# Clocks lose 4 min a day relative to the sun...why doesn't the sun rise at 6pm after 180 days? **URL:** \ **Category:** Factual Questions\ **Created:** [August 3, 2018, 6:45pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062 "2018-08-03T18:45:21Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Sigene](https://avatars.discourse-cdn.com/v4/letter/s/df788c/32.png) [@Sigene](https://boards.straightdope.com/u/Sigene)\ **Post date:** [August 3, 2018, 6:45pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/1 "2018-08-03T18:45:21Z")
partly inspired by a response in this thread that the earth make one rotation in 23.97 hours [https://boards.straightdope.com/sdmb/showthread.php?t=859519](https://boards.straightdope.com/sdmb/showthread.php?t=859519) If the earth rotates once every 23 hours 56 minutes it is rotating faster than our clocks which are set for 24 hours. So it seems that everyday our clocks are off by 4 minutes. After 180 days they will be off by 720 minutes or 12 hours. Why don’t we see the sun rising progressively 4 min earlier than predicted everyday, such that after 180 days the sun is rising at 6pm instead of 6am? I’m going to guess… This discrepency is countered by the orbit of the earth, such that the earth essentially progresses roughly 4 min further along its orbit relative to the sun everyday. And if we take the sun out of the picture the stars will be rising 12 hours off after 180 days. Is this at all correct? Am I explaining it with any success? ---
**Author:** ![friedo](https://avatars.discourse-cdn.com/v4/letter/f/8edcca/32.png) [@friedo](https://boards.straightdope.com/u/friedo)\ **Post date:** [August 3, 2018, 6:55pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/2 "2018-08-03T18:55:14Z")
Your guess is right. A solar day is around for minutes and change longer than a sidereal day, because the rotation of the earth has to “catch up” with the sun which has moved relative to the earth’s rotation. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [August 3, 2018, 7:32pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/3 "2018-08-03T19:32:29Z")
To put it another way, clocks don’t lose 4 minutes per day relative to the sun. They lose 4 minutes per day relative to the stars, which adds up to exactly 1 extra revolution per year. Since we care more about the position of the sun in the sky than we do about the position of Betelguese or Rigel, we calibrate our clocks to display solar time rather than sidereal time. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 3, 2018, 9:27pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/4 "2018-08-03T21:27:53Z")
But there are clocks which are calibrated according to the stars, instead of the Sun. They’re used, unsurprisingly, by astronomers. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [August 3, 2018, 11:57pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/5 "2018-08-03T23:57:03Z")
This was a plot device in Jules Verne’s _Around the World in Eighty Days_ … The protagonist, Phileas Fogg, traveled east … thus he gained an hour for every 15º longitude he covered … the surprise ending is he took 81 days to travel around the world, but the Reform Club had only experienced 80 days … so Phileas Fogg won the bet after all … ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 4, 2018, 12:16am UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/6 "2018-08-04T00:16:43Z")
Not quite the same thing, since he was going around the Earth, while this thread relates to the Earth going around the Sun. But it’s a similar effect. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [August 4, 2018, 12:25am UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/7 "2018-08-04T00:25:25Z")
A lot of modern clocks are synced to National Time Services, like the WWV signal from the US National Institute of Standards. That is adjusted periodically for leap-years (and even leap-seconds, now). So even if the day was short a few seconds or minutes, many clocks would be automatically reset to the agreed-upon national time standard. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 4, 2018, 4:11am UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/8 "2018-08-04T04:11:19Z")
> [@Tim\_T-Bonham.net](#): > > A lot of modern clocks are synced to National Time Services, like the WWV signal from the US National Institute of Standards. That is adjusted periodically for leap-years (and even leap-seconds, now). So even if the day was short a few seconds or minutes, many clocks would be automatically reset to the agreed-upon national time standard. That is an interesting issue, because the current standard only allows for at most one leap second per month, or 12 seconds per year. With the Earth’s rotation slowing down, this will “soon” become a problem for the (solar) day. ---
**Author:** ![The\_Stafford\_Cripps](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_stafford_cripps/32/7167_2.png) [@The\_Stafford\_Cripps](https://boards.straightdope.com/u/The_Stafford_Cripps)\ **Post date:** [August 4, 2018, 8:58am UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/9 "2018-08-04T08:58:24Z")
Am I missing something here? 23.97 hours is only 108 seconds short of 24 hours, not 4 minutes. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 4, 2018, 10:53am UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/10 "2018-08-04T10:53:23Z")
Must be a typo. The current revolution rate is once per 24 hours minus 236 seconds, or approximately 23.93 h. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [August 4, 2018, 2:37pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/11 "2018-08-04T14:37:47Z")
> [@watchwolf49](#): > > This was a plot device in Jules Verne’s _Around the World in Eighty Days_ … Pietro Martire d’Anghiera, historian and chaplain to the King of Spain, commented on this in his _De Orbe Novo_: > [@Peter Martyr D' Anghera](#): > > When the _Victoria_ [the only ship to return from Magellan’s famous voyage] reached the Cape Verde Islands, the sailors believed the day to be Wednesday, whereas it was Thursday. They had consequently lost one day on their voyage, and during their three years’ absence. I said: `Your priests must have deceived you, since they have forgotten this day in their ceremonies and the recitation of their office [prescribed religious service].' They answered: `Of what are you thinking? Do you suppose that all of us, including wise and experienced men, could have made such a mistake? It often happens that an exact account is kept of the days and months, and moreover many of the men had office books and knew perfectly what had to be recited each day. There could be no mistake, especially about the office of the blessed virgin [Mary], at whose feet we prostrate ourselves each moment, imploring her assistance. Many passed their time reciting her office and that of the dead. You must, therefore, look elsewhere for an explanation, for it is certain that we have lost one day.’ > > Some gave one reason and some another, but all agreed upon one point, they had lost a day. I added: `My friends, remember that the year following your departure, that is to say, the year 1520, was a bissextile [leap] year, and this fact may have led you into error.’ They affirmed that they had taken account of the 29 days in the month of February in that year, which is usually shorter, and that they did not forget the bissextile of the calends of March of the same year. The 18 men who returned from the expedition are mostly ignorant, but when questioned, one after another, they did not vary in their replies. > > Much surprised by this agreement, I sought Gaspar Contarino, ambassador of the illustrious Republic of Venice at the court of the emperor [Charles V]. He is a great sage in many subjects. We discussed in many ways this hitherto unobserved fact, and we decided that perhaps the cause was as follows: > > The Spanish fleet, leaving the Gorgades [Cape Verde] Islands, proceeded straight to the west, that is to say, it followed the Sun, and each day was a little longer than the preceding, according to the distance covered. Consequently, when the tour of the world was finished-which the Sun makes in 24 hours from its rising to its setting the ship had gained an entire day; that is to say, one less than those who remain all that time in the same place. Had a Portuguese fleet, sailing towards the east, continued in the same direction, following the same route first discovered, it is positive that when it got back to the Gorgades it would have lost a little time each day, in making the circuit of the world; it would consequently have to count one day more. If on the same day a Spanish fleet and a Portuguese fleet left the Gorgades, each in the opposite direction, that is to say one towards the west and the other towards the east, and at the end of the same period and by different routes they arrived at the Gorgades, let us suppose on a Thursday, the Spaniards who would have gained an entire day would have called it Wednesday. The Portuguese who would have lost a day would declare it to be Friday. Philosophers may discuss the matter with more profound arguments, but for the moment I give my opinion and nothing more. > [@DPRK](#): > > Must be a typo. The current revolution rate is once per 24 hours minus 236 seconds, or approximately 23.93 h. 24_60_60/(365.24+1) = 235.9 seconds. Coincidence? 🙂 ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [August 4, 2018, 5:31pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/12 "2018-08-04T17:31:35Z")
> [@Gorsnak](#): > > To put it another way, clocks don’t lose 4 minutes per day relative to the sun. They lose 4 minutes per day relative to the stars, which adds up to exactly 1 extra revolution per year. Since we care more about the position of the sun in the sky than we do about the position of Betelguese or Rigel, we calibrate our clocks to display solar time rather than sidereal time. Or to rephrase the OP’s question: A star that sets shortly after sunset will 180+ days later be rising shortly after sunset.\* \*Assuming you are at the equator, or the days in question are equinoxes, or some other hand wave to ignore issues about when the sun sets. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [August 4, 2018, 6:14pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/13 "2018-08-04T18:14:53Z")
When Karl Jansky invented radio telescopy, he noticed a loud source he couldn’t quite track down - but he noticed that the loudest part of the noisy area was setting 4 minutes earlier every day, which led him to realize (eventually) that he was picking up the signal from the center of the Milky Way. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 4, 2018, 10:14pm UTC](https://boards.straightdope.com/t/clocks-lose-4-min-a-day-relative-to-the-sun-why-doesnt-the-sun-rise-at-6pm-after-180-days/819062/14 "2018-08-04T22:14:47Z")
His first discovery was that the source rose and set at all, thus proving that it was astronomical in origin. But as it happens, those first observations were in late December, when the Sun is very close to that point in the sky, so he initially assumed that the Sun itself was the source (a reasonable hypothesis, really). If it hadn’t been for that coincidence, he wouldn’t have needed the fact that it set according to sidereal time.