# Leap Minutes? **URL:** \ **Category:** Factual Questions\ **Created:** [January 15, 2001, 5:26am UTC](https://boards.straightdope.com/t/leap-minutes/50092 "2001-01-15T05:26:07Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Sputnik](https://avatars.discourse-cdn.com/v4/letter/s/ce73a5/32.png) [@Sputnik](https://boards.straightdope.com/u/Sputnik)\ **Post date:** [January 15, 2001, 5:26am UTC](https://boards.straightdope.com/t/leap-minutes/50092/1 "2001-01-15T05:26:07Z")
I know that we have leap years because the time it takes the earth to rotate on its axis (one day) and the time it takes the earth to orbit the sun (one year or 365.24 days) are both components of our calendar and we have to reconcile them somehow - thus the leap year. Now for my question: doesn’t this concept apply to _days_ as well? Since a day is really 23 hours and 56 minutes long, and our clocks measure 24 hours, don’t we have to somehow make up for that four minute daily differential? How is that done? I checked out Cecil’s leap-year columns - they don’t talk about it. My first thought was that all clockmakers are party to a worldwide conspiracy to divide up a 23h56m day into 24 parts, but I dismissed that idea because such a scheme would screw up all kinds of other time measurements that take place _within_ a day (an “hour” wouldn’t really be an hour long). So there must be some way to deal with it - otherwise midnight would occur four minutes earlier every night until it happened during daylight. I think. Am I missing something? ---
**Author:** ![schief2](https://avatars.discourse-cdn.com/v4/letter/s/cab0a1/32.png) [@schief2](https://boards.straightdope.com/u/schief2)\ **Post date:** [January 15, 2001, 6:25am UTC](https://boards.straightdope.com/t/leap-minutes/50092/2 "2001-01-15T06:25:18Z")
You’re confusing **sidereal days** , which is what astoronomers use as a point of reference in making their observations, with **solar days** , which are what we use as the basis for our clocks and calendars. Sidereal days equal 23 hours 56 minutes; Solar days equal 86,400 seconds, or exactly 24 hours. From [NASA](http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/sidereal.html): > [@](#): > > A sidereal (pronounced sigh-dear’-real) day refers to the rotation of the Earth measured relative to the stars. It is the time it takes the Earth to rotate 360 degrees and is equal to 23 hours, 56 minutes and 4 seconds. > > A solar day is 24 hours long. This is the time it takes from one noon (sun overhead) to the next noon. > > The difference in the two “days” arises from the fact that during a day the Earth also travels nearly a degree further on its yearly trek around the Sun. However, the earth is actually slowing down ever-so-slightly, and as a result it’s (_very_ gradually) taking longer than 86,400 seconds to equal one solar day. As a result, the Powers That Be will declare an extra “leap second” every couple years or so, to keep all the atomic clocks accurate. [This](http://tycho.usno.navy.mil/leapsec.html) is a fairly technical page from the Naval Observatory with more than you would ever want to know on the subject. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [January 15, 2001, 6:33am UTC](https://boards.straightdope.com/t/leap-minutes/50092/3 "2001-01-15T06:33:18Z")
One solar day is almost exactly 24 hours. The 23 hour and 56 minute period is a sidereal day. A solar day is the rotation period relative to the sun, and the sidereal day is the rotation period relative to the stars. Since the earth orbits the sun once a year, an observer on the sun would only see the earth rotate 365.24 times a year, while an observer outside the solar system would see it 366.24 times. Same reason the moon doesn’t rotate at all when seen from the earth, but when seen from outside the earth/moon system, it rotates once a month. Another way to think about it is that during one day, the earth travels along its orbit by 1 degree. So the earth must spin once plus 1 degree to point the same face to the sun. So the solar day is a bit longer. But we do occasionally have leap seconds. This is because the spin of the earth isn’t perfectly stable. Our clocks are accurate enough to notice this, so we sometimes insert or remove a second or two. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [January 15, 2001, 8:47am UTC](https://boards.straightdope.com/t/leap-minutes/50092/4 "2001-01-15T08:47:12Z")
> [@](#): > > \*Originally posted by scr4 \* > \*\*But we do occasionally have leap seconds. This is because the spin of the earth isn’t perfectly stable. Our clocks are accurate enough to notice this, so we sometimes insert or remove a second or two. \*\* They’ve never removed a leap second, and unless they redefine how they do things, there is little chance that they ever will. That is sometimes seen as a good thing–less confusion. Primarily, the leap seconds aren’t inserted because the day is slowing down, it is because our definition of day is too long–we use one from a hundred years ago. The reason that the day of a hundred years ago is too long is because the earth is slowing down, true, but using a more recent standard would have kept us from adding so many leap seconds. ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [January 15, 2001, 12:06pm UTC](https://boards.straightdope.com/t/leap-minutes/50092/5 "2001-01-15T12:06:24Z")
Try this: [http://www.ieee-uffc.org/freqcontrol/fc\_utc.html](http://www.ieee-uffc.org/freqcontrol/fc_utc.html) ---
**Author:** ![Sputnik](https://avatars.discourse-cdn.com/v4/letter/s/ce73a5/32.png) [@Sputnik](https://boards.straightdope.com/u/Sputnik)\ **Post date:** [January 15, 2001, 6:12pm UTC](https://boards.straightdope.com/t/leap-minutes/50092/6 "2001-01-15T18:12:03Z")
Thank you all for relieving me of another one of those nagging questions. Now I have time to think up some more.