Why are the time zones calculated as 360°/24 and not 361°/24 or 360°/23.933?

  • 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?

    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?

    The 361 ° are wrong anyway, it is 361.002785. But with reference to the sun the Earth rotates 360.0000 ° in 24 h.

    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...

    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".)

    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.

    Frame challenge: There are clearly way more than 24 time zones, ranging from -12:00 to +14:00 (ignoring potential DST complexities).

    @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.

    I'd recommend watching this video: How Earth Moves. 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.

  • James K

    James K Correct answer

    one year ago

    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.

    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?°

    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.

    @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.

    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.

    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.

    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?

    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?

    @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.

    @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 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.

    @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.

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