think could cause this. The rotational slowing is certainly not going to be uniform IMHO and I don't see that it has to be precisely slowing all the time. That level of details can wait for the astrophysicists though.

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Common sense isn't

Ned's right and CNN wrong - it's Earth's rotation that's erratic. The "Observer's Handbook" of the Royal Astronomical Society of Canada has a detailed explanation - six pages' worth - but it doesn't appear to be online. Odd that the reporter or the guy at NIST would get it so bollixed up, though. Could it be an antiterrorism red herring?

Hmmm...what about the sidereal motion? That is, due to the fact that the earth is moving around the sun as it is rotating on its axis, the actual amount of time it takes for the earth to return to its previous orientation with respect to its axis is shorter than the amount of time it takes for any one given spot to return to facing the sun. If there were a perturbation in the orbit around the sun, might that have an effect on how quickly it takes to get to see the sun again?When it's midnight, you are (at least theoretically), facing directly opposite the sun. Suppose that over the next 12 hours, the earth were to move quickly to the opposite side of the sun. Well, according to our standard definition of a "day," you'd expect to see the sun in the sky, but you'd actually never see it...you'd remain at midnight. Since our definition of "day" is the solar day (the time it takes to see the sun again or pretty damned close to 24 hours) and not the sidereal day (the time it takes for the earth to rotate 360 or 23 hours, 56 minutes, 4.1 seconds), might perturbations in the orbit also be responsible for an adjustment in the length of a solar day?I'm not sure if this has any bearing.

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Rrhain
WWJD? JWRTFM!

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(the time it takes for the earth to rotate 360 or 23 hours, 56 minutes, 4.1 seconds)

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Quite a while back, I was thinking about asking the trick(?) question "How many degrees does the earth rotate in 24 hours?". Now the right topic had come along, but you beat me to it.I was thinking of offering up the possible answers -
a) Close to 360 degrees
b) Close to 359 degrees
or
c) Close to 361 degreesI knew the answer wasn't (a), but I didn't know if it was (b) or (c). According to your information, (c) is the answer.Moose

From the NIST website. Page not found | NISTwe have this. So there is probably a hugely complex phenomena at work here. It will be interesting to track this story over time.

The guy Sidelined quotes is correct, and CNN wrong - tides and weather don't much faze our orbital motion, just rotation.Rrhain, I wish that the RASC handbook were online. It gets into all those different sorts of year - tropical, siderial, draconic, anomanolistic....each measured in a different frame of reference. It makes one's teeth hurt to try to learn them all.

It does seem to be the case that the author of the story does seem to be confusing orbital motion with rotational motion.And since Fred McGehan is one of the media contacts for NIST, which has been quoted to contradict the article's claim of orbit, I am wondering why the article seems to indicate he said "orbit."And yeah, crash, I know that there are plenty of different ways of calculating a year, all depending upon how you define your starting and ending positions. If I recall correctly, the "draconic" year has to do with the moon and the sun going around the zodiac...it's connected to calculating eclipses.

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Rrhain
WWJD? JWRTFM!

What is a Leap Second? from the U.S. Naval Observatory.

And if you really feel the need to twist your brain around time keeping and such then this website Gravity Probe B: Testing Einstein's Universe ought to help bring about new information to cause us mere mortals headaches.An excerpt from the site as follows. Lauch date april? 2004.

Huh? I didn't say anything. (If this was a pre-emptive anti-nit-pick, I'd say you have more to worry about from DNAunion these days than from me.)

This discussion clears up my confusion. I mistakenly believed, out of a hopefully temporary failure of logical thinking, that every year the sidereal year would always end at the same time of sidereal day. Can't explain how I fell into this misconception. I'm better now!In checking out a couple of the links and doing some hardcopy reading I found that the tropical year from one spring equinox to the next and the sidereal year measured against the fixed stars differs in length by 20 minutes. The difference is due to precession. 20 minutes is a huge amount and startled me. I did some quick calculations and discovered that every 72 years or so the constellations move by a whole days worth at the spring equinox, approximately a degree. That means that every 2100 years the spring equinox changes constellation, so that in the time of Christ the spring equinox occurred with the Sun in Ares, not Pisces. Wow!While I knew about precession of the equinoxes, it never impressed itself upon me that it was so incredibly rapid. I had it classified in my mind as a "millions of years" type of process.--Percy

In fact, Percy, the position of the sun (relative to the stars) at the equinox is still called the first point of Ares. (Fun astronomy fact.)And, yes, it is rapid. When I was doing telemetry for a spacecraft in orbit about Venus, we had to enter the date of the star charts we were using so that the programs could calculate the positions correctly. A few decades would make a big difference.[This message has been edited by Chiroptera, 01-03-2004]

crashfrog responds to me:

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Huh? I didn't say anything.

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Nah...just me being incapable of distinguishing "Coragyps" from "crashfrog." Gotta get this translator of mine fixed....

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Rrhain
WWJD? JWRTFM!

quote:

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Quite a while back, I was thinking about asking the trick(?) question "How many degrees does the earth rotate in 24 hours?". Now the right topic had come along, but you beat me to it.

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I was thinking of offering up the possible answers -
a) Close to 360 degrees
b) Close to 359 degrees
or
c) Close to 361 degrees

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I knew the answer wasn't (a), but I didn't know if it was (b) or (c). According to your information, (c) is the answer.

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Moose

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Objects orbiting in space have a tendency to gradually change their rotation/spin until the same face faces the object they are orbiting (eg the moon is a classic example; ie its "day" when orbiting around the earth would actually be infinity degrees). The Earth is following this tendency, while that of Venus is in the opposite direction (meaning the change in its rotation will be much greater than say that of earth or other planets).So, if you take say, a position in space (pretend you're looking down) where the earth is moving counter-clockwise around the sun. Then the earth would be spinning also in a counter-clockwise direction, as per the tendency above ^.So, as the earth moves counter-clockwise, the earth would have to spin even more counter-clockwise to compensate.So I think the answer is C, closer to 361 degrees. I might be wrong so don't kill me if I am (just thought this up off the top of my head but anyway...)[This message has been edited by blitz77, 01-10-2004]