<p>It's a matter of how "day" is defined.</p>

<p>Wikipedia's article on Jupiter cites <a href="http://www.hnsky.org/iau-iag.htm" rel="noreferrer">this IAU/IAG paper</a> for the length of a Jupiter day.  In it, footnote (e) of table I has the following:</p>

<blockquote>
  <p>The equations for W for Jupiter, Saturn, Uranus and Neptune refer to the rotation of their magnetic fields (System III)</p>
</blockquote>

<p>The radio emissions of the gas giants have well-defined periodic variations.  These variations are caused by the rotation of the magnetic fields of those planets, and are evidence that they have a reasonably coherent core of some sort that's rotating at a uniform speed.  The periodic variations then represent the rotation speed of that object, which is taken as the rotation speed of the planet.</p>

<p>We're reasonably certain the Sun <em>doesn't</em> have a coherent core.  Measuring the variation of the magnetic field doesn't show a well-defined period, and doesn't provide a useful definition of the Sun's rotation speed. </p>

CC BY-SA 4.0
2019-03-22T08:08:58.137Z
I just add that for Jupiter there is definitively no a fix day based on std rotation. At least no for the outer layer. It does even rotate in stripes and a simulation as in Celestia package is a kind of spectacle.
Alchimista
CC BY-SA 4.0
2019-03-22T13:37:51.880Z
“The assumption is that whatever's generating the magnetic field forms a reasonably coherent mass that's rotating at a uniform speed.” — I’d strengthen this by pointing out that it’s not just an assumption, it’s based closely on empirical facts: the magnetic field has a measurable uniform periodic behaviour, and based our understanding of planetary magnetic fields, we’re confident this corresponds to rotation of a somewhat coherent core.  With the sun, as I understand it, we don’t see *any* uniformly periodic behaviour that we would expect to correspond to some kind of rotating mass.
Peter LeFanu Lumsdaine
CC BY-SA 4.0
2019-03-22T14:06:27.643Z
The [Sun's magnetic field](https://www.nasa.gov/feature/goddard/2016/picturing-the-sun-s-magnetic-field) don't have a period as @PeterLeFanuLumsdaine stated.
Mindwin Remember Monica
CC BY-SA 4.0
2019-03-22T03:14:13.437Z
2019-03-22T19:55:59.160Z
2019-03-22T19:55:59.160Z
Mark
<p>The <a href="https://en.wikipedia.org/wiki/Solar_rotation" rel="noreferrer">Sun's rotation period</a> varies from about 25 days at the equator to about 38 days at the poles. As I understand it, this is because the Sun is not solid, and because of the way centripetal force works, the equator must move faster than the poles.</p>

<p>Question: if this works, why do Jupiter/Saturn/Uranus/Neptune have well-defined days? Why don't the equators of these planets rotate faster than the poles as well? For example, <a href="https://en.wikipedia.org/wiki/Jupiter" rel="noreferrer">Wikipedia's article on Jupiter</a> gives the length of a Jovian day as 9h 55m 30s, which is so precise that it implies Jupiter does not have a rotational period which varies with latitude.</p>

<the-sun><rotation><gas-giants>
Why does the Sun have different day lengths, but not the gas giants?
Allure
# Why does the Sun have different day lengths, but not the gas giants?

(Asked by: username_0 on 2019-03-22)

The [Sun's rotation period](https://en.wikipedia.org/wiki/Solar_rotation) varies from about 25 days at the equator to about 38 days at the poles. As I understand it, this is because the Sun is not solid, and because of the way centripetal force works, the equator must move faster than the poles.

Question: if this works, why do Jupiter/Saturn/Uranus/Neptune have well-defined days? Why don't the equators of these planets rotate faster than the poles as well? For example, [Wikipedia's article on Jupiter](https://en.wikipedia.org/wiki/Jupiter) gives the length of a Jovian day as 9h 55m 30s, which is so precise that it implies Jupiter does not have a rotational period which varies with latitude.

**Tags:** the-sun, rotation, gas-giants


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## 1 answers

### Answer by username_1 on 2019-03-22. Score: 56 | Accepted answer

It's a matter of how "day" is defined.

Wikipedia's article on Jupiter cites [this IAU/IAG paper](http://www.hnsky.org/iau-iag.htm) for the length of a Jupiter day. In it, footnote (e) of table I has the following:

> The equations for W for Jupiter, Saturn, Uranus and Neptune refer to the rotation of their magnetic fields (System III)

The radio emissions of the gas giants have well-defined periodic variations. These variations are caused by the rotation of the magnetic fields of those planets, and are evidence that they have a reasonably coherent core of some sort that's rotating at a uniform speed. The periodic variations then represent the rotation speed of that object, which is taken as the rotation speed of the planet.

We're reasonably certain the Sun *doesn't* have a coherent core. Measuring the variation of the magnetic field doesn't show a well-defined period, and doesn't provide a useful definition of the Sun's rotation speed.

> **Comments:**
> **username_2**: I just add that for Jupiter there is definitively no a fix day based on std rotation. At least no for the outer layer. It does even rotate in stripes and a simulation as in Celestia package is a kind of spectacle.  
> **username_3**: “The assumption is that whatever's generating the magnetic field forms a reasonably coherent mass that's rotating at a uniform speed.” — I’d strengthen this by pointing out that it’s not just an assumption, it’s based closely on empirical facts: the magnetic field has a measurable uniform periodic behaviour, and based our understanding of planetary magnetic fields, we’re confident this corresponds to rotation of a somewhat coherent core.  With the sun, as I understand it, we don’t see *any* uniformly periodic behaviour that we would expect to correspond to some kind of rotating mass.  
> **username_4**: The [Sun's magnetic field](https://www.nasa.gov/feature/goddard/2016/picturing-the-sun-s-magnetic-field) don't have a period as @username_3 stated.  

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