<blockquote>
<p>We teach the students:</p>
<ul>
<li>Sidereal day: In 23 h 56 min the earth rotates 360°</li>
<li>Solar day: In 24 h the earth rotates 361°</li>
</ul>
</blockquote>
<p>You should <strong>not</strong> teach your students that.</p>
<p>You should instead teach your students that it takes the Earth 23 hours and 56 minutes to rotate 360° with respect to the remote stars. So why do we use a 24 hour day? The reason we use a 24 hour day rather than a 23 hour and 56 minute day is that the remote stars do not rule our lives.</p>
<p>There is one star, which is not so remote, that does rule our lives. That star is the Sun. That the Earth orbits the Sun means that it takes a bit longer, about four minutes longer, for the Earth to rotate 360° with respect to the Sun.  In one year, the Earth rotates one more time with respect to the remote stars than it does with respect to the Sun.</p>

CC BY-SA 4.0
2020-09-28T21:37:35.593Z
"remote stars do not rule our lives" - and here I thought this were astrology.SE ;)
Hagen von Eitzen
CC BY-SA 4.0
2020-09-29T10:59:02.200Z
"the remote stars do not rule our lives". I've got a very strict rule : I'm only allowed to drink a sip of rum when Polaris is above the horizon.
Eric Duminil
CC BY-SA 4.0
2020-09-29T14:52:15.487Z
@EricDuminil - Don't go to Australia then. Or to New Zealand, or South Africa, or Antarctica.
David Hammen
CC BY-SA 4.0
2020-09-26T10:59:10.453Z
2020-09-29T14:53:10.323Z
2020-09-29T14:53:10.323Z
David Hammen
<p>All of the other answers are fairly technical, but a decently simple logic chain forces time zones to be 360° / 24. Consider this:</p>
<ul>
<li>Since there are 24 hours in a day, it makes sense to divide the Earth into 24 time zones.</li>
<li>There are 360° in a circle, because that's how degrees are defined.</li>
<li>To completely cover the circumference of the Earth, you must account for exactly 360°. If you account for more, then you will have wrapped the Earth more than once, and end up with some overlapping time zones. If you account for less, then there will be a slice of the Earth that lies outside of all time zones.</li>
<li>Therefore, the average time zone must be exactly 360° / 24 (or 15°).</li>
<li>To be fair, it makes the most sense to just set all time zones to 15°, starting at some line that we call 0°. (Then if some nations want to mess with their own time zones, slight adjustments can be made. And that's precisely what has happened.)</li>
</ul>

CC BY-SA 4.0
2020-09-28T17:45:11.673Z
+1 I really like this answer, I think going into solar vs sidereal is a bit of a red herring for this question.  In practice, time zones are often roughly applied and subject to all sorts of political whims and practicalities.
eps
CC BY-SA 4.0
2020-09-29T13:51:00.880Z
This is really all there is to it. If you want to divide a circle into 24 equal parts, they will each be 15 degrees. The only question is whether it should be divided into 24 equal parts or 23.93 equal parts, but the sidereal day has little use in most day-to-day applications. There's a reason why the solar day is a nice round number and the sidereal day is not, rather than the other way around.
Nuclear Hoagie
CC BY-SA 4.0
2020-09-27T08:06:31.173Z
2020-09-27T08:06:31.173Z
bobsbeenjamin
<p>Here’s a slightly different way to think about it that might be helpful to you and/or the students. To answer the question “how long does it take the Earth to rotate once on its axis?” you have to first answer “rotate with respect to what reference point?”</p>
<p>If you ask how long it takes to rotate enough to bring a given star back to the same position, that is 23 hours and 56 minutes.  If instead you ask, how long does it take to bring the Sun back to the same position, that is 24 hours.   Put differently, in 24 hours the Earth rotates 360 degrees <em>with respect to the Sun</em>.  (And it’s equally true that in 23h56m the Earth rotates 360 degrees <em>with respect to the stars</em>.)</p>
<p>The reason the two are different is that we are moving with respect to the Sun more than we are moving with respect to the stars.</p>
<p>Which is more relevant for Earth-based timekeeping?  Of course it’s the Sun, so we divide that 360 degrees by 24 hours.</p>
<p>Images like <a href="http://www.sos.siena.edu/%7Ejcummings/teaching/astronomy/lectures/figs/sun-earth-sidereal.png" rel="noreferrer">this</a> show the perspective of an observer outside the Earth, but for an observer fixed to the Earth, the Sun really moves (on average) 360 degrees in our sky in 24 hours.</p>

CC BY-SA 4.0
2020-09-26T19:46:32.393Z
The interval between times when the Sun passes the same longitude varies slightly throughout the year as a consequence of the Earth's elliptical orbit.  Many globes have a figure-eight-like figure that crosses the Equator in one of the oceans, which marks what the location of the Sun would be at 24-hour intervals.  It's pretty tall and skinny, but its width is definitely not zero.
supercat
CC BY-SA 4.0
2020-09-28T10:19:42.343Z
@supercat - There are two factors that result in that figure-eight-like figure you mentioned, which is called an analemma. You only mentioned one. The other is axial tilt, or obliquity, whose contribution is slightly greater than that of eccentricity -- for the Earth. A Mars analemma is teardrop-shaped rather than figure-eight-like because Mars' eccentricity dominates over its axial tilt.
David Hammen
CC BY-SA 4.0
2020-09-28T15:56:39.197Z
@DavidHammen: Thanks for the correction/clarification.  My intended point was that the time between local noons isn't constant, though the stated reason wasn't the main one.  If I'm understanding correctly, the issue is that even if the orbit were perfectly spherical, an observer on a stationary Earth-relative reference point (e.g. a plane flying around the Equator at a celestial-sphere-synchronous 1037.7mph) would perceive the Sun as tracing an elliptical path on a circular cylinder whose axis was parallel to that of Earth, but the path would be in the plane of the Earth's orbit?
supercat
CC BY-SA 4.0
2020-09-26T04:10:47.650Z
2020-09-26T04:20:20.267Z
2020-09-26T04:20:20.267Z
Eric Jensen
<p>Though I see the answer a few times, I feel that they are too complicated for a student. I'm 13 and my dad pointed me to this thread.</p>
<p>Here's how I see it. The Earth spins, and the Earth revolves around the Sun. The amount of time that it takes for a star to spin all the way around and appear again at the same spot is <strong>different</strong> than the amount of time  that it takes for the Sun to spin all the way around and appear again at the same spot.</p>
<p>Which one is interesting to me, when some star is rising and setting? Or when the Sun is rising and setting? Because I wake up according to the Sun, and eat dinner according to the Sun, I really only care about when the Sun is setting or rising. So we measure our days by the Sun, not the stars.</p>
<p>Exactly how many hours and minutes each takes is not interesting.</p>
<p>Tell your students to read Around the World in 80 days. There is a surprise at the end due to this exactly!</p>

CC BY-SA 4.0
2020-09-29T08:10:57.950Z
+1 For the teaser about _Around the World in Eighty Days_. I didn't know about this - now I'm intrigued and will have to read the book!
Oscar Bravo
CC BY-SA 4.0
2020-10-11T05:30:25.917Z
It really is a great read, and that ending is M. Night Shyamalan-level awesome
bobsbeenjamin
CC BY-SA 4.0
2020-09-27T19:50:02.057Z
2020-09-27T19:50:02.057Z
Happy Phantom
<p>This is not quite an answer but more than a simple comment. When I speak about distances I mean (angular) distances in the sky, when i speak about position I mean position in the sky. I adhere to terrestrial observer's point of view.</p>
<p>1, Starry sky (and everything on it, including Sun, Moon, planets) rotates around a point near Polaris (north celestial pole)</p>
<p>2, A given star (and everything which doesn't move relative to stars) gets to its maximum height above horizon at south (by definition). This is called culmination.</p>
<p>3, Interval between two consecutive culminations of the star is reasonably constant and is called sidereal day. Its length is approximately 23 hours and 56 minutes</p>
<p>4, Sun moves relative to stars along a well defined path, called ecliptic. This image shows starry sky along ecliptic on Apr 12, 2020 with Sun and planets. The ecliptic is the horizontal orange line in the middle.</p>
<p><a href="https://i.sstatic.net/eF3Ed.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/eF3Ed.png" alt="Zodiac with Sun and planets Apr 12, 2020" /></a></p>
<p>5, Sun moves along ecliptic to the left and completes its journey in one year. The length of the ecliptic is 360 degrees, year has slightly more than 360 days so Sun moves by nearly one degree in one day.</p>
<p>At the end of May, for example, Sun every year passes between the most prominent groups of stars along the ecliptic, Pleiades and Hyades, both in Taurus. This is real photo from SOHO satellite.</p>
<p><a href="https://i.sstatic.net/BljhW.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/BljhW.jpg" alt="SOHO photo of Sun in Taurus" /></a></p>
<p>6, When you measure culmination of Sun you must get more than sidereal day</p>
<p>7, Since, as many others noted in their comments and answers, our life is (still) controlled by daylight (Sun), we define day according to culmination of Sun, rather than stars. This is because solar (rather than sidereal) day has 24 hours.</p>
<p>6, If you define noon as time when real Sun is at south you get noon at different times for places at different geographical longitudes. This is serious problem when you need to coordinate say railway timetables. For this reason, time zones were introduced.</p>
<p>7, While sidereal day has (reasonably) constant length, solar day measured using real Sun has variable length during a year (motion of Sun along ecliptic is not uniform and different parts of ecliptic have different distance from celestial north pole). So the constant length 24 day is an abstraction - you must &quot;observe&quot; culmination of a fictional point called mean Sun.</p>

CC BY-SA 4.0
2020-09-30T08:50:48.117Z
2020-09-30T08:50:48.117Z
Leos Ondra
<p>If you say that one place is one hour ahead of another, that means that the time of day at which the sun reaches its greatest apparent height for the first place is one hour ahead of the second. Time zones refer to the apparent motion of the sun, not the motion of the earth. So they are calculated as solar day divided by 360 degrees. Put another way, the earliest clocks were sundials. It takes a solar day for the shadow of a sundial to go 360 degrees (it's not visible for the night portion, but we can model it as there being an imaginary shadow that continues to move).</p>

CC BY-SA 4.0
2020-09-26T19:28:31.913Z
2020-09-26T19:28:31.913Z
Acccumulation
<p>The Earth takes 23 hours 56 minutes to rotate once.  But that is not relevant to most people. Sure, the stars will be in the same position again after 23 hours 56 minutes, but the sun will not be in the same position.</p>
<p>It is far more important, for most people, to measure the time from noon to noon. And the average time from noon to noon is 24 hours. This is because the motion of the sun is a combination of both the spinning of the Earth and the orbit of the Earth around the sun. The orbital motion of the Earth adds four minutes. You should also teach the students</p>
<ul>
<li>In twenty-four hours the sun advances 360 degrees. (solar day)</li>
</ul>
<p>Time zones are based on clock time, which is based on the motion of the sun and not the motion of the stars.</p>

CC BY-SA 4.0
2020-09-25T17:42:34.963Z
Thanks a lot for the answer and edits. I can totally follow most of your explanation, but I don't get why: "In twenty-four hours the sun advances 360 degrees" -> Why 360° and not 361?°
mfran
CC BY-SA 4.0
2020-09-25T17:56:26.000Z
The motion of the sun is **not** the same as the motion of the stars.  The sun appears to move relative to the stars.  In 24 hours the stars advance 361 degrees.  In 24 hours the sun advances 360 degrees.  The sun has moved relative to the stars because of the orbital motion of the Earth.
James K
CC BY-SA 4.0
2020-09-26T19:27:56.867Z
@mfran (or other readers) It is good to consider that the number of degrees in a circle (360) is close to the number of days ina year (365.25). Hence roughly the one degree difference.
Vladimir F Героям слава
CC BY-SA 4.0
2020-09-26T20:48:01.680Z
Another way to see this motion is to look at it over larger times.  Start with a point at noon.  Wait six months.  When the Earth/point system have the same orientation with the distant starts, it is midnight.
Eric Towers
CC BY-SA 4.0
2020-09-27T00:19:05.020Z
A nice example of @EricTowers comment is the constellation Orion, one of the most recognizable constellations in the Northern Hemisphere winter sky. It began rising before sunrise in late July / early August. It is now visible high in the sky an hour or so before dawn. By January, it will be visible high in the sky after sunset.
David Hammen
CC BY-SA 4.0
2020-09-27T21:21:07.123Z
Why 360 and not 361? Before international standards and SI units, how do you thing people *defined* the length of a day, and then divided it into hours and minutes?
alephzero
CC BY-SA 4.0
2020-09-28T08:33:11.700Z
I think the OP is not understanding why the stars take 23h56m to return to the same position, and the sun 24h00m. Is that the fundamental problem?
Ben Hillier
CC BY-SA 4.0
2020-09-28T10:08:49.147Z
@BenHillier I suspect that that was the problem at the time the OP asked the question. Hopefully, this answer, along with the others, corrected that misunderstanding.
David Hammen
CC BY-SA 4.0
2020-09-29T02:48:38.650Z
@alephzero: Oh, it gets worse than that. Historically, everyone was on apparent solar time (i.e. time as measured by a sundial). The problem? You don't end up with a precisely fixed duration for your days if you measure them that way. So when you get around to inventing mechanical clocks, they have to be [adjusted](https://en.wikipedia.org/wiki/Equation_of_time) to match the sundial. Eventually you get tired of doing that and declare the sundial "wrong" and the mechanical clocks "right," which we call mean solar time. But it's just a convention.
Kevin
CC BY-SA 4.0
2020-09-29T13:59:51.823Z
@mfram the reason the sun appears to advance more than the stars is that we're orbiting the sun. We're not orbiting the stars (well, barring the slow galactic rotation). In half a year we end up on the other side of the sun. We don't end up on the other side of the stars.
candied_orange
CC BY-SA 4.0
2020-09-25T17:07:27.113Z
2020-09-25T17:07:27.113Z
James K
<p>Background: I'm training to be a geography teacher. Currently I have practice lessons and I'll be discussing solar time and standard time with the class. Now I stumbled over an issue to which I could not find an answer:</p>
<p>We teach the students:</p>
<ul>
<li>Sidereal day: In 23 h 56 min the earth rotates 360°</li>
<li>Solar day: In 24 h the earth rotates 361°</li>
</ul>
<p>We also teach that in order to construct time zones (as a replacement of solar time), one divided 360° by 24 (to have a zone for each hour of the day), which results in 24 zones with 15° each.</p>
<p>Now my question is: Why does one mix the measures for sidereal day and solar day? Or put differently: Why doesn't one calculate 361°/24 or 360° / 23.933?</p>

CC BY-SA 4.0
2020-09-27T07:48:24.323Z
Just out of curiosity: Lets say you make time zones 361°/24. Now you start at 0° and have the first time zone to 15.0416°. And so on. The 23rd time zone is from 345.9583° to 360. And thus only 14.0416° big. Or it overlaps with the 1st again. Neither seems very practical nor desirable. How do you propose in practice time zones of 361°/24 would actually work?
Polygnome
CC BY-SA 4.0
2020-09-27T15:54:15.837Z
The 361 ° are wrong anyway, it is 361.002785. But with reference to the sun the Earth rotates 360.0000 ° in 24 h.
Uwe
CC BY-SA 4.0
2020-09-28T07:01:20.693Z
what polygnome said. the earth is divided into (roughly) 24 time zones just because it's simple. this has nothing to do with the earth's rotation and everything to do with how many degrees there are in a revolution...
somebody
CC BY-SA 4.0
2020-09-28T10:47:17.673Z
The time of day and time-zones are based on the *apparent* movement of the Sun around the Earth. (For any passing flat-earthers, note that I highlighted the word "apparent".)
billpg
CC BY-SA 4.0
2020-09-28T13:20:58.813Z
Time zones are sociopolitical constructs *based* on an idea division of the earth into 24 equal segments. The level of precision you are talking about only exists along a very narrow band inside any given time zone anyway.
chepner
CC BY-SA 4.0
2020-09-29T04:54:40.117Z
Frame challenge:  There are clearly [way more than 24 time zones](https://en.wikipedia.org/wiki/Time_zone), ranging from -12:00 to +14:00 (ignoring potential DST complexities).
Eric Towers
CC BY-SA 4.0
2020-09-29T11:03:44.840Z
@EricTowers: Including [really weird ones](https://en.wikipedia.org/wiki/UTC%2B05:45).
Eric Duminil
CC BY-SA 4.0
2020-09-29T16:26:40.313Z
@billpg - Time of day and time zones are based on the *mean* movement of the Sun around the Earth. Time based on the *apparent* motion of the Sun (time as measured by a sundial) and time based on the mean motion of the Sun (time as measured by a mechanical or atomic clock) can differ by over 16 minutes, per the equation of time.
David Hammen
CC BY-SA 4.0
2020-09-29T17:05:52.173Z
I'd recommend watching this video: [How Earth Moves](https://www.youtube.com/watch?v=IJhgZBn-LHg). He goes on a bit at time but he goes over many of the 'arbitrary' choices human ended up doing in order to keep time counting manageable for non astronomers. He also have animations which explain quite visually the difference between sidereal and solar days.
Hoki
<earth><time>
Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?
mfran
# Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?

(Asked by: username_0 on 2020-09-25)

Background: I'm training to be a geography teacher. Currently I have practice lessons and I'll be discussing solar time and standard time with the class. Now I stumbled over an issue to which I could not find an answer:

We teach the students:

* Sidereal day: In 23 h 56 min the earth rotates 360°
* Solar day: In 24 h the earth rotates 361°

We also teach that in order to construct time zones (as a replacement of solar time), one divided 360° by 24 (to have a zone for each hour of the day), which results in 24 zones with 15° each.

Now my question is: Why does one mix the measures for sidereal day and solar day? Or put differently: Why doesn't one calculate 361°/24 or 360° / 23.933?

**Tags:** earth, time
> **Comments:**
> **username_1**: Just out of curiosity: Lets say you make time zones 361°/24. Now you start at 0° and have the first time zone to 15.0416°. And so on. The 23rd time zone is from 345.9583° to 360. And thus only 14.0416° big. Or it overlaps with the 1st again. Neither seems very practical nor desirable. How do you propose in practice time zones of 361°/24 would actually work?  
> **username_2**: The 361 ° are wrong anyway, it is 361.002785. But with reference to the sun the Earth rotates 360.0000 ° in 24 h.  
> **username_3**: what polygnome said. the earth is divided into (roughly) 24 time zones just because it's simple. this has nothing to do with the earth's rotation and everything to do with how many degrees there are in a revolution...  
> **username_4**: The time of day and time-zones are based on the *apparent* movement of the Sun around the Earth. (For any passing flat-earthers, note that I highlighted the word "apparent".)  
> **username_5**: Time zones are sociopolitical constructs *based* on an idea division of the earth into 24 equal segments. The level of precision you are talking about only exists along a very narrow band inside any given time zone anyway.  
> **username_6**: Frame challenge:  There are clearly [way more than 24 time zones](https://en.wikipedia.org/wiki/Time_zone), ranging from -12:00 to +14:00 (ignoring potential DST complexities).  
> **username_7**: @EricTowers: Including [really weird ones](https://en.wikipedia.org/wiki/UTC%2B05:45).  
> **username_8**: @billpg - Time of day and time zones are based on the *mean* movement of the Sun around the Earth. Time based on the *apparent* motion of the Sun (time as measured by a sundial) and time based on the mean motion of the Sun (time as measured by a mechanical or atomic clock) can differ by over 16 minutes, per the equation of time.  
> **username_9**: I'd recommend watching this video: [How Earth Moves](https://www.youtube.com/watch?v=IJhgZBn-LHg). He goes on a bit at time but he goes over many of the 'arbitrary' choices human ended up doing in order to keep time counting manageable for non astronomers. He also have animations which explain quite visually the difference between sidereal and solar days.  

---

## 7 answers

### Answer by username_20 on 2020-09-25. Score: 53 | Accepted answer

The Earth takes 23 hours 56 minutes to rotate once. But that is not relevant to most people. Sure, the stars will be in the same position again after 23 hours 56 minutes, but the sun will not be in the same position.

It is far more important, for most people, to measure the time from noon to noon. And the average time from noon to noon is 24 hours. This is because the motion of the sun is a combination of both the spinning of the Earth and the orbit of the Earth around the sun. The orbital motion of the Earth adds four minutes. You should also teach the students

* In twenty-four hours the sun advances 360 degrees. (solar day)

Time zones are based on clock time, which is based on the motion of the sun and not the motion of the stars.

> **Comments:**
> **username_0**: Thanks a lot for the answer and edits. I can totally follow most of your explanation, but I don't get why: "In twenty-four hours the sun advances 360 degrees" -> Why 360° and not 361?°  
> **username_20**: The motion of the sun is **not** the same as the motion of the stars.  The sun appears to move relative to the stars.  In 24 hours the stars advance 361 degrees.  In 24 hours the sun advances 360 degrees.  The sun has moved relative to the stars because of the orbital motion of the Earth.  
> **username_21**: @mfran (or other readers) It is good to consider that the number of degrees in a circle (360) is close to the number of days ina year (365.25). Hence roughly the one degree difference.  
> **username_6**: Another way to see this motion is to look at it over larger times.  Start with a point at noon.  Wait six months.  When the Earth/point system have the same orientation with the distant starts, it is midnight.  
> **username_8**: A nice example of @EricTowers comment is the constellation Orion, one of the most recognizable constellations in the Northern Hemisphere winter sky. It began rising before sunrise in late July / early August. It is now visible high in the sky an hour or so before dawn. By January, it will be visible high in the sky after sunset.  
> **username_22**: Why 360 and not 361? Before international standards and SI units, how do you thing people *defined* the length of a day, and then divided it into hours and minutes?  
> **username_23**: I think the OP is not understanding why the stars take 23h56m to return to the same position, and the sun 24h00m. Is that the fundamental problem?  
> **username_8**: @BenHillier I suspect that that was the problem at the time the OP asked the question. Hopefully, this answer, along with the others, corrected that misunderstanding.  
> **username_24**: @alephzero: Oh, it gets worse than that. Historically, everyone was on apparent solar time (i.e. time as measured by a sundial). The problem? You don't end up with a precisely fixed duration for your days if you measure them that way. So when you get around to inventing mechanical clocks, they have to be [adjusted](https://en.wikipedia.org/wiki/Equation_of_time) to match the sundial. Eventually you get tired of doing that and declare the sundial "wrong" and the mechanical clocks "right," which we call mean solar time. But it's just a convention.  
> **username_25**: @mfram the reason the sun appears to advance more than the stars is that we're orbiting the sun. We're not orbiting the stars (well, barring the slow galactic rotation). In half a year we end up on the other side of the sun. We don't end up on the other side of the stars.  

---

### Answer by username_8 on 2020-09-26. Score: 29

> We teach the students:
>
> * Sidereal day: In 23 h 56 min the earth rotates 360°
> * Solar day: In 24 h the earth rotates 361°

You should **not** teach your students that.

You should instead teach your students that it takes the Earth 23 hours and 56 minutes to rotate 360° with respect to the remote stars. So why do we use a 24 hour day? The reason we use a 24 hour day rather than a 23 hour and 56 minute day is that the remote stars do not rule our lives.

There is one star, which is not so remote, that does rule our lives. That star is the Sun. That the Earth orbits the Sun means that it takes a bit longer, about four minutes longer, for the Earth to rotate 360° with respect to the Sun. In one year, the Earth rotates one more time with respect to the remote stars than it does with respect to the Sun.

> **Comments:**
> **username_10**: "remote stars do not rule our lives" - and here I thought this were astrology.SE ;)  
> **username_7**: "the remote stars do not rule our lives". I've got a very strict rule : I'm only allowed to drink a sip of rum when Polaris is above the horizon.  
> **username_8**: @EricDuminil - Don't go to Australia then. Or to New Zealand, or South Africa, or Antarctica.  

---

### Answer by username_11 on 2020-09-27. Score: 14

All of the other answers are fairly technical, but a decently simple logic chain forces time zones to be 360° / 24. Consider this:

* Since there are 24 hours in a day, it makes sense to divide the Earth into 24 time zones.
* There are 360° in a circle, because that's how degrees are defined.
* To completely cover the circumference of the Earth, you must account for exactly 360°. If you account for more, then you will have wrapped the Earth more than once, and end up with some overlapping time zones. If you account for less, then there will be a slice of the Earth that lies outside of all time zones.
* Therefore, the average time zone must be exactly 360° / 24 (or 15°).
* To be fair, it makes the most sense to just set all time zones to 15°, starting at some line that we call 0°. (Then if some nations want to mess with their own time zones, slight adjustments can be made. And that's precisely what has happened.)

> **Comments:**
> **username_12**: +1 I really like this answer, I think going into solar vs sidereal is a bit of a red herring for this question.  In practice, time zones are often roughly applied and subject to all sorts of political whims and practicalities.  
> **username_13**: This is really all there is to it. If you want to divide a circle into 24 equal parts, they will each be 15 degrees. The only question is whether it should be divided into 24 equal parts or 23.93 equal parts, but the sidereal day has little use in most day-to-day applications. There's a reason why the solar day is a nice round number and the sidereal day is not, rather than the other way around.  

---

### Answer by username_14 on 2020-09-26. Score: 9

Here’s a slightly different way to think about it that might be helpful to you and/or the students. To answer the question “how long does it take the Earth to rotate once on its axis?” you have to first answer “rotate with respect to what reference point?”

If you ask how long it takes to rotate enough to bring a given star back to the same position, that is 23 hours and 56 minutes. If instead you ask, how long does it take to bring the Sun back to the same position, that is 24 hours. Put differently, in 24 hours the Earth rotates 360 degrees *with respect to the Sun*. (And it’s equally true that in 23h56m the Earth rotates 360 degrees *with respect to the stars*.)

The reason the two are different is that we are moving with respect to the Sun more than we are moving with respect to the stars.

Which is more relevant for Earth-based timekeeping? Of course it’s the Sun, so we divide that 360 degrees by 24 hours.

Images like [this](http://www.sos.siena.edu/%7Ejcummings/teaching/astronomy/lectures/figs/sun-earth-sidereal.png) show the perspective of an observer outside the Earth, but for an observer fixed to the Earth, the Sun really moves (on average) 360 degrees in our sky in 24 hours.

> **Comments:**
> **username_15**: The interval between times when the Sun passes the same longitude varies slightly throughout the year as a consequence of the Earth's elliptical orbit.  Many globes have a figure-eight-like figure that crosses the Equator in one of the oceans, which marks what the location of the Sun would be at 24-hour intervals.  It's pretty tall and skinny, but its width is definitely not zero.  
> **username_8**: @supercat - There are two factors that result in that figure-eight-like figure you mentioned, which is called an analemma. You only mentioned one. The other is axial tilt, or obliquity, whose contribution is slightly greater than that of eccentricity -- for the Earth. A Mars analemma is teardrop-shaped rather than figure-eight-like because Mars' eccentricity dominates over its axial tilt.  
> **username_15**: @DavidHammen: Thanks for the correction/clarification.  My intended point was that the time between local noons isn't constant, though the stated reason wasn't the main one.  If I'm understanding correctly, the issue is that even if the orbit were perfectly spherical, an observer on a stationary Earth-relative reference point (e.g. a plane flying around the Equator at a celestial-sphere-synchronous 1037.7mph) would perceive the Sun as tracing an elliptical path on a circular cylinder whose axis was parallel to that of Earth, but the path would be in the plane of the Earth's orbit?  

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### Answer by username_16 on 2020-09-27. Score: 4

Though I see the answer a few times, I feel that they are too complicated for a student. I'm 13 and my dad pointed me to this thread.

Here's how I see it. The Earth spins, and the Earth revolves around the Sun. The amount of time that it takes for a star to spin all the way around and appear again at the same spot is **different** than the amount of time that it takes for the Sun to spin all the way around and appear again at the same spot.

Which one is interesting to me, when some star is rising and setting? Or when the Sun is rising and setting? Because I wake up according to the Sun, and eat dinner according to the Sun, I really only care about when the Sun is setting or rising. So we measure our days by the Sun, not the stars.

Exactly how many hours and minutes each takes is not interesting.

Tell your students to read Around the World in 80 days. There is a surprise at the end due to this exactly!

> **Comments:**
> **username_17**: +1 For the teaser about _Around the World in Eighty Days_. I didn't know about this - now I'm intrigued and will have to read the book!  
> **username_11**: It really is a great read, and that ending is M. Night Shyamalan-level awesome  

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### Answer by username_18 on 2020-09-30. Score: 1

This is not quite an answer but more than a simple comment. When I speak about distances I mean (angular) distances in the sky, when i speak about position I mean position in the sky. I adhere to terrestrial observer's point of view.

1, Starry sky (and everything on it, including Sun, Moon, planets) rotates around a point near Polaris (north celestial pole)

2, A given star (and everything which doesn't move relative to stars) gets to its maximum height above horizon at south (by definition). This is called culmination.

3, Interval between two consecutive culminations of the star is reasonably constant and is called sidereal day. Its length is approximately 23 hours and 56 minutes

4, Sun moves relative to stars along a well defined path, called ecliptic. This image shows starry sky along ecliptic on Apr 12, 2020 with Sun and planets. The ecliptic is the horizontal orange line in the middle.

[![Zodiac with Sun and planets Apr 12, 2020](https://i.sstatic.net/eF3Ed.png)](https://i.sstatic.net/eF3Ed.png)

5, Sun moves along ecliptic to the left and completes its journey in one year. The length of the ecliptic is 360 degrees, year has slightly more than 360 days so Sun moves by nearly one degree in one day.

At the end of May, for example, Sun every year passes between the most prominent groups of stars along the ecliptic, Pleiades and Hyades, both in Taurus. This is real photo from SOHO satellite.

[![SOHO photo of Sun in Taurus](https://i.sstatic.net/BljhW.jpg)](https://i.sstatic.net/BljhW.jpg)

6, When you measure culmination of Sun you must get more than sidereal day

7, Since, as many others noted in their comments and answers, our life is (still) controlled by daylight (Sun), we define day according to culmination of Sun, rather than stars. This is because solar (rather than sidereal) day has 24 hours.

6, If you define noon as time when real Sun is at south you get noon at different times for places at different geographical longitudes. This is serious problem when you need to coordinate say railway timetables. For this reason, time zones were introduced.

7, While sidereal day has (reasonably) constant length, solar day measured using real Sun has variable length during a year (motion of Sun along ecliptic is not uniform and different parts of ecliptic have different distance from celestial north pole). So the constant length 24 day is an abstraction - you must "observe" culmination of a fictional point called mean Sun.


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### Answer by username_19 on 2020-09-26. Score: 0

If you say that one place is one hour ahead of another, that means that the time of day at which the sun reaches its greatest apparent height for the first place is one hour ahead of the second. Time zones refer to the apparent motion of the sun, not the motion of the earth. So they are calculated as solar day divided by 360 degrees. Put another way, the earliest clocks were sundials. It takes a solar day for the shadow of a sundial to go 360 degrees (it's not visible for the night portion, but we can model it as there being an imaginary shadow that continues to move).


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